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Museum of Comparative Zoology
HERPETOLOGY LIBRARY
Ernst fJlayr Library ^ ^^^J^ ^^
^■fr-m?>aratve Zoology /J p-' ^-^^ —
A HISTORY
OF
BRITISH FOSSIL REPTILES
SIR RICHARD OWEN, Iv.C.B., E.R.S., Etc.,
FOREIGN ASSOCIATE OF THE INSTITUTE OF FEAKCE (ACADEMY OF SCIENCES).
VOL. III.
LONDON : CASSBLL & COMPANY LIMITED,
LA BELLE SAUVAGE TAED. 1849—84.
PEIMTED BT J. E. ADtARI), BAETHOLOMEW CtOSE.
CONTENTS.
PAGE-INDEX.
FOSSIL REPTILIA OY THE LIASSIC FORMATIONS.
CHAPTER I. Order— SAUROPTERYGTA.
§ 1. Genus— Pi. EsiosAURUs.
Species — Plesiosaiirus doUchodeirus .
— homalospondyhis
— rostratiis
— rugosus
1 12 20 34
CHAPTER II. Order— ICHTHYOPTERYGIA.
|
Genus — Ichtiiyosauhus . |
41 |
|||||
|
A. Introduction |
41 |
|||||
|
B. Osteology |
45 |
|||||
|
Species — Ichthyosaurus breviceps |
67 |
|||||
|
— communis |
68 |
|||||
|
— intermedius |
70 |
|||||
|
— platyodon |
73 |
|||||
|
— lonchiodoH |
75 |
|||||
|
— longifrons |
76 |
|||||
|
— latifrons |
76 |
|||||
|
— acutirostris |
78 |
|||||
|
— teniiinostris |
. 7Q |
|||||
|
— longirostris |
82 |
|||||
|
— latimanvs |
83 |
|||||
|
— brachyspondylus |
85 |
|||||
|
Conclusion |
. 85 |
IV
PAGE-INDEX.
CHAPTER III.
Order— DINOSAURIA.
Genus — Scelidosauuis
Species — Scelidosaurus Harrisunii
PAGE
89 92
(Supplement No. II.)
MeZOZOIC L.\CE11TILIA
Geuus — EcuiNODON Species — Echinodon becclesii
126 12G 126
CHAPTER IV.
Ordkr— CROCODILIA.
|
Fainilv— Protosuciiii |
129 |
||||
|
Geuus — Teleosauuus |
130 |
||||
|
Species — Teleusiiiiriis C/ta/jinauni |
130 |
||||
|
— cudoineiisis |
139 |
||||
|
— brevior |
140 |
||||
|
— tatifrons |
141 |
||||
|
— astheaodeirus |
141 |
||||
|
Geuus — Steneosaukus |
143 |
||||
|
Species — Sleiieosuurus Geoffroyi |
144 |
||||
|
— luticeps |
145 |
||||
|
— temporalis |
145 |
||||
|
Geuus — Plesiosuouus |
146 |
||||
|
Species — Plesiosuchus Mansellii |
146 |
CHAPTER V.
Order— SAUROPTERYGIA.
|
Genus — Pliosaukus .... |
. 152 |
|
Species — FUosuurus yiandis |
. 164 |
|
brac/ii/deirus |
. 157 |
|
— truchuiUerius |
. 158 |
|
— portlandicus |
. 163 |
PAGE-IiNDEX.
CHAPTER VI. Order— DINOSAURIA.
Genus — Megalosaurus
Species — Megalosaurus Bucklandi
GeuUS — BOTHIIIOSPONDYLUS
Species — Bothriospondylus magnus
PAGE
166 166 172
, 172
ICHTHYOPTEEYGIA. Ichthyosaurus fortimanus — longimanus
(Supplement.)
176 176
Sauuopterygia. Plesiosaurus macrocephalus — brachycephalus
(Supplement.)
177 178
(Supplement.)
Cheloxia.
Genus — Pleurosteenon .
Species — Pleurosternon concinnum
— emarginatum
— ovatum
— latisculatum Genus — Platemys.
Species— PI ateiiiys Mantelli
Dixoni Genus— Chelone. Species^CAe/one costata ■
— gigas
179 179 183 184 185
186
187
187 188
Order— LABYRINTHODONTIA.
Genus — Labyrinthodon . Species — Labyrinthodon Jaegeri
— leptognatkus
— pachygnathua
— scutulatus ■ Footprints of Labyrinthodon
189 190 191 192 195 197
CONTENTS.
SYSTEM-INDEX.
Order— CHELONIA.
Genus — Pleueosternon
— Platemys
— Chelone
PAGE
179 185
187
Order— LACERTILIA.
Genus — Echinodon. Species — Echinodon hecclesii
126
Order— CROCODILIA.
Family-
|
PRGTOSUCHII. |
• •.... |
. 129 |
|
Genus — Teleosaurds. |
||
|
Species — Teleosauru^ |
■ Chapmnnni ..... |
. 130 |
|
— |
cariomensis ..... |
. 139 |
|
— |
brevior ..... |
. 140 |
|
— |
latifrons ..... |
141 |
|
— |
asthenodeirvs ..... |
141 |
|
Genus — Steneosauehs ...... |
143 |
|
|
Species — Steneosaurus Geoffroyi ..... |
144 |
|
|
— |
laticeps ..... |
145 |
|
— |
temporalis ..... |
145 |
|
Genus— Plesiosuchus. |
||
|
Species — Plesiosuchus Mansellii ..... |
146 |
Order— DINOSAURIA.
Genus — Scelidosadkus
Species — Scelidosaurus Harrisonii
Genus — Megalosaueus
Species — Megalosaurus Bucklandi
Genus — Bothriospondylus
Species — Bothriospondylus magnus
89 92 166 166 172 172
SYSTEM-INDEX.
vn
Order— SAUROPTERYGIA.
Genus — Pliosaurus Species — Pliosaurus grandis
— brachydeirus ■ — trochanterius
— portlandicus Genus — Flesiosaukus Species — Plesiosaurus dolichodeirus
— homalospondylus
— rugosMS
PAGE 152
154
157
158
163
1
1
12
34
Order— ICHTHYOPTERYGl A.
Genus — Ichthyosaurus Species — Ichtliyosaurus breviceps
communis intermedius platyodon loncliiudoii longifrons latifrons acutirostris tenuirostris longirostris latimanus bracliyspondylus fortimanus longimanus .
41 67 68 70 73 75 76 76 78 79 82 85 85 176 176
Order— LABYRINTHODONTIA.
|
Genus — Labyrinthodon .... |
. 190 |
|
Species — Labyrinthodon Jaegeri |
. 190 |
|
— leptognathus |
. 191 |
|
— pachynathus |
. 192 |
|
— scutulatus . |
. 195 |
|
Footprints of Labyrinthodon |
. 197 |
A HISTORY
BRITISH FOSSIL REPTILES.
THE
FOSSIL REPTILIA OF THE LIASSIC FORMATIONS.
CHAPTER I. Order— SAUROPTERYGIA, Oim/. Genus — Plesiosaurus, Conybeare. Species — Plesiosaurus doIicJiodeirus, Conybeare. (Tabs. I— IV.)
Of the Plesiosaurus dolichodeirus, Conyb., the first described and the typical species of the genus, three more or less entire specimens have come under my obser- vation, which have been obtained from the Lower Lias of Lyme Regis and Charmouth, Dorsetshire. One of these, formerly in the possession of the late Duke of Buckingham and now in the British Museum, was the subject of Conybeare's original description.* A second, in the British Museum, is figured by Buckland in his ' Bridgewater Treatise,' vol. ii, pi. xix, fig. 2 ; the third, also in the British Museum, is the one which I have selected for illustration in the present Chapter (Tabs. I and II). In this the vertebral series is entire ; there is no break in the long cervical region, as in the other two specimens ; its perfection, in this respect, satisfactorily shows that the head is at, or nearly at, the correct distance from the trunk, with the neck outstretched, in the two former specimens, the greater completeness of which, in regard to the Hmbs, supplies what is wanting in this respect in the present skeleton (see Tab. I, figs. 2 and 3).
The condition of the vertebral column in the originally described or type-specimen of the Plesiosaurus doUcJiodeirus is such as to suggest that the carcass, after it sank to
• 'Transactions of the Geological Society,' 2nd series, vol. i, p. 381, pi. xlviii.
b
2 BRITISH FOSSIL REPTILES.
the bottom, had been preyed upon by some contemporary carnivorous marine animals. It seems as if a bite of the neck had pulled out of place the eighth to the twelfth vertebrae. Those at the base of the neck have been scattered and displaced, as if through more " rugging and riving." Some creature which has had a grip of the spine, near the middle of the back, has pulled to one side all the succeeding vertebrae of the pelvis; their adhesion to that part and, more or less, to each other, being retained. This wrench would expose the abdominal viscera, a tergo, where we now see the upper or inner surface of the abdominal ribs or sterno-costal arches. The intermediate and succeeding portions of the vertebral column retain their natural relative positions, as in the prone position of the carcass ; and the skull, scapular arch and appendages, pelvic arch and appendages, and the tail, show respectively their relative positions as in the entire animal. Many of the otherwise undisturbed vertebrae, however, have turned, so as to present their most extensive surface to the direction of the slow, cosmical, compressing force operating on their imbedding stratum.
This is the case with the first twenty cervical vertebrae in the specimen Tab. I, which appears to have settled in the Liassic mud back downwards, their spines being turned toward the right side ; beyond the twenty-first cervical the vertebrae have rotated in the opposite direction, presenting more or less of a side view, with the neural arch and spine turned to the left ; but most of the spinous processes have been removed with the matrix in the original exposure of the specimen. The trunk preserves the supine position, exposing the broad coracoids (52), and pubes (G4), with scattered, intervening, abdominal ribs. Part of the left pectoral fin (53—56) is in situ; a smaller part of the corresponding pelvic fin (65—67) lies across the pelvis.
No partial force has operated after interment to dislocate any of the vertebrae, save the few terminal ones of the tail, which have disappeared, probably dragged away with whatever tegumentary expansion may have there represented a caudal fin.
In the specimen figured by Dr. Buckland * the skeleton, as it is exposed to view, lies prone ; the vertebra-, whilst their matrix was in the state allowing them to turn, have presented their largest surface to the direction of superincumbent pressure, the spines of those at the basal half of the neck being turned down or toward the right side, while those of the dorsal vertebrae have yielded in the opposite direction, both kinds presenting more or less of a side view. The thoracic ribs have slipped some way from their articulations, yet preserve, in the main, their relative positions, in serial succession. The anterior dorsals overlie the coracoids, and the posterior dorsal and sacral vertebrae overlie the dislocated parts of the pelvis. One of the thickened, short, and straight sacral ribs abuts against the right ilium. Upwards of thirty caudal vertebrae extend, in nearly a straight line, from the sacrum. The vertebrae at the fore part of the neck have been displaced, and in great part lost. Of the head little is visible, save the mandibular rami. The bones of both fore and hind paddles on the
* Op. cit., vol. ii, plate x, fig. 2.
LIASSIC PLESIOSAURS.
3
right side are in an instructive state of preservation, especially those of the hind fin, which exemplifies the slight superiority of length as compared with the fore fin, cha- racteristic of the present species.
The following are admeasurements of corresponding parts of the three skeletons above mentioned, that of " entire length" being now capable of being given by reason of the integrity of the cervical region of the spine between the head and the pectoral or scapulo-coracoid arch.
|
Conybeare's |
Bnckland's |
Specimen |
|||||||
|
specimen. |
specimen. |
Tah. I. |
|||||||
|
Ft. |
ill. |
ines. |
Ft. |
in. |
lines. |
Ft. in. lines |
|||
|
Entire length (two or three inches of the tail wanting ?) |
9 |
5 |
0 |
9 |
8 |
0 |
8 9 0 |
||
|
Length |
of head |
. |
0 |
8 |
6* |
0 |
8 |
2 |
0 8 6t |
|
)> |
neck . |
4 |
0 |
0 |
4 |
10 |
0| |
4 10 0 |
|
|
j» |
trunk, from fo |
re part of sternum to end |
|||||||
|
of ischium |
. |
3 |
0 |
0 |
3 |
6 |
0 |
2 8 0 |
|
|
>) |
tail . |
. |
2 |
0 |
0 |
2 |
4 |
0 |
(imperfect.) |
|
>) |
pectoral limb |
I |
10 |
0 |
1 |
9 |
0 |
(imperfect.) |
|
|
)j |
pelvic limb |
2 |
0 |
0 |
2 |
2 |
0 |
(imperfect.) |
|
|
>» |
humerus . |
. |
0 |
7 |
0 |
0 |
7 |
6 |
0 7 6 |
|
»j |
femur |
0 |
7 |
0 |
0 |
7 |
4 |
(wanting.) |
|
|
») |
•radius |
0 |
3 |
0 |
0 |
3 |
0 |
0 3 6 |
|
|
)) |
tibia . |
0 |
3 |
0 |
0 |
3 |
3 |
||
|
>> |
mauus |
1 |
0 |
0 |
1 |
0 |
o§ |
||
|
jt |
pes . |
1 |
2 |
0 |
1 |
3 |
0 |
||
|
Breadth of pubis (transverse |
diameter) |
0 |
.') |
0 |
(obscured) |
0 6 0 |
|||
|
Length of ischium (longitudinal diameter) . |
0 |
4 |
6 |
(obscured) |
0 5 0 |
From the amount of concordance in the dimensions of the above three skeletons, it may be inferred that the average length of the mature animal of the present species of Plesiosaurus was between nine and ten feet. Two specimens of certain portions of the skeleton, now in the British Museum, one of which is the subject of Tab. Ill, support this conclusion.
Vertebral characters (Tabs. Ill and IV).
Cervical series. — The cervical vertebrae of the Plesiosaurus dolichodeirus, at the fore part of the neck (Tab. Ill, fig. 1, a, c4— 7), have the centrum (c) of a length equalling the breadth of the articular end ; but the dimension of breadth increases in a greater
* Estimated at that of the lower jaw.
■\ Nine inches, if the angle of the jaw he restored, as dotted in Tab. IL
X It may be that the head has been drawn a little forward in the displacement of the anterior cervical vertebrae.
§ Some of the terminal phalanges are here estimated for.
4 BRITISH FOSSIL REPTILES.
ratio as the vertebrae enlarge and recede in position; so that the length of the centrum may be one sixth less than the breadth at the middle (Tab. Ill, figs. 4, 5, 6) and hind part of the neck. There are, however, varieties in this respect, and the equality of length to breadth of centrum is maintained through a greater extent of the neck in some specimens than in others. The vertical diameter of the middle of the terminal articular surface (ib., fig. 6 c) is less by nearly one fourth than the breadth of the same. The sides of the centrum are longitudinally concave (ib.,fig. 5), as is also the under part (ib., fig. 4), but in a minor degree in the middle and posterior than in the small an- terior cervicals (ib., fig. 1, e).
The costal surface is at the lower part of the side of the centrum ; it is narrow vertically, in proportion to its length in the anterior cervicals, but gains in vertical extent without being elongated in the same degree, and consequently occupies a larger corresponding extent in the middle cervicals (ib., fig. 4, ;j/),* in which tlie costal surface exceeds one half the length of the centrum ; it is divided by a longitudinal cleft, and is situated a little nearer the posterior than the anterior surface of the centrum. In the third to the seventh cervicals (ib., fig. 1, c7) the costal surface is separated by a tract exceeding its own vertical diameter from the neurapophysial surface (ib., fig. 1,«); in the succeeding cervicals the intervening tract equals the costal surface (ib., fig. 4, c ) ; and the interval is never less, and is sometimes more, in the cervical vertebra? to near the base of the neck. The terminal articular surface (ib., fig. 6) is moderately convex at its periphery and very gently concave in the rest of its extent, with a small central, often transversely linear, impression in the centre. f
The free surface of the centrum is finely rugose in the smaller anterior cervicals, and is not smooth in any of the others ; towards the articular ends the roughness is more marked, by irregular narrow risings and groovings, which become more longitu- dinal in direction in the succeeding cervicals (ib., figs. 4, 5). The under surface (fig. 5) is concave transversely from the costal pit (pi) to the two venous openings, and is convex between those openings, which divide the surface pretty equally into three parts, Lengthwise, as already stated, the under surface is gently concave. The neurapo- physial surface is less angular than in some other species, the lower angle being rounded off, making the lower border approach to a curve. Anchylosis of the neural arch with the centrum seems to have been complete in the anterior cervicals of the specimen figured in Tab. Ill, fig. 1 ; and I have not yet seen a cervical centrum of the present species from which the neural arch had become detached, save by fracture. The zygapophyses are proportionally large ; the anterior ones (ib., c, z ) extend forward, in the anterior vertebrte almost immediately above the centrum, overhanging the
* Here obscured by the confluent base of the rib.
f " The concavity again slightly swelling in a contrasted curve near the middle of the circular area," (Couybeare's first Memoir, p. 582, April, 1821) is the character of the terminal articular surface in the riesiosaurus arcuatvs, from the Lias in the neighbourhood of Bristol.
LIASSIC PLESIOSAURS. 5
posterior two fifths of the centrum in advance ; the articular surface, the length of which equals two thirds that of the length of the centrum, looks obliquely upward and inward ; that of the posterior zygapophysis has the reverse aspect. As the cervicals approach the back the zygapophyses diminish in relative size, and their articular sur- faces become less horizontal. The posterior zygapophysis (Tab. Ill, fig. 4, y) over- hangs a small part of the end of the succeeding centrum, and the neurapophysis (ib., fig. 4, n ) rises with a deeper concavity at the back than at the fore part. That this anchylosis had not occurred in the similarly sized and in the larger specimens of the cervical vertebrae of the Flesiosaurus described and figured by Conybeare in his first famous Memoir* is due to their having been derived from a younger specimen of a larger species from the Bristol Lias, probably Plesiosaurus arcuafus.
The neural spine (Tab. I, ns; Tab. Ill, fig. A,ns) arises narrow between the bases or back part of the prezygapophyses (^), and its base extends, increasing in thickness gradually to near the back part of the postzygapophyses (j')- The height of the spine averages half the vertical extent of the entire vertebra from its summit to the lower level of the centrum, being rather shorter in the anterior cervicals and exceeding that length at the base of the neck. In the anterior cervicals the contour of the neural spine extends from the fore part of the base, in a curve increasing in convexity at the upper part, and terminating by a rounded apex overhanging, in the foremost vertebrae (as at c4, Tab. Ill), the concave contour of the hinder border. The upper part of the spine becomes more squared as the spine itself gains in height, in the larger posterior cervicals, by the increasing fore-and-aft extent of their upper part, as in fig. 4, ns.
The pleurapophysis of the axis (Tab. Ill, fig. 1, xpl) has its posterior angle extended backward ; that of the third cervical has its anterior angle also produced forward, but in a minor degree. Both angles continue to be more produced in the succeeding vertebrse, but the front one most so, until, in the fifth cervical, they are equal in length ; the hinder one then elongates, but they do not touch or overlap the contiguous pleurapophyses until about the tenth cervical vertebra. The extent of this terminally dilated or extended border of the riblet exceeds that of the diameter of the same from its upper articulation outward or downward. The line of- articulation is discernible in most of the anterior vertebrae, but in fig. 1 coalescence has com- menced, if it be not complete, as in figs. 4, 5, 6, pi in which the expanded part of the pleurapophysis has been broken off, showing the approximated head and tubercle adapted respectively to par- and di-apophysial divisions of the costal surface. In
* " Notice of the discovery of a new fossil animal, forming a link between the Ichthyosaurus and Crocodile, together with general remarks on the osteology of the Ichthyosaurus; from the observations of H. T. de la Beche, Esq. F.R.S., M.G.S., and the Rev. W. D. Conybeare, F.R.S., M.G.S. (Read April 6th, 1821.) Drawn up and communicated by the latter." The observations on the vertebral characters of the new reptile are said to have been made " on the organic remains contained in the Lias in the vicinity of Bristol" (p. 559). 'Transactions of the Geological Society of London,' first series, vol. v.
6 BRITISH FOSSIL REPTILES.
some vertebrae a low and narrow ridge extends from the neur- to the pleur-apophysial surfaces, as at c, fig. 4.
The degree of concavity of the sides of the centrum in the anterior cervicals, exposed in the specimen figured in Tab. Ill, fig. 1, has been exaggerated by the pressure to which it has been subject, the effects of which are more conspicuous upon the skull : the cancellous mid-part of the centrum has opposed less resistance than the compact articular ends.
The atlas (a) has been disarticulated from the occipital condyle (l) ; the hemispheric articular cup is thus well displayed, with its smooth and shining surface. The coales- cence of the centrums of the atlas [a] and axis (a) is complete. A tubercle from the side of the centrum of the atlas represents the pleurapophysis ; its neural arch is broken away ; that of the axis developes a spine similar to and but little smaller than that of the third cervical.
The dimensions of the seven vertebrae here preserved in connection with the skull will be seen in Tab. Ill, fig. 1, where they are figured of the natural size. The average dimensions of a cervical centrum of the present species, from the middle and basal half of the neck, are, as in figs. 4, 5, 6 —
In. lines.
Length . . ; . 14
Breadth of articular surface 16
Height of middle of ditto ......... 13
Length of costal pit ..... . ... 0 6
Transverse diameter of outlet of neural canal ..... 0 6
Dorsal series. — The transition from the cervical (Tab. I, c) to the dorsal series (ib., d) is effected by the usual elevation of the costal surface by gradational steps, continued through about five vertebree, until a single costal surface is presented by a large diapophysis from the neural arch. The number of cervical vertebrae so defined in the specimen figured in Tab. I is forty-one. In the first dorsal, characterised by the dia- pophysial support of the rib (Tab. IV, figs. I and 2, d), the non-articular part of the centrum is smoother than in the cervical vertebrae, the ridges or rugae occupying a smaller extent near the two ends, where they indicate the attachments of the capsular ligaments. The longitudinal concavity between the two ends is uniform and rather more than in the cervicals. The venous foramina are wider apart and not divided by any special transverse convexity on the under surface of the centrum. A vertical ridge leads from the side of the centrum (ib., c) to the under part of the diapophysis (ib., «/), nearer the hind than the fore end of the centrum.
The diapophysis is convex and longest superiorly ; the fore part is rather hollowed, the hind part flattened, and both converge to the ridge forming the shorter under surface. The articular surface C^) of an irregular oval form, with the small end down-
LIASSIC PLESIOSAURS. 7
ward, looks downward, outward, and a little backward, the process being slightly inclined that way. The margin of the articular end of the centrum is better defined than in the neck; about a line's breadth is, as it were, shaved off; the rest of the sur- face (fig. 2, c) is very slightly concave, sometimes undulated, always nearly flat, and with a small central depression, or a tendency there to a tubercle. The length of tlie dorsal region in the skeleton (Tab. I) 8 feet 9 inches long, is 2 feet G inches ; the number of dorsal vertebrae is twenty-one.
Sacrum. — Two vertebrae (ib., s) succeeding the dorsals are distinguishable, through the greater thickness and straightness of their short pleurapophyses, as sacral ; these elements abut against, or afford ligamentous union to, the iliac bones.
Caudal series. — The caudal vertebrae (Tab. I, c d, Tab. IV, figs. 3 — 9) are shorter in proportion to their breadth than the others ; the centrum approaches to a cubical figure, the under surface (figs. 4 and 7) becoming broad and flattened ; and the contour of the terminal articular surfaces shows a similar tendency to flattening, giving a transversely extended quadrate figure, with the angles rounded off (figs. 5 and 8) ; the margin is thicker, more rounded off, less defined than in the dorsal vertebrae. The articular surface itself is more concave than in the antecedent regions of the backbone, and becomes deeper in the terminal subcompressed vertebrae (fig. 9) ; the movements of the tail in swimming having been helped here by a greater amount of yielding intervertebral substance, approaching in the same degree to the condition of the spine in fishes. The costal surface (figs. 3, 6, pi) is elliptic, with the long axis subvertical, the margin prominent, the cavity simple and rough for the ligamentous attachment of the ril:)let ; it is situated on the upper half of the centrum close to the neurapophysis, the outer end of the base of which contributes to the upper part of -the margin in the anterior caudals (fig. 5, d).
The pleurapophyses in this region (Tab. IV, 5, pi) do not expand terminally, as in the neck ; they are short, thick, and straight, simulating transverse processes ; their non-confluence with the centrum exemplifies the minor vigour of vital co-ossifying influences in terminal parts.
The heemapopliysial surfaces (fig. 3 A') impress the inferior angles of the posterior surface of the centrum ; occasionally, where a haemapophysis has become anchylosed and broken off, its adherent base gives the appearance of a process from that part.;t)f the centrum (ib., figs. 6, 7, S, /*')• The venous foramina are at the lower part of the sides of the centrum. The neural arch (figs. 3, 5, «) rapidly diminishes in size and in the length of the neural spine, ns. The zygapophysial surfaces become more vertical, the anterior, z, looking inward ; the posterior zygapophyses, z', are the first to disappear. Tlie haemapophyses (fig. 5, h) are free, and were ligamentously connected with the centrum above and with each other below, circumscribing there the haemal canal. The proximal surface is expanded, with a subtriangular facet cut obliquely at the anterior part for articulation with its own surface, and with a smaller, less definite
8 BRITISH FOSSIL REPTILES.
surface posteriorly for the intervertebral substance and a small part of the succeeding centrum, where a slight expansion of the everted border of the articular surface is the sole indication of such hsemapophysial junction. In the terminal vertebrae these surfaces with the hsemapophyses have disappeared, and the centrum, now showing a compressed form, supports only a contracted, anchylosed, seemingly exogenous neural arch, which finally disappears.
The following are transverse diameters of the centrum in different regions of the spine, in the specimen, 8 feet 9 inches long, of Plesiosaurus doUchodeirus, figured in Tab. I :
In. lines. Tenth cervical vertebra ....... 1 0
Middle dorsal ditto 2 0
Tenth caudal ditto 13
Cranial characters (Tabs. II and III).
The skull in this skeleton presents, what is rare, the side or profile view (Tab. II) like that of the succeeding anterior cervicals. Its upper part is much injured. The following bones are recognisable : — mastoid 8, tympanic 28, squamosal 27, malar 26, maxillary 21, premaxillary 22; the end of the long pterygoid is seen at 24, abutting against the lower end of the tympanic. But little of the composition of the mandible is discernible ; the tightly closed jaws show the extent of the interlocking of the long, slender, curved, and sharp-pointed teeth.
Of those of the lower jaw the crowns of upwards of twenty may be traced ; the longest occupying the middle three fourths of the series, and the largest of these being the foremost. In some parts of the series two teeth pass into the same dental inter- space of the opposite jaw.
The admirably wrought-out specimen figured in Tab. Ill, fig. 1, exhibits the upper surface of the somewhat crushed skull. Of the basi-occipital a part of the upper surface (l) and of the single median convex condyle is shown. The exoccipitals (ib., 2) preserve their connection with the lateral and upper parts of the basi-occipital, and show the . surfaces — seemingly sutural- — from which the superoccipital (ib., 3) has been displaced. These surfaces (2, 2) are thick' and triangular; they are parallel with the midiMe of the foramen magnum, the lower half of which is formed by the basi- and ex-occipitals. From the outer and back part of the exoccipital the paroccipital process (4, 4) is continued ; of subtriedral form, long, slender, and tapering to a thin I'ounded apex : the outer side appears to be sutural, and that of the left side is applied to the tympanic (ib., 28) : the length of this process is 8 lines. The breadth of the occiput, outside of the exoccipitals, is 1 inch .'j lines ; that of the foramen magnum is 6 lines. That the rough triangular upper surfaces of the exoccipitals are natural, not the result of frac-
LIASSIC PLESIOSAURS. 9
ture, I infer from their being on the same level, and from the corresponding surfaces being presented by the single arched bone (3) representing the superoccipital. This has been displaced by the pressure operating not quite vertically, but with an incli- nation from the left to the right, which has turned the spines of the cervical vertebrae to the right, and which has so far displaced the superoccipital in the same direction that it lies with its concavity or arch embracing, as it were, the right superoccipital, this concavity having formed the upper half of the foramen magnum. The apex of the superoccipital lies beneath the right branch of tlie parietal ; the outer surface of the piers of the superoccipital arch is moderately smooth and convex ; the breadth of the base of the arch is 1 inch 3 lines, that of the span of the arch is 5 lines.
The parietal (ib., 7) is thick and transversely extended posteriorly, where it is overlapped by the mastoids (s), anterior to which it contracts to form the crest be- tween the temporal fossae. The crest is interrupted by the parietal foramen (/), an- terior to which it is resumed for a short extent — 3 lines, before the frontal suture. The total length of the parietal is 1 inch 1 1 lines ; the length of the crest is 1 inch 3 lines. Tlie thick and rather rugged hinder bifurcate part of the parietal is overlapped or embraced by the mastoid (s), and these bones curve outward and backward to articulate with the squamosal (,27) find with the tympanic (28), which is continued in the same direction to the joint of the mandible (29). AH these bones together form a strong arch, curved backward in the present specimen, but owing its horizontal position to the posthumous pressure, and having the piers of the arch directed down- ward as well as outward and backward in the natural state.
The sutui'e between the frontals (11) remains, and that between the postfrontals (12) and the expansions of the parietal {'') upon the sides of the cranium may be traced. There is a smooth superorbital (11') between the rougher frontal and the orbit, unless the fissure defining them be a fracture and not a suture. The external facial plate of the prefrontals is rough ; it overlaps the fore part of the frontal and part of the nasal, and extends to the small external nostril. The nasals (15) overlap the fore part of the frontals, and extend about as far in advance of the nostrils as they do behind them, continuing the median ridge from the frontals forward, in which, however, the median suture is visible. The outer surface of the maxillarics and premaxillaries shows a kind of granular rugosity, which subsides in the maxillary as this bone (21) extends beneatli the orbit. The limits of the lacrymal (73) are not definable. The malar (26) forms the hinder half of the suborbital boundary; its surface is smooth, and increases in breadth to beyond the orbit, when it contracts and becomes rugous where it joins the postfrontal (12) and squamosal (27). The bony boundary between the orbital and temporal cavities is crushed and much cracked : but the outer end of a postfrontal or postorbital is wedged into the squamous union of the malar and squamosal. The latter (27), of a tri-radiate form, curves from the malar round the outer and back angle of the temporal fossa, and extends backward upon the tympanic : the ray directed
c
10 BRITISH FOSSIL REPTILES.
mesiad, and overlapping the mastoid (s) and tympanic (28), is the longest, and termi- nates in a point : the surface of the bone is smooth.
The temporal fossae are broader than they are long. At their forepart the parietal side-wall of the cranium expands as it advances, and is continued into the postfrontal or postorbital partition.
The orbits are rounded anteriorly, and both the upper and under parts of the frame make an angular junction with the straight hinder pai't.
The nostrils have the usual small size and backward position. In both orbits some of the thin sclerotic plates of the eyeball («, s) are preserved ; this is the first specimen in which I have had evidence of this structure.
The interlocking of the teeth of the upper and lower jaws, through the singular care and skill devoted by Mr. Harrison to the removal of the matrix, is peculiarly well displayed in this instructive fossil.
The foremost tooth in each premaxillary make a pair, which curve forward and downward between the two foremost teeth of the lower jaw, the premaxillary teeth slightly diverging as they descend, Tab. Ill, fig. 3. The succeeding premaxillary teeth, four in number, alternate with mandibular ones. I cannot make out with certainty the maxillo-premaxillary suture, but the fifth tooth, counting backward, seems to be near to or upon it. The second premaxillary tooth is double the size of the first ; the third, fourth, and fifth gradually diminish ; the sixth (first maxillary ?) is small ; the seventh tooth suddenly resumes- the size of the second ; and the eighth, of nearly equal size, curves down close to the seventh, and the two are interposed between the inter- space of opposite mandibular teeth. From four to five smaller teeth are traceable behind the eighth, and there may have been more in the upper jaw.
Of the lower jaw ten teeth are shown on each side ; the second, third, fourth, and fifth are the longest and largest, as in Tab. II. In general, the teeth of the upper jaw are separated by intervals allowing the passage of those of the lower ; the teeth of the foremost premaxillary pair being closer together ; and those of the foremost mandi- bular pair being wider apart. They all present the usual generic character of crown — long, slender, curved, pointed, circular in transverse section, with the enamel finely but definitely ridged longitudinally. The longest exserted crown measures ten lines, the shortest four lines, the thickness being in proportion.
The true number of the teeth in the lower jaw is yielded by the specimen of the dentary bone. Tab. Ill, fig. 2, in which twenty-five alveoli are shown on one side, and twenty-four on the other. The size of the alveoli, and the extent of their interspaces, are greatest at the anterior half of dentary. The small successional teeth at the posterior part of the series are so advanced as to look like a double row at that part. A longitudinal groove or depression at the inner side of the base of the alveoli lodged the thicker mass of the vascular gum overlying the matrices of the successional teeth. The skull of the Plesiosaurus dolichodeinis is broad in proportion to its length, with
LIASSIG PLESIOSAURS. 11
a broad and short muzzle, of an equilateral triangular figure if the transverse lines across the fore part of the orbits be taken as the base, the two sides converging to the rounded apex in almost straight lines, with a feeble indication of a constriction where the maxillo-premaxillary suture seems to be. The contour is undulated by the expan- sions for the sockets of the larger teeth, which produce risings, with intervening furrows on the granulate alveolar borders of the jaws. The mandibular rami converge to their terminations at the symphysis, which is not prolonged or expanded. The specimen (Tab. Ill, fig. 1) from the Lias of Charmouth was wrought out of its matrix by the estimable discoverer of the Liassic Dinosaur [Scelidosaurus) described in a former Monograph. It is an admirable example of patience, pains, and skill ; in the bestowal of which, for the furtherance of science, upon the fossils roughly wrought out of the quarries in his neighbourhood, Mr. Harrison found solace during the long and trying illness which confined him to his bed, until his final release by death.
Pectoral and pelvic limbs (Tab. I, figs. 2, 3).
To complete the characters of Plesiosaurus dolichodeiriis I have reproduced, in out- line, the bones of the pectoral, fig. 2, and pelvic, fig. 3, limbs, as they are preserved in the type-specimen. The humerus, % shows rather more convexity at tlie anterior border, and a deeper concavity at the posterior border than in some other species {Plesiosaurus Hawhinsii, PL luacrocejjJialus,^ PI. riigosus, e. g.). The radius, 54, and ulna, 55, are of equal length ; the ulna not being shorter than the radius, as in PL Hawlcinsii (Tab. XIV, fig. 6) ; the ulna has not the olecranal process or epiphysis, as in the PL rurjosus (Tab. XIV, fig. 2, 55') ; and both antibrachial bones are less broad^ in proportion to their length, than in the PI. macrocephali(s-\ (Tab. XIV, fig. 4). The carpus shows seven bones, four in the proximal, three in the distal row ; their homo- logies will be pointed out in the description of Plesiosaurus rur/osus. The metacarpal of the first digit (Tab. I, fig. 2, i), answering to "pohex,'' supports at least three phalanges ; that of the fifth digit, seven phalanges ; the metacarpal of each of the others, six phalanges, but the terminal ones may be wanting in some. The pelvic fin (fig. 3) is rather longer than the pectoral one ; in both fins the fifth digit (v) articulates on a more proximal plane than the others, i. e. nearer the trunk, as in most other Plesiosauri. In the same skeleton the pectoral limb equals seventeen of the middle cervical vertebrae in length ; J in PL macrocejjhalus it equals sixteen of these vertebras, in PL rufjosus it equals fifteen.
* ' Geological Transactions,' 2n(l series, vol. v, pi. 43. t lb.
X The artist has drawn the outline of the limb-bones, figs. 2 and .'5, on a larger scale than that of the skeleton, in Tab. I.
12 BRITISH FOSSIL REPTILES.
Plesiosaurus homalospondylus (Tabs. V — VIII).
In the year 1842 I examined, in the Museum at Whitby, Yorkshire, a collection of Piesiosaurian vertebrae, which had been taken out of a heap of rubbish from the old alum works carried on in the upper Alum Shale — a part of the Liassic series on that coast, characterised by the Ammonites lieteropliyllus. Sow.
The vertebrae were divisible into two groups, indicative of two species of Vlesiomurus.
Of one kind there was a series of sixteen consecutive cervical vertebrfe, charac- terised by the unusual concavity of the terminal articular surfaces of the centrum. On making a section of two of these vertebrae cemented by the matrix in their natural state of co-adaptation, the margins of the opposed articular surfaces were two lines apart, showing the thickness of the inter-articular connecting ligamentous substance at that part, while the middle of the articular surfaces left an interval of eleven lines, thus approaching the ichthyosaurian type of vertebral union.
The following were dimensions of the centrum of these cervicals.
In. lines.
Length 19
Breadth of articular surfaces . . . . . . Ill
Height of ditto 1 10
The inferior surface of the centrum showed a median longitudinal convex ridge between the two w'ide elliptical venous foramina. I named the species indicated by these vertebrae Plesiosaurus ccelospondylus* in reference to the hollow terminal articular surfaces. I hope to have, at a future opportunity, further means of illustrating this species.
The second series of vertebrae presented almost flat articular surfaces of the centrum (Tab. V, figs. 3 and C) ; the inferior surface w^as devoid of a median ridge, or had only a slight rising (fig. 4, c) between the venous foramina, which were smaller and more narrowly elliptical (ib., figs. 4 and 7) than in PI. ccelospondylus ; the middle of the surface was bounded laterally by the costal surfaces (ib., pi), and was nearly flattened, being very slightly concave, both lengthwise and transversely. The costal surface is of a narrow elliptical form, with the long axis parallel with that of the centrum; the dividing line or fissure is not conspicuous ; it is situated, as usual, rather nearer the back than the front end of the centrum (Tab. V, fig. 2, ^0 ; and a space more than twice its vertical diameter intervenes between it and the neurapophysis (ib., np), or
* KotXos, hollow, oXovlvXot, vertebra.
LIASSIC PLESIOSAURS. 13
neurapophysial surface. A low, longitudinal rising, or obtuse ridge, traverses the free surface of the side of the centrum midway between the pleur- and neur-apophysial articular surfaces.
The following were dimensions of the centrum of a cervical vertebrae answering, or nearly so, in position, to that of the Plesiosaurus cmlospondylus selected for measure- ment—
In. lines. Length .......... 2 0
Breadth of the articular surface 1 10
Height of ditto 16
These dimensions showed the greater proportional length of the cervicals of the present species ; and, concurring with the more obvious difference in this shape of the terminal articular surfaces, I thereupon devised the name of Plesiosaurus homalo- spondi/his* indicative of the even or level character of those surfaces, for the species so characterised.
I have subsequently received several additional vertebral evidences of both these species of the Upper Lias, or Alum Shale of Whitby, and, finally, have had the opportunity of studying two almost entire skeletons of the Plesiosaurus homaJosponddus from that locality, one of which (Tab. VIII) is now in the Museum of the Philosophical Society of York, and the other (Tab. V) has been purchased by the Trustees of the British Museum, where it is now exhibited in the Geological Department. Both of these specimens exhibit the striking character of the genus Plesiosaurus in a maximised degree, viz., in the length of the neck and the smallness of the head.
I propose, first, to describe the specimen in the British Museum, Tab. V.
This specimen gives indications of the same conditions of interment in its matrix, and of the operation of subsequent gradual pressure, as that of the species last described, from the lias of another part of the kingdom.
It has sunk into the mud, which afterwards became petrified, either prone or supine ; for I have been unable to obtain evidence as to whether the present exposed part of the skeleton was wrought out from the upper or under surface of the block, as re- moved from the quarry ; but we may assume the former, and consider that the animal was originally imbedded with the upper or dorsal surface toward the observer. Both fore and hind paddles were outstretched at right angles with the axis of the trunk, but only their proximal bones or segments have been preserved. The skull and cervical vertebrae have maintained their original position. At the base of the neck, where the neural spines, from their height and breadth, began to afford a surface upon which the dislocating force could operate, they have begun to yield toward the left
* 'O/ioXo's, planus ; aiziivhvKos, vertebra.
14 BRITISH FOSSIL REPTILES.
side, and, in the dorsal region, d i — co, are turned flat in that direction. At the base of the tail, where these flattened surfaces again become diminished in extent, the vertebrae gradually resume tlieir vertical or prone position, the summits of the spines being uppermost, as far as the seventieth (counting from the head), be)fond which some dozen of the terminal caudals are jumbled together in an irregular group, as if that part of the carcass, supporting perhaps a caudal expanse of integument or fin, had been subject to some disturbing influence prior to complete imbedding in the matrix.
I conclude that this partial rotation of the dorsal series took place before the petrifaction of the bones and bed ; because the ribs of the right side have slipped from their attachments to the diapophyses, in a degree corresponding with the extent of the rotation. For, had they been cemented in their natui*al connections by the Lias stone, i. c, after the petrifaction of the mud, and prior to the operation of the extraneous pressure, they might have been expected to have been bent or broken, when pressed into the same plane with the neural spines, without any slipping from their previous joints ; whereas this dislocation implies a rotting away of the articular ligaments, and a certain yielding of the surrounding bed.
The chief characteristics of the skeleton of the Plesiosaurus homalosjjondylus are, the length of the neck, the height and breadth of the dorsal and contiguous cervical and caudal spines, with the smallness of the head. The length of the neck is due both to the number of vertebrae — thirty-eight, and to their proportionate length individually, and chiefly to the latter character, as compared with Plesiosaurus (Jolichodeirus (Tab. I).
I caused to be carefully removed from the matrix of the present skeleton the thirteenth and fourteenth (Tab. V, figs. 2 — 4) of these instructive vertebrae, the length of the centrum in which agreed with that on which I had made notes and drawings in 1842. They corresponded in every other particular with these vertebrae. The low, longitudinal ridge or rising (Tab. V, figs. 2, 5, r) on the side of the centrum may be traced throughout the neck. Fig. 7, Tab. V, gives a view of the under surface of the eighth cervical vertebra ; fig. G gives an end view, and fig. 5 a side view of the centrum of the third cervical vertebra, all of the natural size. The specific characters are well exemplified in these, which may be profitably compared with the figures of the corresponding vertebrae of the Plesiosaurus planus, in a former Volume,* as exemplifying the degree in which vertebral characters are developed in the diS"erent species of the genus.
The cervical ribs, as indicated by the articular surface (Tab. V, figs. 2, 7, pi), are of small size in proportion to the rest of the vertebra, until about the thirtieth, in which the transverse outstanding part of the stem is two inches three lines in length, and the longitudinal part two inches six lines. In the thirty-fourth vertebra this has attained
* Volume I, ' British Fossil Reptiles ;' Supplement No. II to the ' Fossil Reptilia of the Cretaceous Formations.'
LIASSIC PLESIOSAURS. 15
a length of four inches; the production, anterior to the transverse stem, being nearly an inch in length. In the thirty -fifth vertebra (Tab. VII, c 35) the costal surface pro- jects, the rib begins to ascend, the anterior production to shorten, the posterior one to lengthen. In the thirty-seventh (ib., 37) the rib is supported in equal proportions by the centrum and by a diapophysial growth of the neurapophysis. In the thirty - eighth (c, 3S) the rib has passed almost wholly upon the diapophysis, and has assumed a simple rib-like character, slightly bent, with a length of six inches. In the thirty- ninth vertebra (ib., D 1) the transit from centrum to neurapophysis, np, is complete, denoting the first of the dorsal series. In the second dorsal vertebra of the present skeleton the rib has slipped forward from its joint, d. In the forty-third (Tab. V, fig. 1) it is depressed an inch below the diapophysis. In the forty-sixth to the fiftieth vertebree the heads of the ribs lie beneath the centrums, and the side view of the whole of those vertebrae is obtained. In the succeeding dorsals the ribs gradually approxi- mate their suspending processes, and have resumed their articulation at the twentieth dorsal, or the fifty-eighth vertebra, counting from the atlas.
The ribs of the forty-seventh to the fifty-first vertebrae are from sixteen to seventeen inches in length ; they are the longest of the series. The articular head presents a diameter of one inch and a half; the anterior surface is convex transversely; the outer part of the posterior surface is rather concave in the same direction, so that the outer margin of the proximal half of the rib, to near its head, presents the cha- racter of an obtuse rim or ridge. They gradually decrease in size as the vertebrae recede in position from the fiftieth ; and, at the sixtieth, are reduced to a length of four inches : this and the two succeeding ribs seem to have become anchylosed to the diapophyses. In the sixty-second vertebrae the rib suddenly augments in thickness, extends its articulation downward upon the centrum, and represents a sacral vertebra (Tab. I, s). That of the sixty-first vertebra is somewhat less thick, but it may have assisted in affording attachments to the ilium (ib., 02), the proximal end of which bone is in contiguity with the converging terminations of the ribs of the sixty-first and sixty-second vertebrae. The anchylosed condition, with shortening of the caudal ribs, or pleurapophyses, give them the usual chai'acter of transverse processes in the caudal region.
The neural spines, thin and antero-posteriorly extended in the neck (Tab. V, fig. 2, a, 2, ns), have* been more or less broken away, in the operation of exposing the specimen, from the anterior three fourths of the vertebra? in that region. Their height gradually decreased as they approached the head and receded from that of the thirtieth vertebra (ib., 30), which rises four inches from the summit to the neural arch, having a fore-and-aft diameter of two inches three lines, and a thickness of three lines. The former diameter is least a little above the origin of the spine, and gradually increases toward the summit, where the spines are in contact. In the thirty-third vertebra the neural spine is five inches in length, and its breadth of two inches three
16 BRITISH FOSSIL REPTILES.
lines is maintained through nearly the whole of that length, in corresponding* close contact with the contiguous spines. In the thirty-seventh vertebra (Tab. VI [, 37) the length of the neural spine is five and a half inches ; it has a little increased in thick- ness ; the fore-and-aft diameter continues the same. In the second dorsal the neural spine is six inches four lines in length, with a thickness of six lines. These dimensions are continued to the fifty-eighth vertebra, save that, in the posterior half of the dorsal series, the spines have less fore-and-aft breadth at their proximal third, and leave cor- respondingly wider intervals ; they are in contact at their more expanded distal portions. From the fifty-eighth vertebra they gradually decrease in length to the sixty-second, or sacral vertebra, showing a height of less than four inches, with a ter- minal fore-and-aft extent of two inches, and a thickness of six lines. They decrease in all dimensions as the caudals recede from the trunk, and most so in fore-and-aft extent, leaving wider interspaces ; by which character, with the higher position on the centrum, and anchylosed condition of the pleurapophyses, a caudal vertebra may be distinguished from a cervical of similar size. The caudal centrums are also thicker in proportion to their length, and the under surface, if exposed, would doubtless also yield the character of the hfEmapophysial pits.
The dorsal diapophyses progressively increase from the first (Tab. VII, d 1, 2, d), and attain, at the fifth dorsal vertebra (Tab. V, fig. 1), a length of two inches three lines along the upper border. The rib-surface is cut from above downward and inward, shortening the under extent of the process. A low ridge is continued from the pos- terior angle of the neurapophysis upon the back part of the diapophysis, which expands to the truncate articular surface. After the sixteenth dorsal the diapophyses gradu- ally shorten to the sacral vertebrae, where they have almost subsided.
The zygapophyses in the neck (Tab. V, fig. 2, z, z') and greater part of the back are nearly horizontal, the anterior ones looking a little inward as well as upward, the posterior ones the reverse ; they are given off nearer the base of the neurapophysis tlian usual (compare Tab. V, fig. 2, -, y with Tab. Ill, fig. 4, Plesiosaurus dolichodeirus, and Tab. X, fig. 1, PL rostratus), towards the end of the back their aspect gra- dually changes; and, in the tail, the articular surface becomes almost vertical; that of the anterior ones, which are most developed and longest retained in the vertebral series, looking inward. The terminal articular surfaces of the centrum of the last dislocated caudal vertebrse are, as usual, more concave than in the neck.
The development of the neural spines throughout the trunk and base of the neck is such as to impede inflection in the vertical direction. At the anterior half of the long and slender neck this bend would, indeed, take place in some degree ; but the greatest flexibility would be from side to side. The provision for the attachment of the vertebral muscles in the trunk is very great, indicative of corresponding power of regulating the movements and position of the body during the application of the
LIASSIC PLESIOSAURS. 17
lengthened, slender neck, and small head, in the capture of fishes or other active marine prey.
The whole framework of the trunk is singularly massive, and the character of this part of the skeleton, as shown in the specimen (Tab. V), is especially striking in con- trast with the slender neck and small head of the animal.
Of the Skull (Y&h. VI).
The skull (Tab. VI), from the occiput to the end of the snout, is 9 inches long;, it measures 4 inches 4 lines across the middle of the temporal depressions, 3 inches 6 lines across the occiput, which rises but 1 inch in height above the foramen magnum ; the intertemporal part, or parieto-frontal crest, rises into a sharp ridge ; the length of the temporal fossa is 2 inches 9 lines, the breadth is 2 inches. The diameter of the orbit is 1 inch 6 lines ; from the fore-part of the orbit to that of the snout is 4 inches. The elliptical nostril shows a long diameter of about 6 lines, it is situated about 8 lines in advance of the orbit, and about the same distance from its fellow. The inter-narial portions of the nasal and premaxillary bones rise into an obtuse ridge. The teeth are small, slender, slightly recurved at the fore-part of the jaw, where the enamelled crown of the longest does not exceed 10 lines. No sutural evidence of cranial structure is discernible ; tlie bones about and between the orbits show the effects of pressure. Estimating the length of the skull by that of the lower jaw, about two inches should be added to that taken from its exposed and visible part.
This part of the skull (Tab. V) is susceptible of satisfactory comparison with the corresponding region of the skull in the Plesiosaurus dolichodeirus (Tab. Ill, fig. 1), the species which most resembles the Plesiosaurus homalospondylus in the length of the neck and the small proportional size of the head.
By comparing Tab. Ill with Tab. VI, in which the skulls of the two species are figured of the natural size, from probably mature individuals of average size, and from the same aspect, the difference of proportion and form is such, and so obvious, that, were two skulls of existing lizards to be so contrasted, it is probable that some Erpetologists would be led to sever them more widely than by specific bounds. The composition of the cranium, the position and relative size of its principal cavities, and especially of the nostrils, the character of the dentition, are, however, so strictly Plesiosaurian in the two fossil skulls here compared, that there is no suSicient ground for encumbering the Sauropterygian group with one or two additional generic names.
The skull oi Plesiosaurus homalospondi/his is longer in proportion to its breadth, more oblong in shape, more obtusely terminated anteriorly. It is possible that the skull of the Plesiosaurus dolichodeirus compared (Tab. Ill) may have suffered more horizontal pressure, but not such as to have affected its triangular shape due to the
d
18 BRITISH FOSSIL REPTILES.
more rapid convergence of the sides of the upper jaw to the more pointed muzzle. The temporal fossae may appear broader than natural in this crushed skull, but with due allowance this shape was square, not oblong, as in Plesiosaurus homalospondylus. The intervening parieto-frontal crest is relatively longer, and we may infer that the biting muscles were larger and more powerful in relation to the more massive propor- tions of the dentigerous parts of the jaws in Plesiosaurus homalospondylus : the orbits are relatively less ; their antero-posterior diameter is less than one fifth of the same diameter of the skull taken from the back part of the parietal (7) in PI. homalospondylus; it is more than one fifth in PI. dolichodeiriis ; the orbits are equidistant from the two extremes of this diameter in PI. homalospondylus ; they are nearer the back part of the head in PI. dolichodeirus. In PL rostratus (Tab. IX) the temporal fossae present some- what intermediate proportions between those in the two foregoing species ; but the rostral production of the maxillary part of the skull sufficiently distinguishes the cranium of P/. rostratus from that of previously known species in a comparison of- detached skulls ; whilst its greater relative size to the body more especially distin- guishes it from that in PI. homalosjmndylus or PI. dolichodeirus.
In PL Hawkinsii* the longitudinal diameter of the temporal fossa exceeds the transverse diameter, but not in so great a degree as PL homalospondylus, and the upper jaw is relatively narrower than in that species. This is also the case in PL macro- ce]3halus,\ in which there is a more marked constriction of that part, anterior to the orbits, showing a tendency to the " rostral " character, which is exaggerated in PL rostratus.
Pectoral and pelvic arches and limbs (Tabs. V and VIII).
Of the limbs only the humeri and femora have been preserved in the skeleton (Tab. V) ; these bones show the usual form, with their respective characteristic modi- fications, as exemplified in the different contour of the anterior border, which is straight or partly convex in the humerus, and is concave in the femur. The length of the humerus is 12 inches, that of the femur 13 inches; the distal breadth is nearly the same in both, namely, 6 inches. In the right femur, the coarse fibrous texture which pervades the whole thickness of the bone is exposed. A portion of the exten- sive scapulo-coracoid arch comes into view from beneath the anterior dorsals on the right side (Tab. V, 52). The ilium (ib., 62) presents the usual form; straight, slender at its proximate end, with a slightly twisted, subcylindrical shaft, expanding to a breadth of nearly three inches at its acetabular end. The entire length of the
* 'Geol. Trans.,' 2nd series, vol. v, pi. 45. t Ibid.
LIASSIC PLESIOSAURS. 19
skeleton (Tab. V) is fourteen feet, which would be increased by several inches were the tail entire and outstretched.
The specimen of PL homahsjmiilijlus in the Museum of the Yorkshire Philosophical Society is larger than the one in the British Museum, but has been lithographed on a smaller scale in Tab. VIII; it measures IG feet 6 inches in total length. It lies in a somewhat similar posture to that in the British Museum, but with the long and slender neck and anterior dorsals bent so as to give a concavity to the dorsal con- tour of the animal ; the caudal vertebrae, which are better preserved, are also bent in the same direction, and all the vertebras follow in their consecutive undisturbed jiixta-position in both skeletons. The numbers of the vertebrae in the cervical and dorsal series respectively appear to be the same. The diapophysis has got entire possession of the rib at the fortieth vertebra, counting from the head ; and the costal surface begins, with its process, to sink again upon the centrum, at the sixty-seventh vertebra, which the thickness of the diapophysis indicates to be a sacral vertebra. Beyond this may be counted twenty-seven caudal vertebrae, and it is not probable that their number exceeded thirty.
The cervical vertebrae show the same distinctive characters of the species which have been already defined ; the neural spine is preserved in a much greater proportion of the cervical series ; in the fifteenth cervical it shows a height of two inches, and a nearly equal antero-posterior breadth ; with a broadly truncate summit, having the angles rounded off. The vertebrae keep their proportion of length from this point to the end of the dorsal series ; they then grow shorter to the end of the tail, throughout the greater part of which the centrum is deeper, and the neural spines longer and narrower, than in the neck, indicative of the greater mass of muscle operating on the tail, and also its greater flexibility in a given extent. The costal series has suffered much more displacement and loss in the York specimen than that in the British Museum; the larger ribs are a good deal jumbled and broken in the region of the trunk or thoracic abdominal cavity, but they show the same massive character. The ischio-pubic part of the pelvis has been drawn away, at an acute angle, from the ilium and sacrum; its inner or upper surface is exposed at 63, Qh Tab. VIII. The right pelvic limb has been moved forward, with the head of the femur lying upon the lower end of the right coracoid. The right pectoral limb extends forward from near its normal place of articulation with the coracoid ; but it has been turned bodily over, showing its inner or palmar surface. The limbs of the left side are huddled in a dislo- cated and incomplete state beneath the hinder part of the trunk.
The presence of both these limbs, in an excellent state of preservation, supplies the chief deficiency in the specimen in the British Museum previously described.
The pectoral limb, as in PI. clolichoddms, is rather shorter than the pelvic one ; its entire length is 3 feet 8 inches, equalling sixteen vertebrae towards the base of the neck. The humerus, 13 inches in length and 7^ inches in distal breadth, is broader
20 BRITISH FOSSIL REPTILES.
there, in proportion to its length, than in the PI. dolichodeirus or than in the PL rostratiis ; its anterior margin, as in the skeleton Tab. V, is more straight than in those species. The antibrachial bones (54, 55) show intermediate proportions of length and breadth between those in PI. dolichodeirus (Tab. I, fig. 2) and PI. roHtratus (Tab. IX). They present the usual characteristics of radius (54) and ulna (55) in the present genus, and they are of equal length. The hand measures two feet in length, and is somewhat longer in proportion to the arm and forearm than in the two above-cited species ; it also shows rather more breadth. The carpus consists of six bones, three in each row, and with less inequality of size. The distal bones occupy an equal breadth with those of the proximal row, and do not allow the base of the fifth metacarpal to extend backward to the proximal row, as in the species of which the carpus is figured in Tab. XIV. The bases of the five metacarpals (in Tab. VIII, 57) are on the same transverse line ; and if this specimen should truly exhibit the relative position of the bones of the pectoral fin, characteristic of the species, it adds a well-marked distinction of the PI. homalospondylus. The first, or radial, or innermost metacarpal (57), supports a short digit of three phalanges ; the second a digit of seven phalanges ; the third the same ; the fourth has a digit of six phalanges ; the fifth is obviously imperfect.
The pelvic limb (Tab. VIII, G5, 69) is 3 feet 9 inches in length ; the femur (65) is 14 inches long and 7^ inches across the distal end. The tibia and fibula are respec- tively longer than their homotypes the radius and ulna ; the foot is 2 feet in length and 1\ inches in basal breadth. The tarsal bones are similar in number and arrange- ment to those of the carpus ; and as the bases of the five metatarsals (69) are in this limb also on the same transverse line, I have the greater confidence in the natural structure being here shown in both limbs, and that they thus exhibit a distinctive character, of specific value, from the other Plesiosauri described in the present Monograph.
Plesiosaurus rostratus, Owen. Tabs. IX — XIII.
The specimen on which this species is founded was obtained, in 1863, by Edward C. Hartsinck Day, Esq., F.G.S., from the Lower Lias at Charmouth, Dorsetshire, by whom it was transmitted to London for inspection, and it has been purchased by the Trustees of the British Museum, where it is now exposed in the gallery of Geology. It is figured, one ninth of the natural size, in Tab. IX.
This skeleton, like most of the plesiosaurian ones that have come under my obser- vation, indicates the ordinary and tranquil character of the death and burial of the individual ; it has sunk entire, relaxed, and prone, with outstretched limbs, in its matrix, when this was soft and yielding ; and, as decomposition loosened the liga-
LIASSIC PLESIOSAURS. 21
mentous attachments of the vertebrae and of their elements, they have yielded to external pressure or movement of the matrix, and have rotated on their axis — some of the long-spined vertebrae to the right, some to the left — with a slight displacement of the longer ribs from their attachments.
The third cervical vertebra is displaced about three inches below the axis and atlas, which remain in connection with the occipital tubercles, the third to the fifteenth cervicals are prone with the spines uppermost, and the pleurapophyses in natural connection with the sides of the centrum, the lower part of which is buried in the matrix. Except a slight dislocation between the seventh and eighth, these cervicals have retained their natural sequence and relative position. As the spines grew longer and larger they offered a surface upon which the superincumbent pressure could operate, so as to rotate the vertebrae sideways ; and from the sixteenth to the twenty-eighth inclusive, they are turned half round, with the spines downward or to the left ; but all these vertebrae retain their natural mutual connections. The twenty-ninth vertebra is dislocated, exposing the anterior articular surface of the centrum ; the thirtieth has suffered fracture of its spine ; the thirty-first and thirty- second are partly bent to the left ; the thirty-third and thirty-fourth are turned with _the spines to the right side ; that of the thirty-fifth is broken from its neural arch ; the thirty-sixth to the forty-eighth vertebrae have the neural spines turned to the right, retaining almost their natural relative positions. The forty-ninth vertebra has kept the original prone position, as when imbedded ; the next ten show the side view, with the neural spines to the right; the sixty-first to the sixty-fifth are prone, but with a slight deviation of the neural spines, some to the right, some to the left ; the next six vertebrae have yielded in the opposite direction ; there is then a deeper space, equal to the extent of five vertebrae, in which there are the centrums of three vertebrae and some hsemapophyses irregularly scattered. Beyond this part the terminal caudal vertebrae resume their position and natural connections, and are preserved, seven in number, to the last. The antecedent exceptional violence shown in the caudal series has probably been due to the tugging and gnawing of some predatory animal, whilst this part of the dead and partly decomposed Plesiosaur continued to be exposed at the sea-bottom.
The scapulas (51) and articular ends of the coracoids (52) appear parallel with the twenty-fifth to the twenty-seventh vertebra3, the left being rather further back than the right. Both humeri (53) have been dislocated at the shoulder-joint by super- incumbent pressure, and the articular ends of the scapulae overlap their heads. The rest of the bones of the pectoral fins have retained their natural relative position, protected by the tough, closely-fitting dermal sheath, until this slowly dissolved away. The iliac bones (fi2) lie by the sides of the forty-seventh to the fiftieth vertebrae, almost in the axis of the spine, with their proximal ends turned backward, and their acetabular end forward, having become detached from the thick, converging pleura-
22 BRITISH FOSSIL REPTILES.
pophyses of llie forty-seventh and forty-eighth vertebrte (s, s) which overhe the ischium (63) on the left side of the body. The articular ends of the ischium (63) and of the pubis (64) are exposed, retaining their connection with the ilium (62) opposite the fort)'-third to the forty-seventh vertebrte on both sides. The femora (65) have been slightly dislocated forward, and part of the acetabula is thus exposed.
The bones of the hind fins have preserved their natural relative positions; those of the left side, with their part of the pelvic arch, being a little more backward in position than those of the right, agreeing, in this respect, with the pectoral limbs, and indicating some general movement of the matrix as the cause of such displacement.
Including the atlas and axis there are twenty-four vertebrse before that in which the pleurapophyses have risen, to articulate wholly with the diapophyses (Tab. XII, d). At the forty-fifth vertebra the rib again begins to articulate with the centrum ; in the forty-sixth the parapophysis forms the lower half of the costal surface; in the forty- seventh it forms a larger proportion, and the whole costal surface is here suddenly increased in size, giving attacliment to a short, slightly bent pleurapophysis of correspondingly and abruptly increased thickness ; that of the forty-eighth vertebra is thicker and straighter, and, as the preceding riblet inclines towards its extremity,
1 conclude that their thick, abrupt, digital ends were ligamentously connected with the iliac bone, and that they therefore may be regarded as sacral vertebrae (Tab. IX, s, s). The remaining vertebrae, from the forty-ninth to the eighty-fourth, will be caudal ; thus there may be reckoned 24 cervical, 24 dorsal, 2 sacral, and 34 caudal vertebrae, in the present species.
Perhaps the two vertebras antecedent to the sacral, in which the centrum shows part of the costal surface, might be regarded as lumbar vertebrae.
The total length of the vertebral column, from the third cervical to the last caudal, following its slight undulations, is 9 feet 9 inches. The skull, from the hind end of the mandible to the fore end of the symphysis, or snout, is 1 foot 11 inches.
The first five or six cervicals, from the third, are more or less obscured by pyritic matter ; their neural spines show intervals of from three to six hues ; the upper margin of the spine rises obliquely from before backward, with the angle rounded off; it is thickest at the middle part, where it measures two lines ; that of the fourth vertebra has a fore-and-aft diameter of seven lines, the same diameter of the ninth is one inch. The pleurapophyses of the tenth vertebra are about an inch in length, with a subcylindrical body, bent obliquely backward, and slightly tapering to an obtuse end. In the eleventh vertebra, the centrum of which is an inch in length, about five lines of free surface intervene between the costal and neurapophysial articulations. From the pleurapophysis to the summit of the neural spine it measures
2 inches 5 lines. At the twelfth cervical the pleurapophyses begin to send forward the process which marks what may be termed the neck or pedicle of the cervical rib.
LIASSIC PLESIOSAURS. 23
At the fourteenth cervical the length of the centrum is 1 inch 5 lines ; that of its pleurapophysis is 1 inch 9 lines ; the fore-and-aft extent of the base of the neural spine is 1 inch 2 lines ; the height of the spine is 1 inch 6 lines, and its thickness is 3 lines. The total height of the vertebra is 4 inches. These dimensions are gained by gradual increase from the tenth vertebra. In the nineteenth cervical the length of the centrum is 1 inch 6 lines, the space between the pleur- (ib. p/)) and neur- {ih.np) apophysial surfaces is 7 lines. From the lower part of the centrum to the summit of the neural spine ( ns) is 5 inches ; the length of the pleurapophysis is 2 inches.
The P/ef</osaur/is rostrafus ranks with the section of its genus characterised by broad and short cervical vertebrae. The instructive characters derivable from this region will here be described as they appear in the fifteenth of the series (Tab. X, figs. 1 — 3). This vertebra gives the following dimensions :
In. lines.
Length of centrum ........... 1 6
Height of terminal surface of ditto, or vertical diameter .... 1 7
Breadth of ditto 2 6
Breadth of the middle of centrum ........ 2 3
From the under part of centrum to the summit of neural spine ... 4 0
Fore-and-aft extent of neural spine at its middle ..... 1 2
„ „ neural arclij,from the end of one zygapophysis to that
of the other ....... 2 4
„ „ neural arch below the zygapophyses ... 1 1
„ „ costal surface 0 10
From the costal surface of the base of the neurapophysis .... 0 8
The terminal articular surface of the centrum is nearly flat, very slightly convex towards the circumference, and similarly concave at the centre ; it is transversely elliptical, with a rather thin border, pretty closely co-adapted to that of the contiguous centrum. The sides of the centrum are moderately concave, the under surface is more deeply so ; and this is further excavated on each side of an obtuse median ridge (?•), near which the venous canals open into the large and deep ellipsoid fosste. The outer boundary of these fossae is formed by the lower border of the costal articular surface (Tab. X, fig. 3, and Tab. XI, fig. % pi). The costal surface (Tab. X, fig. \, pi) pre- sents an oval form, with the long axis parallel with that of the centrum, 10 lines in length by 8 lines in breadth, situated at the angle between the lateral and inferior surfaces, and divided by a smooth, non-articular trait of the lateral surface, of 8 lines in vertical extent from the neurapophysial surface of the centrum ; this is defined below by a slightly curved subangular border, convex downward. The fore surface of the centrum presents a slightly fibrous character, not so smooth as in some other species, nor so irregular as in the PL rugosus, for example.
The neural arch is broad and low ; the zygapophyses project from nearer the base
24 BRITISH FOSSIL REPTILES.
of the arch and centrum than in the PL dolichodeirus (Tab. Ill, fig. 4) or in PL Bernardi (for example, Monograph, 18G2, Tab. IV, fig. 1 1), and their articular surfaces are more horizontal, the anterior ones (Tab. X and XI fig. 1, s) looking almost directly upward, the posterior ones (ib. s') downward ; in this character the present species resembles the Fl. Iiomalospondj/liis. The neural spine is of subrhomboid figure, its height hardly exceeding the fore-and-aft breadth ; the anterior border is convex, and rounded off into the upper one, with a scarcely marked angle ; the posterior one is slightly concave, the angle between it and the upper border is blunted ; the antero- posterior extent of the base of the spine is 1 inch 4 lines, the height of the spine is 1 inch G lines. The pleurapophyses (Tab. X, figs. 1 and 3, Tab. XI, fig. 1, pi) are not anchylosed to the centrum. Their head, or articular surface (ib. fig. '2. pi), forms the thickest part ; the bone decreases as it stands outward, especially in vertical diameter, becoming flattened or depressed ; it then bends backward, sending a short process forward, like the tubercle of the Crocodile's cervical rib, but developed from the same plane as the head ; the backwanlly contained body of the rib decreases in horizontal and increases in vertical breadth, presenting a broadly convex surface outwardly (Tab. X, fig. 3 pi). The length of the cervical pleurapophysis in the fifteenth cervical here described from the fore part of the head to the postei'ior point, is 2 inches ; from the end of the tubercle to the posterior point is 1 inch 8 lines. The increase in the succeeding vertebrae is most in the pleurapophyses, next in the neural spines, then in the breadth of the vertebra, and least in the length of the centrum; this, indeed, varies somewhat, but not so much as appears in the figure 2 of Tab. XI, in which the matrix is left upon part of the inferior surface in two of the vertebrae.
Resuming the consecutive examination of the spinal column we find, in the twentieth vertebra (Tab. XII, 20), the costal surface rises nearer the neurapophysis {np) ; the rib has attamed a length of 2 inches 8 lines. In the twenty-first (ib. 21) the costal surface reaches the neurapophysis {np), which contributes a little to its upper part by a diapophysial projection. The vertical extent of this costal surface is 1 inch, the length of the pleurapophysis is nearly 3 inches. In the twenty-second vertebra half the costal surface is formed by the diapophysis. The length of the rib {d) is 3 inches 9 lines ; the anterior process or tubercle becoming shortened. It is shorter in the next rib (y), the body of which is longer ; and on the rib of the twenty-fourth vertebra it has disappeared. In the twenty -fifth vertebra (ib. D 1) the diapophysis (rf) is prominent, and forms the entire costal surface. The ribs of this instructive series of six consecutive vertebrae have been dislocated from their articulations, apparently by the operation of the pressure which rotated the rest of the vertebrae from the vertical to the lateral position, but they retain their relative positions to each other, the end of one extending beyond and below the fore part of the next, and, in a greater degree, as the vertebrae approach the back. The sides of the neural spines of these vertebrae are roughened by irregular or granulate ridges, directed toward their
LIASSIC PLESIOSAURS. 25
summit, which is bent backward. The dorsal vertebrae continue to increase in the length and size of the diapophyses, in the height of the neural spines, in the breadth and depth of the centrum, and, by a still greater degree, in the length of the ribs ; in every dimension, in short, except that of the length of the centrum, which, in the tenth dorsal, is 1 inch 10 lines, and in no dorsal vertebra exceeds 3 inches. The breadth of the centrum in the tenth dorsal is 3 inches ; the height, 2 inches 3 lines ; the articular surface is moderately hollow at the middle, and gently convex towards the peripher)^ ; the neural spines gradually attain the height of 3^ inches towards the end of the series, the fore-and-aft extent being about I3 inch near the summit, which is more thickened and truncate than in the neck, measuring, in some of these vertebrae, 9 lines in thickness. Both margins are concave at the lower half of the spine, and the intervals between those of different dorsal vertebrae average about three fourths of an inch at the narrower parts of the spines. The length of the diapophysis is about 1^ inch; it expands to its extremity, which is abruptly truncate, looking obliquely outward, backward, and a little downward ; it is flat, and rather rough, for ligamentous union with the rib ; suljquadratc in form, averaging about an inch across. The ribs attain their greatest length from the twelfth to the fifteenth dorsal, where they are 1 foot 6 inches in length, with a simple expanded end, corresponding in shape and size with the diapophysial surface ; the body of the rib is subcylindrical, then sub- trihedral, and again subcylindrical in shape, about 6 lines in diameter at the narrower part, and gradually enlarging at the distal third to the truncate extremity, which was ligamentously connected with the sternal rib. Some of the longest ribs have suffered fracture, and some contortion at their middle slender part, in the course of the cosraical pressure which has spread them out flat ; but they retain much of their natural curvatures on each side the vertebral column. After the thirteenth, the ribs gradually decrease to a length of 3^ inches, in the last vertebra, in which the rib articulates wholly with the diapophysis (twenty-second dorsal), the breadth of this rib is 5 lines. Where the rib begins again to descend from the centrum, it continues to decrease in length in the first and second, in the latter of which it begins to gain in thickness. In tlie forty-ninth vertebra, counting from the skull, which vertebra I have indicated (Tab. IX, s) as the first sacral, the rib is 2 inches 6 lines in length, and 9 lines in least diameter ; its head is partly buried in the matrix, but the articular surface next the vertebra from which it is detached is 2 inches in vertical and I inch in longitudinal diameter, and the surface projects below, from the centrum, as it does above, from the neural arch. The borders of the terminal articular surfaces of the centrum are thicker and rougher than those of the dorsal or caudal vertebrae, indicating a stronger connection between the vertebrae from which the pelvic arch was suspended. The rib of the second sacral is straight, 2| inches in length, and 13 lines in the smallest diameter.
In the caudal vertebrae the neural spines gradually decrease in length, but more so in antero-posterior breadth, being longer, and with wider intervals at the basal half of
c
26 BRITISH FOSSIL REPTILES.
the tfiil than in the neck. The pleurapophyses continue to articulate in part with the neural arch to the tenth or twelfth caudal vertebra ; the pleurapophyses are straight, and have gradually diminished to a length of 1^ inch in the tenth caudal ; they are flattened, and slightly expand towards the fore extremity, which, in the one above cited, there measures 10 lines across ; the haemapophyses are distinctly shown, those of each pair being separate, beneath the centrums of the seventeenth, eighteenth, and nineteenth caudals, forming part of that series where the neural spines are turned to the left. The first of these hfemapophyses has a length of 1 inch 9 lines, and a fore- and-aft breadth of 6 lines at its compressed, dilated, free extremity. The articular surface of the centrum of the twentieth caudal vertebra is exposed ; it is gently concave, with a central depression, 1 inch 3 lines in vertical and nearly the same in transverse diameter, with an inferior border bevelled off at the fore part, for the articulation of the h^emapophyses. The ten terminal caudals show the lateral com- pression and flattening, with suppression, first of the posterior then of the anterior zygapophyses ; next of the neural spine, and, in the last three or four, of the neural arch itself. Traces of heemapophyses may be distinguished as far as the twenty-ninth caudal. This compression of the centrums would indicate, by cetacean analogy, some development of the terminal dermal expanse, but in a vertical, not horizontal, direction. Reckoning the dorsal series of vertebrse as twenty-four in number, it constitutes rather more than one third of the whole extent of the spinal column ; the thirty-four caudal vertebrae, of smaller proportions, constitute another third; the twenty-four cervicals are rather less than a third. The skull is equal in length to three fourths of the neck and to one sixth of the entire skeleton. The total length of the vertebral column is 9 feet 9 inches, the total length of the skeleton being 1 1 feet 8 inches.
The skull (Tabs. IX and XIII) is 1 foot 1 1 inches in length, 9 inches in breadth across the mastoids, 7^ inches across the back of the orbits, but here it appears to have been somewhat flattened out by pressure. It is 5 inches 3 lines broad in front of the orbits, 2 inches across the narrowest part of the snout, which, from tlie fore part of the orbit, is 11|^ inches in length, and expands at its extremity to a breadth of 2^ inches. This is the proportion of the snout which gives the peculiar and distinctive character to the present species of Plesiosaurus and which suggested rostratus as the specific name ; in fact, the head, from the aspect exposed, resembles rather that of the Muschelkalk Pistosaurus than that of any of our heretofore known Liassic PJesiosauri.
The temporal fossae arc oblong, contracting anteriorly, and are there outwardly rounded off; in length 5 inches ; in breadth, posteriorly, 3 inches. The subcircular orbits are 2 inches in diameter. The narrow elliptical nostrils are 1^ inch in advance of the orbits. The upper and hinder boundary of the cranium, formed by the bifurcate parietal, and strong, overlapping mastoids, is convex superiorly, expanding as it proceeds outward. The middle part of tlie parietal rises into a sharp crest
LIASSIC PLESIOSAURS. 2 7
between the temporal fossas, and a continuation of the same crest, whose sides slope away from each other at a right angle, characterises the upper part of the frontal to midway between the orbits. The postfrontal bar, or flattened tract, dividing the orbital from the temporal fossae, is an inch in breadth. The narrow nasals are divided by a medial suture, and, with the prefrontal and lacrymal, separate the orbit from the nostril.
The lower jaw has slipped from beneath the cranium, and, by the effect of the gradual pressure, has been turned, with the flatter left side upward. The angle projects beyond the articular surface 2 inches 3 lines, terminating obtusely, slightly bent, with the concavity upward, and 1 inch in thickness. The great part of the articular cavity of the right ramus is exposed, showing a transverse diameter of 1 inch 3 lines, and a fore-and-aft diameter of 10 lines: it is concave lengthwise, sinuous across. In advance of the articulation the ramus shows a depth of 1-| inch, gradually increasing to that of 2 inches 3 lines, and then contracting vertically toward the dentary part ; the deepest portion, formed by the angular and sub-angular elements, is situated about four inches in advance of the articular cavity, and there the thin outer parietes of the ramus have been crushed in, yielding to the superincumbent pressure.
The length of the dentigerous part of the jaw is 1 foot 2 inches ; externally the dentary descends vertically from the sockets containing the teeth, but internally it swells out into a strong, convex, longitudinal tract, strengthening the alveoli, until the two dentaries meet at the symphysis. There is a longitudinal groove at the middle of the inner surface, below which the bone again swells out and is continued into the thick under surface of the dentary. The length of the symphysis is nearly 7 inches ; and here the vertical extent increases, and terminates more sharply below. The vertical extent of the dentary, behind the symphysis, is 1 inch, the deepest part of the symphysis is 1^ inch ; the outer surface of the symphysis is coarsely and irregulai-ly rugose ; its upper ])order is scooped out at the alveoli for the larger teeth ; on the left side there are about twenty-two sockets for teeth of different sizes ; the smallest are behind, and the hindmost shows a straight crown (Tab. IX, fig. 6), sloping forward, from 4 to 5 lines long, with the usual longitudinal ridges of the enamel. The tooth in advance is sliglitly bent ; the eighth in advance shows a crown, 9 lines in length ; the fifteenth in advance (Tab. IX, fig. 4) shows a sudden increase of size, and greater degree of backward curvature ; including this, eight teeth occupy the rest of the alveolar surface, which is coextensive with the symphysis. Here the teeth are divided by intervals of rather more than their own basal breadth ; the largest tooth (ib., fig. 3), following the curvature, has a crown two inches in length. The longitudinal enamel-ridges begin at from one to two lines above the base of that covering of the crown, where it is smooth, and they terminate about the same distance from the apex ; they are least developed at the outer, convex part of the upper half of the crown. Two of these large, laniary teeth project from the anterior alveolus, the outer and
28 BRITISH FOSSIL REPTILES.
anterior tooth being about to be shed. The right side of the symphysial part of the jaw (Tab. IX, fig. 2) contains nine teeth, the third, fifth, and seventh being the largest. The fifth (Tab. IX, fig. 5) measures 2§ inches in length of crown, following the curve. The fourth and sixth teeth were emerging from sockets larger than themselves, and arc successional teeth. The unusual length of the symphysis corresponds with the prolongation of the premaxillary above.
In the same locality and formation with the skeleton above described was discovered the portion of skull, corresponding in size, and, so far as it is preserved, in shape with that of the skeleton of the Plesiosaurus rosiratus, to which species, therefore, I pro- visionally refer the specimen (Tab. XIII). It includes the basi-occipital, right ex- occipital, basi-sphenoid, portions of pterygoids, ecto-pterygoids, palatines, with fragments of the maxillary and of the right ramus of the mandible. In the figures of this specimen (Tab. XIII) the mandibular part is omitted. The left ex-occipital is wanting ; the right (ib., 2) is displaced, so that the whole of the upper part of the basi- occipital (ib., 1) is exposed. This shows that the bases of the ex-occipitals are divided from each other by a narrow tract, and that the basi-occipital forms the whole of the condyle and of the median part of the floor of the epencephalic compartment of the cranium; this part of the bone (1) measures across its most contracted portion about three lines. The condyle is subhemispheric, ^Yith the transverse diameter rather the longest, and with a slight and irregular depression a little above its centre. The upper border of the condyle (fig. 1) is on a level with the advanced epencephalic surface (1) of the basi-occipital, from which it is divided by a shallow and narrow transverse channel; the lower border (fig. 2, o) projects abruptly downward, and is divided from the more advanced surface of the basi-occipital by a transverse furrow, three lines wide and four or five lines deep. The sui'face of the basi-occipital is covered by the posterior border of the pterygoids (24) which underlie it, extending backward, so as to leave only parts of the pair of rough and tuberous basi-occipital processes {h,h) exposed. The posterior part of the epencephalic surface of the basi-occipital (Tab. XIII, fig. 1, 1) is smooth and concave transversely; but, as it advances, it becomes irregular and expands, and apparently is divided by an irregular protuberance at the middle part. The neurapophysial surfaces {n,n) on each side are, as in the succeeding centrums, triangular, with the angles rounded ofi". About a line in advance of these is the 'harmonia,' or straight suture, indicating the flat synchondrosis by which the epencephalic unites with the mesencephalic centrum (basisphenoid, ib., .J), thus repeating the kind of union which attaches the centrum of the atlas to that of the axis ' vertebra;. The neural surface (5) expands to a greater breadth than it had attained in the basi-occipital, measuring fourteen lines across ; it has a smooth, undulating surface, moderately concave in the middle, where it sinks below the level of the epencephalic surface (1). The sides of the mesencephalic surface are bounded by a narrow ridge, seemingly an exogenous growth from the centrum, as in some modern
LIASSIC PLESIOSAURS. 29
Lacertilia, to which ' neurapophysial ' ridges (Tab. XIII, fig. 2, n, n) may have been attached tlie neurapophyses, but retaining, as in these Lacertilia, more or less of their primitive histological fibro-cartilaginous condition. The under and lateral parts of the mesencephalic centrum, called ' basisphenoid,' are covered by the largely developed 'pterygoids' (ib., fig. 1, 24), — the diverging appendages of a more advanced cranial segment. The back part of the third or ' prosencephalic' centrum has coalesced with the second, the boundary-line being indicated by a shallow depression, which may have lodged the vascular appendage of the brain called ' hypophysis,' or ' pituitary gland.' The sides of this part of the centrum rise, neurapophysially, and terminate with a fractured and worn surface, and the rest of the centrum, answering to the cetacean 'presphenoid' (9), has been broken away. Beneath the fractured end of the presphenoid is the nasal ' meatus/ or canal (ib., fig. 1, r), which is divided below by the junction of the palatines (ib., fig. 2, 20, 20) with the pterygoids (ib., 2A, 24), so as to open upon the roof of the mouth by a pair of posterior or palatal nostrils ' palatonares' (ib., r,r)- These are of a narrow, elliptical form, with the long axis longitudinal, but slightly, inclined ' mesiad,' or converging anteriorly, pointed behind, 1 inch 6 lines in long diameter, 7 lines across their widest part, and separated anteriorly by a tract of bone 9 lines across; from the posterior end of the ' palatonares' to the same part of the basi-occipital tuberosity is I inch 5 lines. The palatines (ib., fig. 2, 20, 20), where they give attachment to the pterygoids, are narrow, rather thick, with a shallow median channel, bounded by low, lateral, obtuse risings ; external to these the palatines form the anterior ends of the ' palatonares,' and thence expanding, and flattening as they stretch forwards, they unite laterally with the ectopterygoids (25, 25). The pterygoids (24, 24), which form the inner and posterior boundaries of the palatonares, expand as they pass backward, become thinner and flatter, and retain a sutural union along the mesial line until they reach the precondy- loid fossa of the basi-occipital ; underlying and concealing from view, inferiorly, the basi-presphcnoid and major part of the basi-occipital. These ' basilar' plates are slightly concave from side to side, and are divided from the ' tympanic ' pro- cesses (24', 24') by a rising of bone, hardly to be called a ridge ; this is chiefly formed by the concavity or sinking of the surface at the commencement of the under part of the ' tympanic' process, which also gradually contracts in breadth as it extends backward and outward to abut against the tympanic pedicle (28), which is wedged at its upper or cranial half between the pterygoid (24) and par-occipital (3).
The ectopterygoid (ib., fig. 2,25) articulates with the pterygoid near the posterior part of the palatal nostril, where it is about seven lines in breadth ; this joint has been put out by pressure on the right side and the ectopterygoid dislocated downward ; the corresponding part is broken ofi^ on the left side, where it overlaps the pterygoid, forming the outer boundary of the palatal nostril. The ectopterygoid expands as it advances forward ; curving, mesially, round the fore part of the nostril to join the
30 BRITISH FOSSIL REPTILES.
palatine (20), and laterally to join the maxillary (21), of which a fragment is preserved in this crushed specimen. The lower opening of the temporal fossa is bounded mesially in nearly equal proportions by the pterygoid and ectopterygoid.
Estimating the length of the skull in the present specimen at two feet, about six inches in length of the hind part is here preserved; and the palatal nostrils open chiefly upon the hinder half of this part. In their posterior position, therefore, they agree with those in Teleosaurus, but differ in being divided by the medial productions of the palatines and pterygoids, and in not being confluent, as a single aperture, as in the Crocodilia ; thus they exemplify the more general Reptilian character as it is preserved in our modern LaccrtUla.
In the Kothomurus the palatonares are two ; but they open upon the anterior fourth part of the bony palate, having their hinder boundary formed by the palatines instead of their front one. In Pistosaiirus the palatonares are situate about midway between the fore and back part of the long and narrow bony palate. In all 8auroj)ter)j(jia the pterygoids present much of their crocodilian character in their posterior extension and expansion, underlying the posterior cranial centrums and covering, in this way, in PIe--iiosauriis, more of the basi-occipital than in the crocodiles. Some portions of long, slender, subcompressed bones adhering to the present instruc- tive fragment of the plesiosaurian cranium may have belonged to the hyoidean arch ; one of them (40) adheres to the bony palate, and partly conceals the left palatal nostril in fig. 2.
Pectoral arid pelvic arches and limbs (Tab. IX).
The scapula (Tab. IX, 51) is 5 inches in length; smooth and convex externally at its narrow upper part, where it shows a breadth of 10 lines; it rapidly expands to a breadth of 3 inches at its humero-coracoid extremity, which overlaps, as before mentioned, the head of the dislocated humerus. The outer surface of the articular end of the scapula is roughened by longitudinal ridges.
The humerus (ib., 53), showing a breadth of 1 inch 9 lines where it emerges beneath the scapula, expands to a breadth of 4 inches where it is articulated to the antibrachial bones ; its anterior border is straight, less convex at the distal half than in PI. dollchodeirits ; less expanded at the distal end than in PL liomaloHpondiihis.
The radius (ib., 54) is 3 inches 2 lines in length, 2 inches broad at its proximal end, 1 inch 9 lines at its distal end ; with a thin, straight, somewhat irregular anterior border, and a thicker, smooth, concave posterior border. The ulna (ib., 55) presents the usual reniform figure, with the concavity toward the radius ; it is of the same length as the radius, but is flatter ; 2 inches 3 lines across its middle part ; the margin next the radius is rather more concave than that which it opposes; the
LIASSIC PLESIOSAURS. 31
opposite margin of the ulna is very convex ; the distal end is divided by a low angle between the surfaces for the outer and middle carpals of the proximal row.
The carpal bones are six in number, three in each row. The proximal ones are the largest, but, of this row, the radial carpal, or ' scaphoid,' is least ; it is of a trans- versely oblong figure, with its distal border divided by a low angle between the radial and the middle bones of the second row ; this relation is belter shown in the right than in the left pectoral fin. The middle proximal carpal, or ' lunare,' is the largest, of a sub-hexagonal form ; the shortest side, toward the radius, is concave ; the side opposite the radial carpal is rather convex ; the other facets for the ulna, ulnar carpal (' cuneiforme') and the two larger carpals of the second row, are nearly straight. The three carpals of the second or distal row increase in size from the radial to the ulnar side of the waist; the outermost being surrounded by three carpals and three metacarpals ; the metacarpal of the fifth digit extending along its ulnar side to articu- late with the ulnar carpal of the first row. In its relations to the metacarpals, the laro-est of the second row resembles the 'magnum' and 'unciforme' combined, of the mammalian carpus. The next in size answers to the ' trapezoides,' supports the second metacarpal, and, at its opposite border, fills the interspace between the radial and middle carpals of the first row. The radial carpal of the second row is the smallest of all ; it is wedged between that of the first row, the first metacarpal and the middle carpal of the second row : it answers to the ' trapezium.'
The first metacarpal (ib., 56) is the shortest, 1 inch in length, it supports two phalanges, the last of which is 1 0 lines in length ; the whole length of this digit, including the metacarpal, is 2 inches. The second metacarpal is 1 inch 9 lines in length, and supports six phalanges, the last being 4 Hues long; the total length of this digit, including the metacarpal, is 10 inches 6 lines. The middle metacarpal is 1 inch 10 lines in length, it supports eight phalanges ; the total length of this digit, including the metacarpal, is 10 inches 6 lines. The fourth metacarpal is 2 inches in length, it supports seven phalanges ; the total length of the digit, including the metacarpal, is 10 inches. The metacarpal of the fifth digit is 1| inch in length, its proximal breadth is 1 inch; that of the first metacarpal being half an inch, and that of each of the three intermediate metacarpals ranging from 9 to 10 lines ; the outermost metacarpal sup- ports seven phalanges : the total length of the digit, including the metacarpal, is 9i inches ; the fifth metacarpal, besides being shorter and broader than the three middle ones, is more convex, and obliquely bevelled off at its proximal end, the radial side being shorter than the ulnar one. The phalanges of the fifth digit arc more con- cave at their outer or ulnar border than the others. The first metacarpal is more concave at its radial border. All the phalanges are flattened and expanded at their extremities, the outer surface showing linear impressions, the middle part being smooth. The total breadth of the carpus is 4 inches 5 lines. The total length of the hand is 1 foot I inch. The total length of the pectoral limb seems to have been about
32 BRITISH FOSSIL REPTILES.
2 feet. It is much shorter in proportion to the trunk (as reckoned from the first dorsal (ib., fig. 1, d) to last sacral (ib., s) than in the Plesiosaurus homahspondylus (Tab. V), and differs in the more pro.ximal position of the metacarpal of the fifth digit, and in the smaller size of the radial carpal of the distal row.
Like the scapula in the pectoral arch, the ilium (Tab. IX, G2) is the smallest bone of the pelvic one; it is 5 inches in length, I^inch across the obliquely truncate upper (proximal or sacral) end, it contracts to a diameter of 9 lines at its middle, and then expands to a breadth of 2 inches 4 lines, with proportional thickness, at its lower acetabular end. The stem is subcompressed, convex transversely, and also longitudi- nally at the upper half, which shows a low ridge externally, and is longitudinally striate near the margin of the surface connected with the sacral pleurapophyses, from which, as before stated, it has been dislocated. The acetabular end preserves its natural connections with the corresponding thickened part of the ischium (ib., G3), which on both right and left sides is interposed between the ilium and pubis. The rough acetabular surface is nevertheless continued from the ischium upon the pubis, for about two inches of the contiguous border. The head of the femur is applied to the ischio-pubic surface in both limbs, yet the better proportioned articular depression is that formed b)^ the ilium and ischium, from which it seems as if the femur had been dislocated forward. As, however, the mode of attachment has been by a ligamentous mass, this may have converged from the whole of the antero-posteriorly extended acetabular surface to the head of the thigh-bone, allowing a certain freedom of play of the bone forward and backvvard ; the diameter of such acetabular surface, lengthwise, is 5 inches, the greatest vertical diameter is 3 inches.
The ischium (Tab. IX, 63), as it passes from the acetabulum mesiad, loses its thickness and expands into a plate of tlie usual triangular form, the posterior apex of which is seen on the left side behind the two overlying sacral ribs, stretching as far back as the ilium (ib., 62) ; from this apex, or angle, to the acetabular junction with the pubis, the ischium measures 8 inches. Pyritic matter intervenes between tlie ischium and sacral ribs, on the left side.
So much of the pubis (Tab. IX, 64) as is visible on the left side exhibits the usual subcircular discoid shape, with the two facets on the thickened part of the margin, one for articulation with the ischium, the other completing the fore part of the acetabular tract. The broadest part of the exposed pubic disk measures fi inches lengthwise.
The femur (Tab. IX, Gj) is 9 inches 9 lines in length. A longitudinal notch feebly marks out a trochanteric part of the thick, convex, articular head ; this is coarsely pitted for the ligamentous insertions. The shaft contracts, chiefly losing thickness, and becoming lamelliform as it expands in breadth to the distal articular surface. This is convex, curving in a greater degree at its hinder part. Both anterior and posterior borders of the shaft are concave ; the former least so, but to that extent differentiating the femur from the humerus, in which it is straight, or rather convex.
LIASSIC PLESIOSAURS. 33
The distal breadth of the femur is 4 inches 8 lines ; the non-articular surface is smooth, except near the two ends, where there are rough, longitudinal ridges and depressions, indicative of ligamentous insertions.
Both tibia (Tab. IX, Cfi) and fibula (ib., 67) are broader in proportion to their length than their homotypes in the fore limb. The posterior distal angle of the fibula is more decidedly truncate, for articulation with the middle tarsal, than is the corresponding part of the ulna. The inter-osseous space is a long ellipse with pointed ends, about an inch in width across the broadest part.
The tarsus (Tab. IX, G8), like the carpus (ib., 56), consists of six bones, in two rows of three each. The tibial bone of the first row is broader, in proportion to its length, than its homotype in the carpus ; and tliis is the proportional character of all the bones of the tarsus, save that which intervenes between the fibula (67) and the fifth digit (69), the metatarsal of which passes outside the distal tarsal series.
The metatarsal of the innermost tibial, or first digit, is 1 inch 4 lines in length, 10 lines in breadth, and supports three phalanges; the total length of the digit, including the metatarsal, is 4 inches 8 lines. The metatarsal of the second digit is 1 inch 9 lines in length, and supports six phalanges, the last being 4 lines in length ; the total length of this digit, including the metatarsal, is 8 inches 6 lines. The meta- tarsal of the mid-digit is 2 inches in length, and supports nine phalanges ; the total length of this digit, including the metatarsal, is 1 foot. The metatarsal of the fourth digit is 1 inch 11 lines long; the fibular side of its base is more produced than in the others ; it supports a digit of eight phalanges, and this, including the metatarsal, is 1 foot 1 inch in length ; the metatarsal of the fifth digit is 1 inch 10 lines in length, and supports six phalanges, the last of which is broader and flatter than in the other digits ; the total length of the fifth digit, including the metatarsal, is 10 inches 2 lines. The breadth of the leg is 5 inches 5 lines ; the length of the tibia 3 inches ; that of the fibula 2 inches 9 lines. The breadth of the metatarsus is 4 inches 7 lines. The total length of the hind limb is 2 feet 4^ inches. The hind limb, though longer and larger than the fore limb, repeats the character of relative shortness in proportion to the trunk, as engraved with the same parts in Plesiomurus liomalospondijlus. The neural spines of the trunk are shorter, with wider intervals, exemplifying the superior vigour and locomotive power of the longer-necked and larger-finned species (Tab. VIII.)
We see in JPl. rostratus a correlation of the size of the head with that of the anterior laniary teeth, and with the shortness of the neck. But the head is pro- portionally less compared with the trunk, and the neck is shorter, and has fewer vertebrae, than in the PI. macrocepJialus* These characters, with the greater lengthening and attenuation of the muzzle in PI. rostratus, indicate a nearer step in affinity toward the Teleosaurian marine reptiles.
* ' Geological Transactions,' 2nd series, vol. v, pi. .ijliii.
/
34 BRITISH FOSSIL REPTILES.
Plesiosaurus rugosus, Owen. Tabs. XIV and XV.
This species, originally indicated by characters of detached vertebrae,* has received ample elucidation from the fine specimen (Tab. XIV) presented by His Grace the Duke of Rutland, K.G., to the British Museum. It was obtained from the zone of Lower Lias of Leicestershire, characterised by the Ammonites stellaris, in the neigh- bourhood of Granby.
This specimen of the Plesiosaurus rugosus presents a similar condition to that of the PL dolichodeirus decribed by Conybeare,! save that the head is not preserved in advance of the small, scattered vertebrae of the anterior part of the neck ; about five-and-twenty vertebra; of this region preserve their consecutive arrangenient, most of them in almost a straight line. The vertebrae of the trunk have suffered a greater degree of dislocation, and, the specimen having been exposed in a prone position, they are so dispersed as to permit to be seen the upper or inner surface of the coracoids, the abdominal ribs, and the pubic and ischial bones. Two thirds of the caudal vertebrae show the same scattered and dislocated condition ; but nine near the end ot the tail have preserved their natural position, as consecutively articulated, and appa- rently their true relative position to the trunk. The four paddles are preserved outstretched, as naturally articulating with their respective arches of support, with the superior or external surface of the bony framework exposed. ,
The cervical vertebrae, which have retained their natural consecutive arrangement and juxtaposition, have undergone the same partial rotation as is observable in most Plesiosaurian skeletons from Liassic beds, presenting their broadest surface or sides to view ; the neural spines have here rotated toward the left side. Besides the twenty- five cervical vertebrae which are more or less consecutive, three or four are huddled in a heap at the base of the neck, and five or six are scattered at its fore part. From the size of the articular surface of the foremost of these, which measures but six lines jn diameter, as well as from so much of the length of the neck as is demonstrated, it may be inferred that the head was small, as in the PI. dolichodeirus.
Cervical veriehrce (Tab. XV).
One of these vertebrae, corresponding in position to the fifteenth cervical of the PI. rosfratus (Tab. X), was carefully wrought out of the matrix of the specimen Tab. XIV, and is represented of the natural size in Tab. XV. Its proportions show that the present species, like PI. dolichodeirus and PL Hawkinsii, belongs to the section
* ' Report on British Fossil Reptiles,' 1839 ; ' Reports of the British Association,' 8vo, 1840, p. 82. t 'Trans. Geol. Soc.,' torn. cit.
LIASSrC PLESIOSAURS.
35
characterised by intermediate proportions of the centrum, neither "long," as in PL homalospondylus, nor " short," as in PL rostratus and PL planus. The following are the dimensions of this vertebra :
Length of centrum .........
Height or vertical diameter of terminal surface of ditto . Breadth of ditto
,, the middle of centrum .......
From the under part of centrum to the summit of neural spine Fore-and-aft extent of neural spine at its middle ....
,, „ „ arch from the end of one zygapophysis to that of
the other .....
,, ,, „ arch below the zygapophyses
„ ,, costal surface ......
From the costal surface to the base of the neurapophysis
|
In. |
lines. |
|
9 |
|
|
9 |
|
|
101 |
|
|
7 |
|
|
4 |
|
|
4 |
|
|
2 |
6 |
|
1 |
3 |
|
0 |
7 |
|
0 |
11 |
The free or non-articular surface of the centrum shows near the margins of the terminal surfaces the strongly marked rugous character which originally suggested the specific name in the detached vertebrae. The irregular risings of bone lie chiefly in the direction of the axis of the vertebra, and project so as to come into view exterior to the articular surface in an end-view of the centrum, as in Tab. XV, fig. 2. The sides and under part of the centrum are moderately concave lengthwise. The contour of the terminal articular surface is circular; its border is thick and convex, leading to a moderate concavity, with the central part rising into a slighter convexity. The costal pits {pi) are of a full elliptical form, with a slightly prominent margin, situated near the lower surface of the centrum, a little nearer the hind than the fore end, and with twice their own vertical diameter intervening between them and the base of the neurapophysis {np)- This part of the side of the centrum is traversed by a low, longi- tudinal rising, a little nearer the neur- than the pleur-apophysis. The venous orifices on the under surface of the centrum (Tab. XV, fig. 4) are two and a half lines apart, with an intervening low, obtuse, longitudinal ridge, and are not situated in definite depressions. The lower border or base of the neurapophysis has not the angular form seen in PL Hawkinsii and PL dolichodeirus, but is curved. The neurapophysis rises, with both fore and hind borders vertically concave, transversely convex, about five lines above the centrum before giving off the anterior zygapophyses {:) ; the posterior arcs [z') come off, as usual, a little higher. The neural spine is subquadrate, more angular between the fore and upper margins than in PL rostratus (Tab. X, fig. 1); the thickness of the spine is shown in Tfib. XV, fig. 2 ; the front border is sharp. The chief variety observable in cervical vertebrae of the present species is the presence of the longitudinal groove bisecting the costal surface. The articular surface of the centrum in every cervical vertebra where it is exposed in the present skeleton repeats
|
Ft. |
In. |
|
. 10 |
6 |
|
. 4 |
0 |
|
. 3 |
6 |
|
. 3 |
3 |
|
1 |
If |
|
. 0 |
51 |
36 BRITISH FOSSIL REPTILES.
the character of the one described ; but some, at the base of the neck, have the central rising rough, and with a small pit in the middle. The same character is continued throughout the dorsal series, and the concavity is exaggerated in the vertebrae of the tail, which are, however, more concave than the others in all Plesiosauri.
The total length of the specimen preserved is . . . . The length of the cervical region preserved is ... .
From the fore part of the coracoid to the hind part of the ischium From th^ ischium to the end of the tail as far as preserved . The transverse hreadth of the pubic bones across their broadest part is The fore-and-aft diameter of pubis at its middle part is
Scapular arch and appendages (Tab. XIV).
The humerus (Tab. XIV, 53) is 10 inches long, 2^ inches across its middle nar- rowest part, and 4| inches across its distal broadest part. The outer part of the head is somewhat produced, with a slight longitudinal depression on each side ; its surface is tuberous and rough ; there is a low tuberosity on tlie hind part of the humerus, below its head. The contour of the anterior border of the bone is nearly straight, slightly wavy, first concave, then convex, again concave and more convex as it is rounded off to the lower border. The posterior margin is more deeply concave from the upper tuberosity to the posterior angle, which is rounded off. The distal margin, convex in a general way, has its two surfaces sufficiently defined for the radius and ulna ; they do not, however, meet at so well-defined an angle as in some species ; a space of about an inch intervenes here betweeri the radius and ulna in both right and left limbs, whereas they meet and touch each other in the PL doUcJwdeirus (fig. 5) and PL Hawhhmi (fig. 6). The shaft of the humerus shows a tolerably smooth and longi- tudinally fibrous surface, but has a rough tuberculate character for about an inch and a half from the distal articulations.
The radius (Tab. XIV, figs. 1 and 2, 54), 4 inches in length, is 2 inches 9 lines across the proximal end, 1 inch 10 lines across the distal end, 1 inch 6 lines across the middle. The radial or anterior margin is produced and somewhat thickened below the head, making the margin beyond it concave half way towards the distal end ; the posterior or ulnar border is uniformly and moderately concave ; the distal border of the radius is straight from its ulnar angle to near its radial one, where it becomes convex, that angle being, as it were, cut off. This distal border is most closely articu- lated, seems, indeed, partially confluent, with the scaphoid. The ulna (Tab. XIV, figs. 1 and 2, 55) would present its usual reniform shape, were it not that the proximal angle of the. posterior or ulnar border is produced into a sort of olecranon (fig. 2, 55')- This process is separated by a fissure or fracture from the body of the bone ; but as this
LTASSIC PLESIOSAURS. 37
occurs in the same place and with the same course in both forearais, I infer it to be natural, and that the quasi-olecranon was of the nature of a sesamoid, closely articu- lated with the body of the ulna. The extreme length of the ulna, from the apex of this part or process (55') is 5 inches ; the greatest breadth of the ulna 3 inches. The proximal articular surface joins the ulnar facet of the humerus, beyond which the olecranon projects. The distal surface of the ulna articulates with the lunare (/), the cuneiforme (c), and with an ossicle (p) wedged into the interspace posteriorly between it and the ulna, which ossicle may represent the pisiforme. With the excep- tion of about one third of the middle of the shaft, the exposed surface of both radius and ulna is roughened by coarse rugae and small tubercles.
The carpus includes eight ossicles. The scaphoid (fig. 2, s) is an oblong bone, with its dimension greatest transversely, viz., 2 inches, longest at its ulnar side, which is 1 inch 3 lines, with a slight angular projection at its free radial border. The lunare (ib., i) is subreniform, with the concavity, representing the " pelvis of the kidney," com- pleting the lower part of the inter-osseous space, for the radius and ulna are separate below as well as above, and for a greater extent, the radius extending for nearly an inch below or beyond the ulna. The rest of the circumference of the lunare is divided more or less distinctly into its articular surfaces for the radius, scaphoides^, trapezoides Q, magnum (m), cuneiforme (c), and ulna. The ulnar surface has a sigmoid form. The lunare {1) is larger than the scaphoid. The cuneiforme (c), about the size of the lunare, has a subhexagonal shape ; the two proximal sides articulate with the ulna and pisi- forme (p), the radial side with the lunare, the ulnar side with the unciforme («), the two distal sides with the magnum (m), and the base of the fifth metacarpal {„), which, as in some other Plesiosauri, ascends above the rest to this connection, severing, so to speak, the unciforme («) from the magnum [m). The pisiforme {p) is a subtriangular small bone, wedged into the outer interspace between the ulna and cuneiforme. In the distal row .of carpals the trapezium {t) is subquadrate, broader toward the scaphoid, narrower where it supports the first metacarpal (i) ; it is about half the size of the scaphoid. The trapezoides (r) is also subquadrate, but of larger size, articulating with the scaphoid, trapezium, the ulnar angle of the base of the first metacarpal, wholly sustaining the second metacarpal, articulating with the radial side of the base of the third metacarpal, with the os magnum (»«), and with the lunare (/). The os magnum (?«), of similar size, is hexagonal ; the two proximal surfaces articulate with the lunare and cuneiforme, entering the angle which they leave ; the two distal surfaces articulate with the third and fourth metacarpals; the radial side with the trapezoides, the ulnar side with the corresponding part of the base of the fifth metacarpal ; the displaced outermost bone of the distal row is limited to its articulation with the ulnar border of the cuneiforme (c) ; it is the smallest of the carpal series.
The bones of the digits are small in comparison with those of the carpus and fore- arm. The entire breadth of the carpus being 6 inches 5 lines, tliat of the middle of
38 BRITISH FOSSIL REPTILES.
the metacarpus is 5 inches in the left hand, and but 4^ inches in the right, both being preserved in their natural connections as they were buried, but one showing a more expanded, the other a more contracted, condition of the fin.
The first metacarpal (Tab. XIV, fig. 2, i) is 1 inch 9 lines in length, and supports two phalanges ; the total length of that digit, including the metacarpal, being 4 inches 3 lines. The second metacarpal (ib., ii) is 3 inches 3 lines in length ; it supports five phalanges ; the total length of the digit, including the metacarpal, is 9 lines. The third metacarpal (ib., ill) is 2 inches 8 lines in length ; it supports five phalanges ; the total length of the digit, including the metacarpal, is 9 inches 9 lines. The fourth metacarpal (ib., iv) is 2 inches 5 lines in length ; it supports four phalanges, and is the same length with the preceding. The fifth metacarpal (ib., v) is 3 inches 3 lines in length, with its ulnar margin more deeply concave than in the others ; it supports four phalanges, most of which show the same deeper concavity, with a greater production of the ulnar ends of the articular expansions ; the total length of this digit is 8 inches 6 lines ; a distal phalanx is wanting in it, and the same may likewise be the case with the others.
There is a want of precise symmetry in the proportions of the right and left fore paddles, those of the right being longer and somewhat slenderer than those of the left.
The whole of the outer surface of the carpal bones is rugose, as is the chief part of that of the metacarpals and phalanges. The total length of the bones of the right pectoral limb, as here preserved, is 2 feet 3 inches; the breadth of the antibrachiura is 6 inches 6 lines ; that of the carpus 6 inches ; that of the metacarpus 5 inches ; the interspace between the heads of the two humeri is 1 foot 6 lines.
Pelvic arch ajid limbs (ib.).
The iliac bones (Tab. XIV, fig. 1, 62), dislocated by pressure, lie in tha axis of the trunk, parallel with the ischia (ib., 63); the vertebral end of the ilium is broader but less thick than the acetabular one ; the length of the bone is 4 inches 10 lines, the breadth of the vertebral end is 3 inches 6 lines ; the breadth of the acetabular end is 2 inches. The surface here exposed, probably the outer or posterior one, shows a slight concavity on the vertebral expansion, where the bone is smooth ; beyond, it becomes longitudinally striate, and rugose or tuberculate near the acetabular extremity ; this is thickened and obscurely divided into the rough synchondrosal surface for the ischium, and the cor- responding somewhat smaller surface for the ligamentous attachment of the femur.
The ischium (ib., 63) is flat, and of the usual elongate, triangular form ; it joins its fellow by its straight inner side having the posterior angle rounded off; the outer, obtuse, non-articular border presents a sigmoid curve, concave near the ilium. The anterior shorter border is emarginate in the middle, where it forms the posterior boundary of the obturator foramen (ib., o), the straight articular parts of this side
LIASSIC PLESIOSAURS. 39
joining the corresponding parts of the pubis ; the outer acetabular angle is produced, and terminally expanded and thickened to form the articular surfaces for the ilium and femur.
The pubis (ib., 64), as in other Plesiosauri, is broader and larger than the ischium, with the medial or symphysial margin straight, measuring six and a half inches in extent ; the anterior and external free margin is convex ; the posterior margin is more deeply excavated than the opposite one of the ischium, forming a greater part of the circumference of the obturator foramen ; the angle between the posterior and outer borders is thickened, to contribute the anterior part of the acetabulum. This rough and ill-defined articular surface for the femur is thus formed, as usual, by the three constituents of the pelvis.
The femora (ib., 65) here, as in some other Plesiosauri, have the head resting against the ischio-pubic part of the acetabulum, the ilia being placed about an inch further back. The femur, 10 inches in length, is 1 inch 9 lines across the narrowest part of the shaft, and expands to a breadth of 4 inches 9 lines distally ; the outer (here the upper) part of the head is produced, and behind it is a longitudinal depression. The surface, for two inches or more from the distal end, is rugose, with longitudinal ridges breaking up into tubercles ; both anterior and posterior borders are concave ; the latter is the shorter border. Tlie distal border is more regularly convex, and in a greater degree than in the humerus.
There is an interval between the proximal ends of the tibia and fibula, and a wider one between their distal ends, the interosseous space being considerable, as in the forearm. Here, also, the tibia (ib., 6G), like its homotype (ib., 54), has a more distal extension. Its length is 3 inches 1 1 lines, its proximal breadth '2 inches 8 lines. The anterior proximal angle is somewhat produced ; the anterior orbital border is slightly concave ; the posterior one is more so. The fibula (ib., 67), like the ulna, departs from the oi'dinary reniform figure by the production of its fibular proximal angle (67') ; this is not separated from the rest of the bone in the left leg, but it is so by what appears to be a crack in the right leg, and yet so as to indicate that such crack is in the place of an original epiphysial junction. The length of the fibula, including this process, is 4 inches 4 lines ; the length of the concave tibial border of the fibula in a straight line is 2 inches 3 lines. As great a proportion of the exposed surface of the leg bones is rugose as is that in the bones of the forearm. Between the tibia and the tarsal bone supporting the first metatarsal there is a vacant space in both limbs, which, in the right limb, is partially occupied by a tubercle of bone. This we may regard as the beginning of ossification of a fibro-cartilaginous homologue of a naviculare (Tab. XIV, fig. 3, «). The homotype of the lunare (a) completes, by a free concave part of its border, the distal end of the inter-osseous space. This tarsal («), which we may call " astragalus," articulates with the tibia (66) ; but to a greater extent and by a more definite straight border, with the fibula (ib., &7) ; posteriorly with the
40 BRITISH FOSSIL REPTILES.
calcaneum (cz), distally with the two outer bones of the distal row of tarsals. The bone (d) wliich I have called " calcaneum" is the homotj^pe of the cuneiforme in the carpus, which it resembles in size and shape ; it articulates chiefly with the astragalus (a) and ecto-cuneiforme (ec) ; it seems to touch the fibula {67) by a small part of its periphery ; and in the angle between it and the fibula is wedged an ossicle [d' ), answering to the pisiforme in the wrist and to the apophysial part of the calcaneum in the higher Vertebrates. The distal row of tarsals includes but three bones. The first ici), the homotype of the trapezium, I call " ento-cuneiforme ;" it articulates with the rudiment of the naviculare («), and supports the metatarsal of the tibial or first toe G). The next bone, " raeso-cuneiforme " (cm), of larger size, supports the second and part of the third metatarsal, articulates with the ento-cuneiforme (ci), and more intimately and largely with the ecto-cuneiforme [ce]- This (ce) is the largest of the three ; it supports the fibular half of the base of the middle metatarsal, the whole of the base of the fourth metatarsal, and the tibial side of the base of the fifth meta- tarsal; it articulates also with the meso-cuneiforme {cm), astragalus (a), and calca- neum {d) ; it is plainly the homotype of the os magnum in the wrist. The homotype of the unciforme, if it existed, must have articulated with the posterior or fibular margin of the calcaneum, but it is not present in either limb.
If we regard the largest of the distal tarsal series, supporting the fourth and part of the third and fifth metatarsals, as the " cuboides," we must then consider its obvious homotype in the wrist (m) to be the unciforme. Tlie bone here called " meso- cuneiforme" {cm), which articulates with both second and third metatarsals, will then be the " ecto-cuneiforme," and the bone («') will be the two other cuneiform bones con- nate ; in like manner its homotype, called " trapezoides" in the wrist, which has a similar relation to the second and third metacarpals, will be the trapezoid and trapezium connate, but in that case the outermost ossicle (fig. 2, u) of the distal part of the carpus would be a supernumerary without a name. I therefore prefer and adopt my first homologies.
The outer surface of the tarsals, except the middle of the calcaneum, is rugose. The first metatarsal (ib., i) is 1 inch 9 lines in length ; it supports two phalanges ; the total length of the toe, including the metatarsal, is 4 inches 9 lines. The length of the second metatarsal is 2 inches 3 lines ; three of its phalanges are preserved ; it is more distal in position than the first by about 4 lines. The middle metatarsal is 2 inches 5 lines in length ; four of its phalanges are preserved. The fourth meta- tarsal, 4 inches 6 lines in length, has the fibular part of its base more extended, and that margin of the shaft is more concave ; four phalanges are preserved, and the length of the digit, including the metatarsal, is 10 inches. The fifth metatarsal is 2 inches 5 hues in length ; the tibial angle of its base is truncate, the fibular one is much produced and tuberous ; the fibular margin is deeply concave ; only one phalanx is preserved.
THE
FOSSIL REPTILIA OF THE LIASSIC FORMATIONS.
CHAPTER II. Order— ICHTHYOPTERYGIA, Owen. Genus — Ichthyosaurus, Kionig}
A. Introduction.
Remains of the extinct mai'ine Reptiles, now known as Iclithyosaurs, have attracted the attention of collectors and describers of organic fossils for nearly two centuries past.
In Scheuchzer's ' Querelge Piscium,' 1708, tab. iii contains figures of the biconcave vertebree of an Ichthi/osaur from the Lias of Altdorf, supposed to be a fish. Knorr, also, in his ' Naturgeschichte der Versteinerungen,' vol. ii, represents, in figs. 5 — 7 of tab. i, vertebrae of the same Reptile, as " Ichthgosjjondi/len."
So, likewise, when the attention of more modern palaeontologists was awakened to remains of the remarkable subjects of the present Monograph, as in the paper by Sir Everard Home, Bart., F.R.S. (' Philos. Trans.,' 1814), we find such described as "Fossil Remains of an animal more nearly allied to Fishes than any of the other classes of animals."
In this paper, however, as in succeeding ones by the same author, which appeared in the 'Philosophical Transactions' for the years 1816, 1818, 1819, and 1820, the accurate and beautiful engravings of the drawings of the several subjects by William Clift, F.R.S. , enabled contemporaiy investigators, more capable than Home in determining the true nature and affinities of the fossils, to contribute a durable and rich accession to their science.
In this work the names of Contbeare and De la Beche^ stand pre-eminent, and with them must be associated that of Charles Konig, whose appreciation of the affinities of
1 'Icones Fossilium Sectiles,' fol., pi. six, fig. 250, 1825.
* " Notice of a Discovery of a new Fossil Animal forming a link between the Ichthyosaurus and Croco- dile ; together with general remarks on the Osteology of the Ichthyosaurus," ' Transactions of the Geological Society,' 4to, vol. v, 1821, p. 559, pis. Ix, Ixi, Ixii.
6
42 BRITISH FOSSIL REPTILES.
the animal, the fossil remains of which he figured in the work above cited, is exemplified by the generic name which the extinct Reptile has subsequently borne.^
Baron Cuvier amply confirmed the conclusions to which the above-cited authors, and, subsequently, Conybeare^ arrived, and introduced copies of figures illustrative of their papers in the concluding volume of his great work on ' Fossil Remains. '^
In the same year Prof George Fred. Jaeger recognised fossils as Ichthyosaurian in the Lias of Boll, to one of the plates in whose Work reference will be subsequently made.*
Subsequent additions to the history of the genus Ichthyosaurus will be found in my ' Report on British Fossil Reptiles,' in the volume of the British Association for the year 1839, 8vo., p. 86.
Before entering upon the details of structure and specific characters I may remark that whenever the antecedent representatives of a class or order may be known, to which an extinct genus is referable, the characters of the genus should be compared with those of its predecessors in such class, rather than with its successors or with existing forms, to gain an insight into its true affinities.^
The Labyrinthodont order, prevalent from the Carboniferous to the Triassic forma- tion, manifests the tendency to dermal or peripheral ossifications which was carried out to greater extent in older and lower vertebrate forms. The Ichthyopterygian order, pre- valent from the Liassic to the Cretaceous period, continues to show the supplementary ' prosquamosals ' (Tabs. XIX and XX, fig, 1, 27') and ' postorbitals ' (ib. ib., 12); and the vertebral centrums retain the biconcave character (Tab. XVIII, fig. 6). The 'foramen parietale ' (Tab. XIX, fig. 1,/) is common in Carboniferous/ Permian, and Triassic Reptiles ; and, in some, is larger than in Ichthyosaurus'^ whence it has been continued on to modern Lacertilia, but has become obliterated in the Crocodilian order. Both diapo- physes and parapophyses* appear in Reptilian vertebriE at the same geological period, and are carried on in the Crocodilian modification of the class, but are lost in existing Lacertilia.
' " We have retained in these observations the name Ichthyosaurus, originally applied to this animal by Mr. Konig, of the British Museum, feeling convinced that on a full and careful review of its whole structure it will not be found to possess analogies sufficiently numerous or strong with the peculiar organi- sation of Proteus to authorise the change of this appellation into Proteosaurus, as subsequently proposed." — Tom. cit., p. 563, Conybeare and De la Beche.
" " Additional Notices on the Fossil Genera Ichthyosaurus and Plesiosaurus," ' Trans, of the Geolo- gical Society,' 2nd series, vol. i (1824), p. 103.
^ ' Recherches sur les Ossemens Fossiles,' 4to, tome Seme, 2de partie, 1824, p. 447, pi. ii.
* ' De Ichthyosauri sive Proteosauri fossilibus specimiuibus in Agro Bollensi repertis,' 4to, 1824.
5 Owen, ' Palaeontology,' 8vo, 1860, p. 206.
^ " Ueber Arcliegosaurus Declienii, Goldf.," von Dr. G. Jiiger, 4to, ' Miinchen Abhandl.,' Bd. v, 1847, p. 415, tab. xxvi, fig. 1.
' 'Descriptive and Illustrated Catalogue of the Fossil Reptilia of South Africa in the British Museum,' 4to, 1876 ; Galesaurus pi. .xviii, fig. 8, '" ; Petrophryne, pi. xx, fig. 18, ^' ; Gorgonops, pi, xxi, fig. 3, 7'; Dicynodon, Ptychognattius, Oudenodon, Kisticephalus, pis. Ixiv, Ixv; Procolophon, pi. xxiij figs. 4, 8.
^ Op. cit., Pareiasaurus, pi. x, figs. \, 3, d, p ; Dicynodon, pi. lii, fig. 1 ; pi. liii, fig. 3, d.
LIASSIC ICHTHYOSAURS. 43
They are conspicuous Ichthyopterygian characters, and are associated, as far along the spine as they are distinctly developed, with the double-jointed ribs, showing ' capitulum ' and ' tuberculum ' (Tab. XVII, fig. 2, a, b). The prezygapophyses of the atlas converge, descend, and aid in forming the anterior cup, which receives a corresponding convex joint-surface of the occipital vertebra ; the change from the double condyle of the oldest air-breathing Vertebrates to the single condyle in Triassic Reptilia is retained in the IchthyoiJterygia. The teeth in this order show a trace of the older Labyrin- thodont character in the converging folds of cement penetrating their base,^ but the alveolar partitions of their native groove ai'e not complete in any part of the tooth-bearing tract. Anchylosis of the tooth-root to the jaw, seen in Mosasauroids and modern Lizards, is not effected in any Ichthyosaur. The teeth retain this freedom, as in Crocodiles, with a similar repeated succession and Shedding ; as in Crocodiles, also, they are confined to the maxillary, premaxillary, and premandibular (" dentary ") bones, but with the ordinal character of much greater length of the premaxillaries than of the maxillaries. The orbits, in Ichtliyo^iterygia, are conspicuous for their size; the circle of sclerotic plates usually found fossilised in them exemplifies a primitive vertebral character under a modification continued on in Chelonia, Lacertilia, and Aves. The nostrils are distinct, and antorbital in position. The limbs are natatory, with many- jointed digits, and these exceed, in some Ichthyosaurian species (Tab. XXVI, fig. 3), five in number. The scapular arch (Tab. XXIV, fig. 4, Ich. latimanus, Ich. communis, e. g.), includes an episternura (46) and clavicles (53), with a well-developed coracoid (52) and scapula (51), the latter near to, but detached from, the occiput. The hinder part of the vertebral column is as free for natatory work as in Whales ; there is no sacrum, but a pair of pelvic fins is constant, and these, usually smaller than the pectoral ones, ar"^ supported by iliac, ischial, and pubic bones. The terminal caudals are modified for thi support of a tegumentary fin, but are compressed, not depressed, the fin being vertical, not horizontal.
The adaptive modifications of the Ichthyopterygian skeleton, like those of the Cetacean, relate to their medium of existence; they are superinduced, in the one case upon a Reptilian, in the other upon a ]\Iammalian type, and both show analogies to the Vertebrates which the " waters first brought forth." But that the Ichthyopterygians did not breathe by means of gills is shown by the absence of the branchial framework, and by the presence, position, and structure of passages leading from the nostrils to the palate for the course of currents of air on their way to lungs, which were protected and worked by movable thoracic-abdominal costal girdles. Herein these old Sea-reptiles rise higher in structure than some modern cold-blooded air-breathers, such, e.y. as Batrachians and Chelonians.
An Ichthyosaur by the shortness, one may say absence, of neck, and equality of
1 A transverse section of the base of the tooth of an Ichthyosaur gave the first clue to the structure of that of the Labyrinthodon. 'Odontography,' 1840, p. 201, pi. Ixiv b, fig. 3.
44 BRITISH FOSSIL REPTILES.
width of the back of the head with the front of the chest, shares witli the Wiiale a resemblance to Fishes, but pushes the Ulceness closer in the greater number and less length of the vertebrae, and in the indication of the main joints of the backbone being elastic bags filled with fluid, occupying the intervertebral spaces of the biconcave centrums, as in Fishes, Labyrinthodonts, and modern perennibranchiate Batrachians.
Being cold-blooded, and with a small brain needing a much less supply of oxygen for its work, the Ichthyopterygians, like Fishes, had this advantage over Whales, that their stern-propeller could have the form best adapted for a swift straightforward course through the water.^ The horizontality of the tail-fin of the Whale tribe relates to their need, as large-brained, warm-blooded air-breathers, to have easy and speedy access to atmospheric air. Without the means of displacing a mass of water in the vertical direction by such broad tail-fin the head of the Whale could not be brought with the needed rapidity to the surface for the purpose of breathing. Nevertheless the Cetaceans are restricted to their element as closely as Fishes, and perish almost, if not quite, as soon when cast ashore, whilst the Ichthyosaurs were less limited in regard to medium, and had a power upon dry land which neither of the other aquatic vertebrates enjoy.
That our Sea-lizards occasionally sought the shore is to be inferred from the strong inverted osseous arch supporting their fore fins, spanning across the chest from one shoulder-joint to the other. In structure this arch closely resembles that in a group of aquatic Mammals {Ornithorhpichus), which similarly surpass Cetacea in having a command of both land and water, although, by their low position in the mammalian class, they have closer alliance to the Reptilia.
There is reason to infer, from examples of diminutive Ichthyosaurs fossilized within the abdominal cage of larger ones, and with the snout directed toward, or partly protruding from, the pelvic outlet, that they were ovo-viviparous and, as a rule, uniparous, reptiles.' Others may have sought the shore for sleep or copulation, and have been enabled, by reaction of their large and strong fore paddles against the scapular arch, to have crawled or dragged themselves along with the belly resting on the ground.
In outward form an Ichthyosaur (Tab. XXIV, fig. 1) resembled a huge predatory abdominal fish, with a longer tail and smaller or shallower tail-fin ; scaleless, moreover, being clothed by a smooth, probably finely wrinkled, skin,' something like that of the whale-tribe.
The mouth was wide and the jaws long, armed, as a rule, with numerous pointed and, in some species, trenchant teeth. Masses of comminuted bones and detached ganoid scales
* " Note on the Dislocation of the Tail at a certain point ohservable in the Skeleton of many Ichthyosauri," 'Trans, of the Geological Society,' 2nd series, vol. v, 1838, p. 511, pi. xlii.
2 As first shown in the specimen of Ichthyosauri from the Lias of Boll, described and figured by George Frieurich Jager, op. cit., tab. i, fig. 4 ; subsequently noted by Quenstedt in specimens in the Tubingen Museum ; also by Channing Pearce (' Report of the British Association,' 1874).
' " Description of some of the Soft Parts with the Integument of the Hind Fin of the Ichthyosaurus, &c.," 'Trans, of the Geological Society,' 2nd series, vol. vi, 1840, p. 199, pi. xx.
LIASSIC ICHTHYOSAURS. 45
of coeval fishes have been found within the costal cage of the fossil specimens in the situation where the stomach may be judged to have have been. Small, hard, and un- digested bodies, containing fish bones and scales, and bearing impressions of the folded surface of the intestinal membrane, have received the name of " coprolites."^
Such were the air-breathers which governed the seas of our planet from the Liassic to the Cretaceous period inclusive. At the later epoch the Ichthyopterygians became extinct, and appear to have been superseded by the Mosasaurians. In these the vertebrae have become proccelian ; their modified dentition both as to position and attachment is continued on in existing Lizards, but the liiiihs were fins. The transition, if there was such, is, however, abrupt, and the links are, as yet, unknown which connected the Tertiary cetaceous Zeuglodonts with antecedent whale-like reptiles.
The Cretaceous Ichthyosaurus canqryhdon - retains the characters of its order as definitely as they are shown in the species of the ]\Iuschelkalk or Lias ; and the com- mencement of this type of Reptile seems to be as abrupt as its close. Much remains to be, and may be, discovered indicating the antecedent forms which linked on more closely the Ichthyopteryyia with earlier air-breathing vertebrates. But with later ones there is no evidence of transitional alliance ; they seem to have passed away under the type of structure which I next proceed to explain as far as study of the fossil remains has made it known to me.
B. Osteology.
a. Bones of the TrunJc. — In the vertebrse (Tabs. XVII and XVIII), according to the regions of the column, are to be noted: the centrum (c) neurapophyses {n), neural spine (n«), pleurapophyses {pi), haemapophyses {h), haemal spine {hs), zygapophyses {z, z), dia- pophyses (rf), parapophyses {p), and hypapophyses [hy). Some of these are autogenous, others exogenous parts.
The centrum is more or less antero-posteriorly compressed (Tab. XVIII, figs. 1 to 7, 9, 13), with concave terminal articular surfaces (ib., fig. 6) not intercommunicating; on each side of the shallow myelonal canal (ib., figs. 2 and 4, »?) is the deeper, usually triangular, articular surface {np) for the neurapophyses (w). These, in each vertebra, converge, and, save in the atlas (Tab. XVII, fig. 1, «), coalesce at their summits with each other and with the neural spine (ib. figs. 2, 3, 4, n, ns). In most Fishes the neural arch coalesces with the centrum, as in Cetaceans ; its separate state is a saurian, chiefly crocodilian, modification ; it is such in the Ichthyosaurs, and adds to the power of inflecting the spine vertically, as in the specimen (Tab. XXV, fig, 2).*
^ BucKLAND (Dr. Wm.), "Discovery of the Faeces of the Ichthyosaurus," 'Trans, of the Geological Society,' 2nd series, vol. iii, 1835. « Vol. I, p. . * More extreme and abrupt vertical flexures, shown in two specimens in the British Museum, may be
46 BRITISH FOSSIL REPTILES.
Most of the neurapophyses interlock by means of coadapted zygapophyses (Tab. XVII, fig. 6, r,y). The hfcmapophyses are developed beneath the abdominal ribs (ib., fig. 2, h,h'^ and beneath the bodies of most of the caudal vertebrse (ib., figs. 4, 5, A) ; they are always distinct from their centrum (e), and do not coalesce below with each other, or with a haemal spine. The hypapophyses remain detached in the first two or three vertebra? (ib. fig. 1, hy), and have advanced to the interspace between their own. and the antecedent centrum. That of the atlas (Tab. XIX, fig. 5, hya) is wedged between it and the basioccipital (o) ; that of the axis (ib., hyx) between it and the atlas, and so on (ib., hy, 3). CoNYBEARE, who first noticcd this structure, describes it as follows : — " We have only seen the inferior piece or body (if it can be so called) of the atlas ; and the odontoid process (which in all reptiles forms a distinct piece) of the axis ; they very nearly resemble those of the turtle."^
In the trunk the centrum of the atlas (Tab. XIX, figs. 2 — 5, c a) is the most modi- fied of that series of vertebral elements. Its fore surface (ib., fig. 2) presents at its upper two thirds a concavity (c a), occupying the medial two fourths of its transverse extent, the cavity gradually changing to convexity {b) in the lateral fourth. Beneath this smooth concavo-convex articular surface is a rough, flat, triangular surface [t], inclining from its upper base backward. The upper joint-surface [c a), is for the basioccipital (ib., figs. 1, 2, fig. 5, 0, in dotted outhne), the lower one {t), is for the hypapophysis (Eig. 5, hya). The hind surface of the atlantal centrum (ib., fig. 4) is fiat, and with a few feeble ridges at its peripheral and commonly lower third part. It is rare to find this element unanchylosed with the succeeding centrum (ib., fig. 5, e x)- The neural surface of the atlantal centrum (ib., fig. 3) is divided equally between the medial quadrate tract [m) for the myelon,^ and the lateral subtriangular depressions (« j,) for the neurapophyses. The length or fore-and-aft diameter of the atlantal centrum is usually relatively less than in the trunk- or tail-vertebrae of the same individual. The neurapophysial surface bends down upon the side of the centrum, forming a prominence (fig. 5, d) on its upper part, representing the diapophysis ; beneath this, with a non-articular interval, projects a low obtuse parapophysis (ib., p).
The neurapophyses of the atlas (fig. 6, «), as far as I have been able to infer from this
posthumous, due to disturbance of the decomposing carcase prior to final burial in the Liassic mud, in and with which the skeleton subsequently became petrified.
1 'Trans. Geol. Soc.,' vol. v, 1821, p. 574. The homology of the "odontoid process" as the " centrum of the atlas," and that of the anthropotomical " body of the atlas " with the hypapophysial part of that vertebra, is shown in my "Description of the Atlas, Axi-s and Subvertebral Wedge-bones in the Plesiosaurus," 'Annals and Magazine of Natural History,' vol. xx, 1847, p. 217, figs. 1 — 6.
In 183.5 Sir P. de M. Grey Egerton communicated to the Geological Society his discovery of not only Conybeare's " inferior piece of the atlas," but the homotypal parts of the two succeeding vertebrse ('Proceedings of the Society,' vol. ii, No. 41, p. 192), and subsequently gave a detailed description, with figures, of these parts under the name of " subvertebral wedge-bones ; " ' Trans. Geol. Soc.,' 2nd series, vol. V, 1836, p. 187,pl.xiv.
- ' Spinal marrow,' ' spinal cord,' of ' Anthropotomy.'
LTASSIC ICHTHYOSAURS. 47
commonly mutilated or much disturbed part of the fossil skeletons, were not united together atop, or there developed into an exogenous spine, but retained their distinct- ness, like their antecedent homotypes the exoccipitals (Tab. XXII, fig. 1, 2). In the best preserved specimen each atlantal neurapophysis is bent back at the middle of its length, the upper compressed portion overlapping the fore part of the base of the neural spine of the axis, as shown in Tab. XIX, fig. 6.
Of the existence of the atlantal pleurapophysis (Tab. XIX, fig. 5, pi, a), each being joined by a bifurcate proximal end to the di- and par-apophyses of the centrum, there is better evidence. Such rib was short, directed outward and backward ; and is uncon- nected, distally, with any hsemapophysis (Tab. XVII, fig. 1, pi).
The hypapophysis of the atlas (Tal:). XIX, fig. 5, hy, a, and Tab. XXIII, fig. 2) is an irregular triangular robust ossicle, smooth and convex on its inferior and free surface, with the opposite articular surface divided into three facets. The anterior of these (ib., fig.
2, a) is smooth and concave, completing, with the concave part of the atlantal centrum, the cup for the basioccipital ball; an almost flat rough tract (ib., J) next below articulates synchondrosaly with the corresponding rough surface of its centrum. Beneath this is a smaller flat roughish surface (ib., c), sloping backward from the one above, for articulation with the succeeding hypapophysis. Such are the complex characters of the first trunk- vertebra of Ichthyosaurus.
The centrum of the second vertebra (Tab. XIX, fig. 5, ex, and Tab. XXII, figs.
3, 4, 5), has a flattened, roughish, anterior surface (fig. 4), like the posterior one of the first vertebra, with which it sooner or later coalesces. The hind surface of the axis- centrum (fig. 5) is more deeply and entirely concave, with a sharpish circumferential margin. On the upper surface of the centrum (fig. 3) the myelonal surface (»«) is similar in size and shape to that of the atlas, but is shallower. The neurapophysial surfaces (k, w) are less excavated, and the diapophysial productions (fig. 4, </, d) upon the sides of the centrum are more prominent, better defined as processes. The same may be said of the parapophyses (ib., p, p), which project close to the fore border of the side surface and show a more distinct facet for the head of the axial rib than do those of the atlas. The length of this rib is a little more than the vertical diameter of the centrum. The lateral surfaces of the centrum of the axis-vertebra are antero-posteriorly concave, of greater extent behind the rib-processes ; vertically they describe a convex curve converging from each side to an inferior medial ridge. This ridge is interrupted, anteriorly, as if that end had been obliquely cut ofi", forming a roughish subconcave facet for the hind half of the base of the second or axial hypapophysis (Tab. XIX, fig. 5, hy, x) This element is barely half the size of the one in front, is conical, the apex downward ; the base is divided into the surfaces respectively joined to the contiguous hypapophysial facets of the atlas and axis. The neurapophyses converge as they rise and coalesce to form the base of a neural spine (ib., fig. 6, n s), the antero-posterior extent of which equals the height — a proportion which distinguishes that part of the second vertebra. Postzyga-
48 BRITISH FOSSIL REPTILES.
pophyses (ib., ;) are developed from the base of the spine which overlap and articulate with the prezygapophyses (,) of the third cervical vertebra.
Thus, at the fore part of the vertebral column, the neural arch presents the three following modifications : — In the atlas the neurapophyses remain distinct and develop neither post-zygapophyses nor neural spine ; in the axis they coalesce, develop the post- zygapophyses and a lofty spine, broader than those in the succeeding vertebrae ; the neural arch of the third cervical develops both pre- and post-zygapophyses (ib., fig. 6, z, z) and a neural spine (««), sul)compressed like that of the axis, but narrower antero- posteriorly : in both vertebrae the neural spine inclines rather backward.
The above descriptions and figures are from an immature specimen of Iclithi/osaurus lonr/ifrons.
In the vertebrfe along about a third or more of the trunk, the neurapophysial surface is continued on to the diapophysial process (Tab. XVIII, figs. 1 — 3, np, d)- This process next becomes distinct (ib., fig. 4, d) \ and, as the parapophysis continues to be developed, the presence of the pair of tubercles, d, p, near the fore margin of the side of the centrum cha- racterises that part as far as the fortieth or forty-fifth vertebra in Ichthyosaurus communis. In this course both processes gradually descend (ib., figs. 5, 7, rf,^.), but the diapophysis more rapidly, until it coalesces with the parapophysis, forming therewith an oblique ridge or rising. In the caudal vertebrae the ridge gradually contracts to a rounded tubercle (ib., figs. 9, 11, ^, p.), and finally disappears at about the eightieth vertebra (ib., fig. 13). At this part of the column, in Ich. communis, the abrupt bend or dislocation of the caudal series commonly occurs ; and here three or four of the centrums become more compressed than either those that precede or those that follow them, and their lateral margins are raised, as if by forcible compression.
The neurapophysial facets become detached from the diapophyses ((/) by contracting in breadth, and take the form of narrow longitudinal grooves (ib., fig. 5, np), bounding laterally the myelonal surface (?«). This surface sinks a little deeper into the centrum as the vertebrae recede in position, and in the caudal region it contracts both vertically and laterally, until it loses definition in the extreme vertebrae.
The articular bases of the neurapophyses undergo corresponding modifications ; the joint-surface is sul)triangidar, somewhat protuberant in the anterior vertebrae ; but, after the diapophysis or rib-surface gets independent, that for the neurapophysis becomes longitudinal, narrow, and grooved. The neurapophysis rises, with a slight receding from the vertical position, for a height usually equalling its fore-and-aft breadth ; it develops a short prezygapophysis and inclines backward, with a postzygapophysial surface at its under and hinder part. The pair of neurapophyses having then coalesced send upward and slightly backward a subquadrate compressed neural spine, usually twice the height of the the subzygapophysial part or pedicle (Tab. XVII, fig. 0, ») of the neurapophysis, and with gradually augmenting height and antero-posterior breadth as far as the midpart of the trunk. Towards the hind part the spines begin to lose height, but not breadth, until
LIASSIC ICHTHYOSAURS. 49
they enter the caudal region, when they gradually decrease in all dimensions, and disappear at or near the bend of the tail. Feeble emarginations at the fore and hind part of the pedicle form or bound the nerve-outlets.
The contour of the centrum (Tab. XVIII) varies with the number and position of its lateral processes. At the fore part of the column it is more or less shield-shaped (fig. 3), with the angles of the upper border rounded off; at the hind part, where the rib-processes have descended (fig. 8) or have coalesced (fig. 11, dp), the base is below and the apex trun- cate for the neural arch ; further on, where those processes have disappeared, the contour becomes ellipsoid or elliptic, with the long axis vertical.
The centrum is always short in proportion to its breadth and depth, but this varies in different species ; beyond the atlas and axis it is always biconcave (ib., fig. 6), but the contour of the concavity varies specifically. In some the sinking begins at the periphery ; in others in a feebler degree there ; in others a slight and narrow marginal convexity (fig. 3) precedes or leads to the central concavity ; in others, again (fig. 8), a peripheral portion of the joint-surface is flat before it sinks into the central hollow ; in exceptional instances, the fore and hind concavities blend at a small central perforation, as they do in the Triassic ' Tretospondilia ' of the Cape.^
The pleurapophyses (dorsal ribs, Tab. XVII, fig. 3, i^l) are developed, as free movable elements, over a larger proportion of the vertebral column than in most other four-limbed Meptilia, extinct or existing (Tab. XXIV, fig. 1). They commence at the foremost seg- ment as shown in the description of the ' atlas ' (Tab. XIX, fig. 5, pi), gain slightly in length on the axis, in a greater degree on the third vertebra, and acquire their extreme length between the tenth and thirtieth (Tab. XVII, fig. 2, vh and Tab. XXIV, fig. 1) ; beyond this they shorten, but continue as free elements, though short and straight (Tab. XVII, fig. 4), along a great part of the caudal region ; their existence, as such, being attested in detached centrums by the single sessile process (Tab. XVIII, figs. 9 — 11, dp) on each side : with the disappearance of the di-parapophysis the ribs cease. All the ribs are comparatively slender, commonly subcylindrical ; the longer ones, or those of the trunk, are longitudinally grooved on their outer surface (Tab. XVII, fig. 7), as if each rib con- sisted of two confluent more slender ones.^ This structure is not common to all the species. A transverse section which I made exposed a small central cavity.
The haemal or costal arch is complete along the major part of the trunk. Here the hsemapophyses are each in two partially overlapping pieces (ib., fig. 2, h, K) ; they are more slender than the pleurapophyses.* The median piece (haemal spine, u) is transversely extended, symmetrical, slightly produced at its thickest midpart, forward and backward, but more extended laterally, there becoming slender and diminishing to a point. A similar slender piece, pointed at both ends (A'), is spliced as it were, to the fore part of each
1 ' Quarterly Journal of the Geological Society,' vol. xxxii, p. 43 (1875).
2 Clift, 'Phil. Trans.,' mdcccxiv, pi. xix (1814, Ichthyosaurus platyodon). 8 lb., 'Phil. Trans.,' mdcccxix, pi. xiv (1819, Ichthyosaurus communis).
7
50 BRITISH FOSSIL REPTILES.
lateral production of the medial piece. A second styliform haemapophysis (h) is similarly adapted to the fore part of the foregoing style ; but as it approaches its pleurapophysis it is slightly thickened, and is joined by a truncate end to that of the pleurapophysis (pi). Of the five bones which thus constitute the haemapophysial part of the thoracic-abdominal hsemal arch, the two on each side I regard as a divided haemapophysis, and the trans- versely extended medial piece as the haemal spine, or abdominal " sterneber.'' A small but strong triradiate haemal spine (" episternum ") closes the dislocated occipital haemal arch formed by the modified pleurapophyses (" scapulae '') and hajiuapophyses ("clavicles").
In the caudal region the centrum, save at the terminal pinnigerous part, shows a small tubercle at or near each of the four angles of the quadrilateral space (Tab. XVIII, figs. 9, 12), forming the lower surface, the anterior tubercles (hy, hy) being the largest. Each has an articular surface, and the contiguous ones of coadapted centrums give attachment to the base of a short and slightly bent haemapophysis. These elements, of the same pair, converge as they descend, but do not coalesce to form a " chevron-bone," nor is a haemal spine developed. These inferior or haemal arches cease on the terminal twenty or more vertebrae ; in most of these the centrum is subcompressed, especially where the seeming fracture or abrupt bend (Tab. XXII, fig. 8) takes place.
b. Bones of the Head. — The resiilts of my studies in the craniology of the species of Ichthyosaurus, subsequent to the ' Report on British Eossil Reptiles ' of 1839, were given in the ' Hunterian Lectures,' at the Royal College of Surgeons, London, 1855; and in those delivered at the jMuseum of Practical Geology, Jermyn Street, in 1858.^ I have found but little to add or alter in the course of subsequent researches in the preparation of the present Chapter.
The general form of the skull of the typical species. Ichthyosaurus communis (Tab. XXIV, fig. 1), as in Ich. breviceps, Ich. intermedius, Ich. lonchiodon, and Ich. platyodon^ resembles that of the Cetaceous Dolphins {Delphinus tursio and Delphinus delphis). In Ich. acutirostris (ib., fig. 2) the beak is produced to the shape of that of a gigantic stork, while Ichthyosaurus tenuirostris and Ich. longirostris (ib., fig. 3) rival or surpass the Delphinus [IniaJ yanyeticus in the length and slenderuess of the jaws.
The main difference in the Sea-reptiles lies in the restricted capacity of the brain-case, the seeming expanse of the cranium being due to the great depth and breadth of the post- orbital part of the zygomatic arches or outer walls of the temporal fossae. A more essential departure from the warm-blooded Vertebrates is the persistent individuality of those cranial elements which, though primitively distinct, become blended into single bones in
1 Notes of these Lectures were published iu the ' Annals and Magazine of Natural History,' 3rd series, vol. i, 185S, p. 388 et seq.
2 See Clift's excellent figure in ' Philos. Trans.,' mdcccxiv, pi. xvii.
LIASSIC ICHTIIYOSAURS. 51
the higher and later developed forms. The Ichthyosaurs further differ from the marine mammals in the great extent of the premaxillary and the small size of the maxillary bones, in the great capacity of the orbits and the circle of sclerotic plates lodged therein, and, finally, in the antorbital vacuities serving as external bony nostrils.
The occipital region of the skull (Tab. XXII, fig. 1) is of great breadth and of mode- rate height. In its formation there enter not only the basi-, ex-, super-, and par-occipitals, but also part of the parietals, mastoids, tympanies, zygomatics, prosquamosals, and pterygoids.
The chief feature is the large proportional size of the basi-occipital (ib., i), the outer surface of which is divided into an articular (Tab. XXI, fig. 1, 1') and non-articular part (ib., 1). The articular portion is in the form of a hemispheric, convex condyle, in some species showing a subcentral depression, but deriving no contribution from the exoccipitals, and divided from the ' foramen magnum ' by a narrow, upper non-articular tract ; the lateral tract gains breadth as it descends along the sides of the condyle, below which it shows an extent of two thirds the diameter of the condyle ; but this part of the basioccipital, in extending forward, deviates little from the perpendicular, and belongs rather to the hinder than the under surface of the cranium. The upper non-articular part of the basi- occipital, dividing, in the specimen under description, the condyle from the foramen magnum, is one eighth the diameter of the condyle ; then come the depressions for the sutural joints with the exoccipitals, a mere crest dividing them. The exoccipitals (Tab. XXII, fig. 1, 2, 2) are small andreniform ; their bases almost meet above the basi- occipital; their obtuse summits are divided by the base of the super-occipital (3), which contributes about a fourth part of the circumference of the foramen magnum (/).
The fore part of the basioccipital presents, in some species, a slight notch or groove, as if for the outlet of an Eustachian canal. This canal, in Crocodiles, traverses the basi- sphenoid, close to its suture with the basioccipital.
The basioccipital articulates, below or in front, with the basisphenoid (Tab. XXI, fig. 1,5) laterally with the paroccipitals (Tab. XXII, fig. 1,4); but between these and the basisphe- noid it joins the mesially inclined hinder end of the pterygoids (ib., 24). The apex of the superoccipital (3) is wedged into the interspace of the hinder bifurcation of the parietal bones ("'), which it underlaps and partly supports ; its base forms the upper border of the foramen magnum. The paroccipitals (ib., 4, 4), broadest where they join the basioccipital, contract as they extend outward into a strong triedral bar, which abuts against the tympanic (28), at the interval between the mastoid (8) and pterygoid (24).
The centrum (l), neurapophyses (2), neural spine (3), and parapophyses (4) of the hindmost cranial vertebra are instructively demonstrated by the Ichthyosaurian condition of the ' occipital bone ' of Anthropotomy.
The basisphenoid (Tab. XXI, fig. 1, 5) presents, on its under and outer surface, the form of an irregular, subquadrate plate, narrow^est behind, where it joins the basioccipital, expanding as it advances, the anterior border presenting a rough, sutural, notched
52 BRITISH FOSSIL REPTILES.
surface, at its middle third, for the presphenoid (9), and a smooth emargiuation on each side forming the hind border of the sphenopterygoid or ' interpterygoid ' vacuities («, «).^ The hinder half of the under surface of the basispheuoid presents shallow rough depres- sions and feeble risings for muscular attachments, and, like the basioccipital, it is imperforate. Of the alispheuoids I have been unable to determine more than their presence and their small size. The side vk^alls of the brain-case proper seem to have been mainly cartilaginous.
The parietals (7) in most Ichthyosaurian skulls retain their median (sagittal) suture (Tab. XIX, fig. 1, 7), which usually opens out anteriorly to form the hind end of the ' foramen parietale ' or fronto-parietal fontanelle' (/), the chief part, or whole, of which is bounded by the frontals (11).
The upper surface of the parietals seems, by reason of the aspect of the occipital por- tion, to be divided by a ridge (r) extending from the mastoids (8"), and continued upon the parietals to their mid-suture, into an anterior (7) and posterior (/') surface. This masto- parietal ridge (s" r) properly bounds, above, the occipital surface, to which the parietals thus contribute about a fifth part of their length above the superoccipital bone (Tab. XXII, fig. 1, 7'). Anterior to this ridge each parietal slopes to the temporal fossa (Tab. XIX, fig. 1, t), the parietal surface being divided by a low longitudinal rising continued forward from a posterior convexity into two facets, both of which are concave across. The dividing ridge is overlapped by a postero-mesial angle of the postfrontal (12), between which and the frontal a narrow forward continuation (7") of the parietal is exposed, which overlaps the hind part of the frontal. The margins of the sagittal suture usually rise into a low ridge, which is continued upon the occipital part (;') of the parietal. Of this part (Tab. XXII, fig. 1, 7') the surface on each side of the mesial ridge is feebly concave, almost flat, laterally overlapped by the mastoids (s") ; it seems to rest upon, without sutural junction with, the superoccipital (3). The postero-lateral extension of the parietal (ib., fig. 1, 7x) curves down beneath the mastoid (s) to within a third part of the lower end of that bone, contributing therewith to the upper and lateral parts of the broad occipital surface.
The mastoid (8,8', 8") is a large and strong triradiate bone, the rays inclining forward from the outwardly obtuse centre or body forming the prominence at each postero-lateral angle of the skull. The upper and inner ray or branch (Tab. XIX, fig. 1, 8'') is three- sided, one facet looking upward, a second backward, the third forward and outward, contri- buting, with the parietal, to the outer and hinder wall of the temporal fossa (t). The angle at which the anterior joins the superior facet is continued forward upon the ridge (s'V), dividing the hinder and upper facets of the parietal bone. The lower mastoid branch
1 'Anatomy of Vertebrates,' vol. i, p. 157, fig. 9S, B, s.
2 "This foramen, or ' fontanelle,' is common in the Tnassic Reptilia. It is described and figured in Galesaurus, Petrophryne, Dicynodon, Ptyehognathus, Oudenodon, Kisticephalus, and Procolophon ; iu the latter it is large. It is wholly ' parietal ' in Kisticephalus and Ptyehognathus, in which it is placed far back." — ' Descriptive and Illustrated Catalogue of the Fossil Reptilia of South Africa in the British Museum,' 4to, 1876.
LIASSIC ICHTHYOSAURS. 53
(Tab. XXII, fig. 1, 8) is a broader plate, smooth, and almost flat externally, forming the upper sides of the occipital region, articulating outwardly with the prosquamosal (27') and tympanic (28), and below with the pointed process of the pterygoid (24), wedged between the mastoid and tympanic, and here overlapped by the paroccipital (4) in its way to abut agauist the tympanic. The outer branch of the mastoid, smooth and subconvex outwardly (Tab. XX, fig. 1, 8'), extends forward to form the hinder half of the upper zygoma, overlapped by the post-frontal (Tabs. XIX, XX, figs. 1, 12), and articulated along its lower border with the broad sclerodermal plate ('prosquamosal,' 27')' occupying the interval between the upper and lower zygomata.
The tympanic is abruptly divided into an upper auditory or proper tympanic portion (Tab. XXII, fig. 1, 28) and a lower articular portion (ib., 28')- The former is a narrow, subcompressed, outwardly subconvex, bony piece, and is wedged between the mastoid (8) and prosquamosal (27') ; its hind or mesial border extends from the mastoid junction to articulate with the pterygoid (24) ; the outer or lateral border is smoothly rounded and concave, forming more than the hinder half of the auditory meatus (»«). The suddenly expanded, thick, articular portion (28') joins the pterygoid (24) mesially and the zygomatic (27) laterally, then descends obliquely backward, for an extent equal to the auditory portion, to thicken and terminate in the surface for the articular element of the mandible. This surface is obliquely suboval, convex from before backward, slightly concave trans- versely at its fore part.
The hyoid, or haemal arch of the second cranial segment, is represented by a basihyal and by a pair of rib-like bones (Tab. XXI, fig. 3, 38), homologous with the thyrohyals in the Crocodiles, which elements they resemble in their small relative size, but are of more simple structure. Each is feebly and regularly bent, the convexity (in the petrified skull) turned towards each other, converging forward to their junction with the small flattened basihyal, which junction seems to have been by much ligamentous tissue. I noted in a fossil cranium of a full-sized Iclitliyosaurm communis that the hind ends were 4f inches apart while the fore ends were but \\ inch apart. The total length of fche bone was 4 inches 2 lines ; the breadth of the hind end was \ an inch, that of the mid-part 5 lines. In the Ich. loncModon (Tab. XXVII, fig. 5) each thyrohyal is a fifth part the length of the mandible. The stylohyals appear to have retained their fibro-cartilaginous or cartila- ginous tissue, and have consequently disappeared.
The presphenoid (Tab. XXI, fig. 1, 9) is a long, slender, trihedral bone, broadest where it joins, and commonly coalesces with, the basisphenoid (5) and, along the narrower part, with the two lower sides, converging to a median obtuse angle. It divides the long and narrow, pear-shaped interpterygoid vacuities («,«).
Of the orbitosphenoids I have no exact knowledge ; they may not have been
1 This osseous plate is described in my "Report on British Fossil Eeptiles," 'Reports of British Association,' 1839, 8vo, p. 9, as the " squamous element of the temporal bone;" it is analogous therewith bnt not homologous.
54 BRITISH FOSSIL REPTILES.
ossified, there is no trace or sign of the lacertian columellar bone near that part of tlie cranium.
The frontals, or raidfrontals (Tab. XXIII, fig. 1, ii), are small, transversely convex in greater or less degree according to the species, curving in toward the mid-suture in most, and bending outward and downward to the obtuse angle intervening between the nasal (15) and the postfrontals (12). At the hind end of the frontal suture each border curves outwardly to contribute their large share to the 'foramen parietale,' then converging and, in some species {Jch. latifrons, Tab. XXIII), wholly encompassing it ; here, also, as in some other species, the midfrontals distinctly join the postfrontals. In all the species the hind border joins the parietal (7), the fore border the nasal (15). The midfrontals are widely separated from the orbits by the postfrontals and nasals. I have not had clear evidence of their touching the prefrontals, or of the presence of distinct superorbitals. In Icldhyosaurus latifrons (ib., fig. 1) the frontal suture becomes obliterated, and the bone (11) is convex lengthwise as well as across.
The postfrontals (ib., and Tab. XIX, fig. 1, 12) exceed the midfrontals in size. Each extends from a mesial angle outwards, expanding horizontally, and inclining to form its share of the superorbital border ; thence the postfrontal is continued backward, bending down to join the postorbital (12') and prosquamosal (27'), and contracting to a notched and pointed end, which receives and overlaps the fore end of the zygomatic ray of the mastoid (Tab. XX, fig. 1, 8'). The similarity of character in the postfrontal (12) and mastoid (s) is worthy of note in regard to their general homology as cranial diapophyses. The haemal or pleurapophysial arch of the third or frontal segment of the cranium is modified to constitute the lower jaw.
In the mandible of Ichthyosaurus the articular element (Tab. XXII, fig. 2, 29)^ is scantly visible in an outside view, being covered by the largely developed and backwardly extended surangular^ (ib. 30). With this the angular coextends or slightly surpasses behind (ib., 31)-^
Of both elements the outwardly exposed surfaces (Tab. XX, fig. 2), as they advance, gradually diminish to a point ; the surangular (30), in Ichthyosaurus covimunis, disappearing between the dentary (33)* and splenial (32),^ in advance of the hinder half of the ramus; the angular (31) terminates between the surangular (30) and splenial (32), as far behind the fore half of the ramus. The hind part of the outer surface of the surangular shows a triangular, almost flat, but feebly concave surface, finely sculptured with linear impres- sions, converging forwards to the apex of the triangle, and indicative of a muscular inser- tion. In advance of this part the surface is smooth and feebly convex, gaining in depth by the convex curve of the lower border, and a similar one at the upper border, which
' CuviER, ' Oss. Foss.,' V, pt. ii, p. 272, pi. xvi, figs. 4 and 5 {Faranits), d (' articulaire ').
2 lb., ib., ' surangulaire,'y.
^ lb., ib., 'angulaire,' e.
* lb., ib., (Faranus), ' os dentaire,' a.
^ lb., ib., ' operculaire,' b.
LIASSIC ICHTHYOSAURS. 55
simulates a coronoid process. Opposite, or beneath the slender lower boundary of the orbit, the surangular is overlapped by the hinder-jointed end of the dentary (33). An oblique canal opens forward below the coronoid rising, beneath the hind border of the orbit, whence extends forward a channel, becoming shallow and ending beneath the over- lapping point of the dentary. The angular (3l) gradually diminishes in breadth or depth as it extends forwiird.
From such views as [ have been able to obtain of the inner surface of the mandibular ramus, the articular element (Tab. XXII, fig. 2, 29), after developing the concavo-convex broad surface for the lympanic, seems to be continued forward as a thinner, deeper plate, corresponding to the ' complenientaire ' of Cuvier.^ It, however, develops no coronoid process in Ichtiiyopterygians, but in some specimens seems to be a detached scale (ib., figs. 2 and 3, 30'), simply applied to the inner surface of the ramus over the line of junction of the surangular and angular elements touching or joining anteriorly the hind end of the splenial.
This element (Tab. XX, fig. 2, 32 ; Tab. XXII, figs. 3 — 6, 32) begins behind by a point between the surangular and angular, gains breadth as it advances to become applied to the inner side of the dentary, which it also underlaps and strengthens usually to M^ithin the anterior fourth part of the length of that element. The splenial in Icli. communis contributes a small part of the outer side of the jaw beneath the anterior pointed end of the surangular.
There are no other indications of lack of outer sui-face of the ramus than the neuro- vascular foramen and groove of the surangular and smaller irregular foramina in the dentary.
This important element (Tab. XX, fig. 2, 33 ; Tab. XXII, figs. 4—7, 33) is the longest, if not largest, constituent of the composite mandibular ramus ; it seems not to have carried in any species its symphysial articulation with its fellow to actual confluence. It affords for the lodgment of the mandibular teeth at the hinder half or more of that series only the outer wall and more or less of the floor of a broad and shallow alveolar channel, the inner wall being here supplied by the splenial element (Tab. XXII, figs. 4 and 5, 32). As the dentary advances it supplants the splenial by developing an inner Avail, which finally rises so as to exceed in height the outer one (ib., figs. 6 and 7, 33). The inner surface of the outer wall of the dentigerous groove shows feeble vertical ridges, indicative of alveolar compartments, like those seen along the hinder terminal part of the same channel in Crocodilia.^
On the outer surface of the dentary, a little below the alveolar border, a series of vas- cular foramina and grooves leading thence forward is seen in most species of Ichthyo- saurs ; in a few species the same surface is indented by a narrow longitudinal furrow.
^ Loc. cit., p. 272, c.
^ ' Descriptive Catalogue of the Osteological Series contained in the Museum of the Royal College of Surgeons of England,' 4 to, 1853, vol. i, p. 167, Specimen No. 765.
56 BRITISH FOSSIL REPTILES.
The longitudinal central vacuity of the mandible or interspace between its several con- stituents is considerable, as is shown in Clift's figure of a transverse section of a Tamus.^
After comparison of the foregoing structure with the homologous bone in Beptilia, I may remark that the mandible of Ichthyosaurus differs from that in Lacertilia in the minor relative size and backward extension of the articular (29) f it resembles more the Crocodilia in the major relative size of the angular (31), but exceeds in its proportion and its position as forming the angle of the jaw, though it is less produced backward. It differs from both the Crocodilian and Lacertian jaws in the larger relative size of the surangular, which extends backward so as almost to conceal the articular from outward view. It resembles the Crocodilia in the absence of any coronoid process from a coniplementary element; also in the larger relative size and length of the ' splenial ;' in this character the Monitors depart less than do the Iguanas from the Ichthyosaurs. In the dentary element Ichthyosaunis shows the important affinity to Lacertilia in the dental groove, devoid of alveolar partitions ; but in the rudimentary indication of these there is a resemblance, as before remarked, to the short non-alveolar tract at the back of the dental series in some Crocodilia. The mandible of Ichthyosaurus notably differs from that of Crocodiles, and resembles that of Lizards, in the absence of the vacuity between the angular, surangular, and coronal elements, and in the presence of the longitudinal series of neurovascular foramina or outlets along the outer side of the dentary element.
In the greater extent of the mandibular symphysis Ichthyosaurus differs from both Crocodiles and Lizards ; in these, it is limited to a small part of the dentary element, but in Ichthyosaurus it includes both dentaries and splenials, and the ' symphysis dentarii ' (Tab. XXI, fig. 2, 33') is to be noted together with the ' symphysis splenii ' (ib. ib., 32') in the discrimination of species. Thus, the latter is longer than the dentary symphysis in Ich. intermedius, but is shorter in Ich. conmmnis, and, relatively, much shorter in Ich. tenui- rostris (ib., fig. 3), Ich. longirostris (ib., fig. 2), and other long and slender-jawed kinds.
The vomerine bones are narrow, longitudinal, vertically disposed plates, with their lower margins rarely visible at the interspace of the palatines and pterygoids.
The prefrontal (Tab. XX, fig. 1, 14) I, as yet, know only by its external or facial part. This is a narrow, moderately long, bony tract, extending from the postfrontal to near the nostril (»*), there receiving the upper angle of the lacrymal (73) in a notch, the upper branch of which notch is wedged between the lacrymal and nasal, (1.5) ; the lower boundary is bent inwards to form part of that of the orbit. The extensive upper and inner border of the prefrontal articulates with the nasal, the short hinder border with the postfrontal.
The position and relations of the prefrontal in Ichthyosaurus, as in some Fishes,
1 ' rhiloB. Trans,' 1820, pi. xvi. See also PI. XXII, figs. 4 and 5, of the present Chapter.
2 CuviEB, torn, cit., pi. xvi, figs. 4, 8, 13, d.
LIASSIC ICHTHYOSAURS. 57
instructively illustrate its general homology as an element of a. cranial segment distinct from that to which the frontal belongs, of which bone it has been regarded as a mere dismemberment. As the neurapophyses of the nasal segment they lend, in Icldhyosaurus, a large share of their longitudinal extent to the support of their neural spine, the nasal bone. The large size of both pre- and postfrontals relates to that of the eye and of the cavity destined to contain it in the IcIdJii/osanrus.
The nasals (Tabs. XIX, XXIII, figs. 1, l5) are the longest and largest bones of the cranium proper, but contribute only a small part to the side face ; each sends a pointed process backward into a corresponding notch of the frontal which it partially overlaps. The apex of the process is in contiguity with the parietal ; a notch on each side the base of the process receives the anterior part of the frontal ; the angle of the outer notch touches the postfrontal. By its outer border the nasal unites with prefrontal, lacrymal, and pre- maxillary (22). The latter overlaps and conceals the naso-maxillary suture in the species in which such may be traced. The upper horizontal parts of the nasals gradually diminish to a point between the nasal portions of the premaxillaries. The exposed extension of the nasals terminates, in Ich. tenuirostris, about as far in advance of the nostrils as the fore- boundary of these is in advance of the oi'bits.
The palatines (Tab. XXI, fig. 1, 20) are long, slender bones, commencing behind at the anterior notch between the pterygoid (24) and ectopterygoid (25), forming, as each advances, the mesial boundary of the small palato-naris {pn). The palatine then extends forward, joining mesially the pterygoid, until this diminishes to a point, when the palatines come into contact or near contact with each other at the midline of the palate. Externally the palatine unites with the maxillary (21), but the suture is hidden by the underlnpping extension of the palatal process of the premaxillary (22), which reaches to the palato-naris.
The maxillary (Tab. XX, fig. 1, 21) begins behind, parallel with the anterior third of the orbit, from which it is divided by the slender fore part of the malar (26). As it advances the maxillary expands into a facial, an alveolar, and a palatal plate, articulating mesially with the ectopterygoid (Tab. XXI, fig. 1, 25), and, in advance of the palato-naris, with the premaxillary (22). The palato-alveolar part of the maxillary is divided from the facial part by the outer alveolar wall. The facial part (Tab. XX, fig. 1, 21), coming into view beneath the fore part of the malar (26), expands slightly to unite with the lacrymal (13), and, in the species affording the subject of fig. 1, Tab. XX, it contributes the middle third of the lower border of the nostril ; but, as a rule, it is separated there- from by the junction of the lacrymal (73) with the premaxillary (22), as in fig. 2. In advance of this the maxillary is overlapped by the premaxillary, which conceals it from view at about the fifteenth tooth, counting forward, in Icldhyosaurus tenuirostris. In no species does it support more than about one third of the series of teeth on its own side of the upper jaw.
The premaxillary (Tabs. XIX— XXI, XXV, figs. 1, 22) is characterised by its great 8
58 BRITISH FOSSIL REPTILES.
relative length. Its facial part begins behind by an expanded bifurcation bounding anteriorly the nostril ; in advance of which the bone gradually expands, overlapping the nasal and maxillary, descending to the alveolar border to form the major part of the upper jaw, and narrowing to the end. The palatal portion (Tab. XXI, fig. 1, 22), long and narrow, is continued backward between the maxillary and palatine to terminate in a point penetrating the palatn-naris {p n).
The pterygoid (Tabs. XXI, XXII, fig. 1, 24) is also a bone of considerable size, especially length. It begins behind by a triradiate expansion ; the outer short subquadrate branch abuts against the tympanic (28) ; the upper narrower pohited ray is wedged between the paroccipital (4), mastoid (g), and tympanic (28) ; the inner and longest branch is broad, wedged between the paroccipital and basisphenoid, reaching with its blunt-pointed end the basioccipital. In advance of this triradiate expanse the pterygoid contracts, pre- senting a concave inner border, articulating with the side of the basisphenoid (5), and a more concave outer border, which forms the inner one of the cavity which was occupied by the gristly petrosal. As it continues to advance, the pterygoid again expands, its outer border forming an angular process, to the fore part of which the ectopterygoid (25) unites ; the pterygoid then extends forward and gradually inward (mesiad), decreasing in breadth, and is continued as a long and narrow strip of bone in contiguity with its fellow, bounding anteriorly the long interpterygoid vacuity («), articulating outwardly for one half of its extent with the ectopterygoid, and for the other half with the palatine.
The ectopterygoid (Tab. XXI, fig. 1, 25) is an elongate plate of bone, though less than half the length of the pterygoid ; its rounded hind end and much of its mesial margin articulate with the pterygoid, the rest with the palatine, with which it combines to form the hind boundary of the palato-naris {p n). The outer margin contributes to bound the pterygomaxillary vacuity (y) posterior to its junction with the maxillary, along which it extends to opposite the fifth upper tooth or thereabouts, counting forwards, and there terminates in a point.
The malar (Tab. XX, fig. 1, 20) is a long and slender, moderately bent bar of bone, commencing anteriorly in a point wedged between the lacrymal and maxillary, thence receding to form the lower boundary of the orbit, and expanding to be partially over- lapped by the postorbital bone (12'), behind which it terminates as a wedge between that bone and the zygomatic.
This bone (ib., 27) is subquadrate, its vertical equalling its longitudinal extent ; the angles are more or less produced. The upper anterior one is wedged between the pro- squamosal (27') and postorbital (l2x) bones; the lower anterior one underlaps the end of the malar ; the front border articulates with both malar and postorbital, the upper border with the prosquamosal ; the hind border is rounded and concave, forming the fore part of the 'meatus auditorius extern us' (Tab. XXII, fig. 1, m) ; the hinder half of the under surface of the zygomatic expands, and is slightly excavated to articulate with the outer and upper part of the expanded articular end of tlie tympanic (28')-
LIASSIC ICHTHYOSAURS. 59
The lacryraal (Tab. XX, fig. 1, 73) forms the lower two thirds of the anterior border of the orbit ; it sends off from the middle and inner part of this border a short process protecting the lacrymal orifice. The bone contracts vertically as it approaches the nostril, of which it forms the hind concave border. The npper part of the lacrymal sends a process which fits into a notch of the prefrontal (ij), anterior to which it joins both the prefrontal and the nasal. Anteriorly, it unites in some species with the maxillary (ib., fig. 1), in most with both maxillary and premaxillary (ib., fig. 2) ; its lower and longest margin articulates with the maxillary and malar.
The two supplemental skull-bones in Ichthyosaurs, which have no homologues in Crocodiles, are the postorbital {l2 x,y and prosquamosal (27');" both are present in Labyrinthodonts. The postorbital is the homologue of the lower division of the post- frontal in those Lacertians (<?. y. Iguana, Tejns, Ophisaurus, Anguis) in which that bone is said to be divided. The postorbital most resembles a dismemberment of an ascending process of the malar ; its lower end overlaps and joins by squamous suture the hind end of the malar ; whence it sliglitly expands, rising to the middle of the back of the orbit, thence, gradually contracting as it curves upward and forward, it articulates with the pro- squamosal (27') and postfrontal (12).
Were the prosquamosal (27') connate with the zygomatic (27) as in Chelotie, the resemblance to those parts of the Mammalian ' temporal bone ' would be close, save that the squamous portion would be removed from the inner to the outer wall of the tem- poral fossa. The prosquamosal holds the place of the temporal fascia in Mammals, and should be viewed as a sclerodermal ossification closing, in Ichthyosaurus as in Lahyrin- thodon, the vacuity between the upper and lower zygomatic arches, such as exists in Croco- dilia. In Ichthyosaurus the prosquamosal (27') is a broad, thin, flat, irregularly-shaped bony plate, smooth and subconvex outwardly, wedged into an interspace between the postfrontal, postorbital, zygomatic, tympanic, and mastoid bones.
The chief vacuities in the skull are : — In the occipital region (Tab. XXII, fig. 1), the ' foramen magnum ' or neural canal of the occipital vertebra ("), the ' occipito-parietal,' (0^:1), and the 'auditory' (?«) ; on the upper surface (Tab. XIX, fig. 1), the 'foramen parietale ' (/) and the ' temporal fossae ' (t) ; on the sides (Tab. XX), the ' orbits ' (o) and the ' nostrils ' («) ; on the lower surface (Tab. XXI) the ' palato-nares ' (p «), the ' interpterygoid ' («), and the ' pterygomalar ' (y)^ apertures.
The ' foramen magnum ' is formed by the basi-, ex-, and super-occipitals, the last having a nearly equal share with the exoccipitals ; the basioccipital contributes the least
1 Described as " apparently a distinct and peculiar bone " of the orbit in the ' Report ' for 1839.
- This is termed "squamosal" in the Lectures above cited (1858), p. 392. The recognised distinctness of this bone in Ictithyosuurus inclined tne in 1839 to view the zygomatic and squamous parts of the temporal bone of anthropotomy as essentially distinct elements ; a view which subsequent extensions of comparison enforced me to abandon.
^ ' Pierygomaxillary ' in Crocodiles and Lizards, ' Anat. of Vertebrates,' vol. i, pp. 15G and 1J7, fig. 98, y.
60 BRITISH FOSSIL REPTILES.
part of any. The occipito-parietal vacuities are larger than in Crocodilia, smaller than in Lacertilia ; they are bounded mesially by the ex- and superoccipitals, laterally by the parietals and mastoids, below by the paroccipitals.
The auditory aperture, or ' meatus ' (Tab. XXII, fig. 1, m), is bounded by the tympanic and zygomatic. The tympanic takes a greater share in the formation of the ' meatus auditorius ' in many Lizards ; in Crocodiles it is restricted to that which it takes in Ichthyosaurs.
The orbit is remarkable for its large relative size and backward position : in the former character the Lizards, in the latter the Crocodiles, approach the Ichthyosaurs. The cavity is formed by the pre- and postfrontals above, by the lacrymal in front, by the postorbital behind, and by the long and slender malar below. In Crocodiles, and in most Lizards, the frontal or mid-frontal enters into the formation of the orbit, and in some Lizards (Sieliio, Jj/avia) the maxillary also. In Chameleons, both the frontal and the maxillary are excluded from the orbit.
The external nostrils are not homologous with the single medial one in the Crocodiles, but answer to the parial nostrils in Lizards, and to the supplementary aper- tures bounded by the nasal, lacrymal, and maxillary bones in the Teleosaurs. In Lizards the lacrymal is usually excluded by the maxillary from the nostril. In Iclithyo- saimis the nostril («) is a longish triangular aperture, with the narrow curved base behind ; it is bounded by the lacrymal, nasal and premaxillary (22), sometimes also by the maxillary bones, and is usually distant from the orbit by less than its own long diameter. Like the orbit, the plane of its outlet is almost vertical.
The interpterygoid vacuity (Tab. XXI, fig. 1, s) is a very long and narrow triangular one; its base is behind, and is bounded by the anterior concavities of the basisphenoid. As it advances it is divided for a certain extent by the presphenoidal rostrum ; the lateral boundaries are due exclusively to the pterygoids, which, converging, reduce the aperture to a point. In this formation, Ichthyosaurus resembles Iguana, but in Varantis the palatines enter into the formation of the palatal vacuity in question.
The pterygomalar vacuity (Tab. XXI, fig. 1, y) may be described as the lower outlet of the temporal fossa ; it is bounded laterally by the zygomatic and malar, chiefly the latter bone; posteriorly by the tympanic, anteriorly by the maxillary, mesially by the ectopterygoid and pterygoid ; the outer deep emargination of the latter bone relates to the passage of the large temporal muscles for insertion into the mandible. The larger proportion which the maxillary contributes to this vacuity in recent Saurians suggested the term ' pterygo- maxillary,' applied to it in the undercited work.^
The palato-nares (ib., pn) are relatively smaller than in most Lizards, and are circum- scribed by the palatine, ectopterygoid, maxillary, and premaxillary bones. In Lizards the ectopterygoid is excluded from its formation, and the position of the palato-naris is more
1 'Anatomy of Vertebrates,' vol. i, p. 156.
LIASSIC ICHTHYOSAURS. 61
advanced than in Ichthyosaurus. The Crocodilia depart widely from the Ichthyosauri in their single and backwardly placed palatal nostril.
The 'foramen parietale ' (Tab. XIX, tig. 1,/) receives a much smaller proportion, if any, of the parietal bones than of the frontals in its formation (Tab. XXII, fig. 1).
The upper outlet of the temporal fossa (Tabs. XIX and XXIII, fig. 1,t) is bounded mesially by the parietal, laterally by the mastoid and postfrontal," behind by the mastoid and parietal, in front by the postfrontal ; the lateral or outer wall of the fossa is formed by the mastoid, postfrontal, prosquamosal, postorbital, malar, and zygomatic bones.
More or less of the circle of sclerotic |)lates are commonly preserved in the fossil skulls of Ichthyosauri. They are of an irregular, olilong, quadrate form, joined together by squamous or overlapping sutures at their longest sides. The hind part is usually about half the length of the plate, and is very thin, ending in a trenchant border; the front or pupillary corneal border is thicker, shorter, and nearly straight. From this border each plate extends, raylike, outward, for more than half its length, then suddenly bends towards the back of the eyeball, defining and encasing its periphery, and indicating the extreme oblateness of that visual spheroid. In the Ichthyosaurus communis I have counted seventeen of these sclerotic plates.
c. Pectoral and Pelvic Arches and Appendages. — The limbs of Ichthyopterygia, as the name of the order implies, resemble the fins of Fishes in the number of digital joints or segments, and, in some species, the seeming excess of digits beyond the typical ' five.' With the parial ones of Fishes these Reptilian fins also correspond, the anterior pair with the ' pectorals,' the posterior pair with the ' ventrals.'
The inverted arch^ supporting the ' pectorals ' is detached from the occiput, as in the Plagiostomes ; that supporting the ' ventrals ' is also detached from the ' sacrum,' but retains the position beneath the vertebrae, which, when coalesced, receive that anthro- potomical name. The hinder arch^ has gained a structure determinative of the homology with the hsemovertebral elements called ' pelvis,' and the limbs so supported are called ' pelvic'
The pectoral arch (Tab. XXIV, fig. 4) consists of a pair of scapulae (51), a pair of coracoids (52), a pair of clavicles (08), and an episternum (-1&). In some specimens there appears a trace of a pair of precoracoids.
The correspondence with the same arch in Ornithoryhnchus was pointed out and figured by Clift.^ I have not seen an Ichthyosaurus in which the clavicles were confluent mesially as a single bony arch, resembling the Avian ' furculum ; ' but such confluence does take place in the full-grown or aged Monotremes. No sternebers succeed the episternum in Ichthyosaurus as they do in Ornithorhynchus.
The episternum is small ; each clavicle exceeds the length of the anterior transverse
1 The definition of ' girdle ' in our Dictionaries is inapplicable to these parts of the skeleton.
2 'Philos. Trans.,' mdcccxviii, p. 32, pi. ii.
62 BRITISH FOSSIL REPTILES.
ray ; tlie medial longitudinal ray or stem does not exceed the transverse portion in extent. The clavicles are powerful bones, pointed at each end, overlying the transverse rays of the episternum, and continued along the anterior border of the scapulae towards or near to the ' base ' or free extremity of those bones ; the joints are rough or sutural. The scapulae are oblong, subconipressed, truncate at the free or basal end, thickened and broadened at the opposite or articular end for the two joint-surfaces of the coracoid and humerus (53) respectively.
The two pairs of limbs (ib., fig. 1, s, p) have been found in every sufficiently preserved skeleton, and where such fins have been lost their supporting arches or some elements thereof have usually indicated their existence. Of these limbs the anterior or pectoral («) surpass in size, but in different degrees according to the species, the posterior or pelvic (p) pair. They appear to be most nearly equal in size in the skeleton, in part restored, of the Ichtliyosaurus jjlatyodon (Tab. XXVII, fig. 1), but confirmatory evidence of the degree of difference is desirable in regard to this species. The pelvic pair is the smallest relatively in Ichtliijosaurus latimanus, Ich. communis, and Ich. breviceps, but the inferiority is nearly the same in Ich. intermedins (Tab. XXVI, fig. 1).
In all the species the digits are supported by flattened, subquadrate, hexangular, pentangular, transversely quadrate, or rounded phalanges, exceeding in number in each digit that known in any other Reptile, and recalling the many-jointed rays of the pectorals and ventrals of Fishes.
The shorter-snouted species have the greater number of digits, with more and smaller phalanges ; as the jaws proportionately elongate the number of digits decrease, and their plialanges become relatively larger and fewer.
In all Ichthyosaurs the pectoral limb includes a humerus (see the typical restoration, Tab. XXIX, fig. 1, 53), two antibrachials (54, 55), three proximal carpals (55), and four distal ones (56'), from which the more numerous series of ossicles (57) are continued. I shall here limit the description of this part of the skeleton to the modification presented in the IcJdhyosaurus commnnis (Tab. XXIV, fig. 1, «).
In this species the length of the humerus is but one fourth more than its breadth, and this is greater at the proximal than the distal end. The joint-surface of the head of the bone is subconvex, produced outwardly or anconally upon a thick deltoid ridge, subsiding half w' ay down the shaft ; there the ancono-thenal compression becomes more marked and is continued to the distal end, which is pretty equally divided into two sub- concave, almost flattened, surfaces for ligamentous union with the antibrachials.
Assuming the prone position of the fin, which presents to outside view its anconal surface, as in Fishes, the anterior antibrachial represents the radius (54), the posterior one the ulna (55). Both bones are pentagonal by reason of the truncation of their distal approximated angles, which give lodgment to the proximal angle of the middle hexagonal carpal bone ; the radial and ulnar carpals (56) are transversely oblong, and the quadran- gular shape is but slightly disturbed by the production of their contiguous borders into the
LIASSIC ICHTHYOSAURS. 63
intervals between the midcarpal and the two distal carpals, which it partly supports. The radial and ulnar ossicles (56', 56') of this third series from the humerus are extended trans- versely ; the four of the following series articulate each with its corresponding carpal.
The series of three bones (Tab. XXIX, fig. 1, 56) presents the same relation to the anti- brachials as does the proximal row of carpals in Tesfudo ; ^ and the series of four ossicles which follows may be homologised with the distal series of carpals in the same number in Testudo. In this case the next transverse row of four ossicles, the third from the antibrachium, may be regarded as metacarpals (.)/). According to this view the radial metacarpal (57'), not the ulnar one (57) supports two digits, and the normal digits in Ichtliyomurus communis are thus five in number (i, 11, iii, iv, v). Each consists of a series of flattened, somewhat transversely extended ossicles, of which I have counted tliirty and upwards in the two ulnar digits of the present species ; they are rather fewer in the two radial and the mid-digits. But, in addition to these multiplied digital joints there are two superadded marginal series of ossicles ; that (1') on the radial border of the fin begins between the second and third joints of the radial digit, and is continued to near its extremity. The series (v') along the opposite, ulnar, margin, begins at the interval between the ulnar proximal and distal carpals, and is also continued to near the extremity of the fifth normal digit. These supplementary ossicles are more rounded in shape than the normal phalanges, but, like these, progressively decrease in size to the tapering end of the fin. At first view, apart from the preceding homological analysis of the bones of the fore limb of IcUhyosaurus, they seem to show that seven digital series are present in that fin of IcTi. communis.
All are enveloped in one sheath of smooth integument, which is continued beyond the framework to an obtusely pointed end, and likewise extends some way beyond the hinder or ulnar border of the bones, where it seems to have been supported by numerous fine cartilaginous rays unless the appearance be interpreted as due to parallel bifurcate wrinkles of the skin (Tab. XXIV, fig, 5).'
The ilium (Tab. XXVI, fig. 5, 62) is an oblong, subcompressed bone, slightly enlarging at the acetabular end to form the articular surfaces for pubis, ischium, and femur ; some- times, also, more slightly expanding to the opposite free flattened end. This part, if attached to any of the vertebrae, would be so by ligamentous or soluble tissue. The length of the iUum does not exceed, usually, that of three contiguous centrums.
The ischium (ib. 63), of minor length, is thickened at the acetabular end, which shows facets for the ilium, the pubis, and its share in the shallow cotyloid cavity for ligamentous attachment of the head of the femur. The body of the ischium expands, more or less in different species, to its free non-articular border, anterior to which it unites synchoudro- sally with the pubis. This (ib., Ci) is usually a somewhat longer bone; it also expands
^ ' Ar.at. of Vertebrates,' vol. i, p. 174, fig. 108, a, d, c.
2 ' Transactions of tlie Geological Society,' 2nd series, vol. vi, p. 199, pi. .\.x.
64 BRITISH FOSSIL REPTILES.
to join the ischium at its lower end ; the opposed borders of the two Ijones are concave, and inclose a long and large ' obturator ' vacuity.
Modifications of these pelvic elements are shown in Ichthvosaurus communis and Ich. latifrons, and will be noticed in some other species.
The femur (Tab. XXVII, fig. 1, 65) resembles the humerus, but is smaller and usually rather less broad. Its distal end supports two cnemial bones, repeating the shape and relative proportions of the antibracliial ones. To these succeed three tarsal ossicles, which support four series of digital ossicles. But in Ich. communis the anterior or tibial series soon bifurcates, and supplementary ossicles are applied to those of the fibular or hinder digit. The shape of the ventral or pelvic fin thus supported is similar to, but relatively less broad (in Ich. communis) than, the pectoral one.
The rare illustration given in Tab. XXIV, fig. 5, is of a pelvic fin.
d. Osteohgical Summary. — At the risk of some repetition the following remarks, the result of such comparisons as I have had time or means of extending over the cold- blooded air-breathers, may not be unacceptable. The comparison with existing Reptilia is almost restricted to the Lacertian and Crocodilian modifications.
In Crocodiles the exoccipitals leave an intervening tract of the basioccipital, which thus takes its share in support of the ' medulla oblongata'; in Lizards the exoccipitals meet upon the basilar element, as in Ichthyosaurus, but the extinct Reptile differs from both in the exclusive formation, by the basioccipital, of the condyle articulating with the atlas. Such joint in Eishes is afforded by the basioccipital only, but the articular surface, as a rule, is concave instead of convex. The superoccipital is excluded from the foramen magnum in Crocodiles, but contributes, in Lizards, a share thereto, as in Ichthyosaurus. The paroccipital is confluent with the exoccipital in both Crocodiles and Lizards, as it is in Plesiosaurs.^ It remains distinct in Chelonians as in Ichthyosaurs.
The extension of the mastoid upon the occipital region of the skull gives it an aspect of solidity more like that in Crocodiles than in Lizards ; but this is an adaptive con- formation, and depends on the need of an extent of bony surface for the implantation of the powerful nuchal muscles mainly concerned in wielding a head produced into long and heavy jaws, beset, as a rule, with formidable teeth ; it also relates to the stabihty of the prow of the Fish-lizard in cleaving the watery element. The occipital aspect of the diverging extensions of the parietals, and the presence of the ' fontanelle,' called ' foramen parietale ' in Plesiosaurs" as well as Ichthyosaurs, are more decisive instances of the closer affinity of Lizards, than of Crocodiles, to the antecedent marine types of Beptilia. No part of the parietal extends upon the occiput in Crocodiles, but this extension holds in Plesiosaurs as in Ichthyosaurs.
In adaptive relation to the mandible and its armature I regard the relative size and
1 Ante, p. 8.
2 lb.. Tab. IV, fig. 1,7.
LIASSIC ICHTHYOSAURS. 65
shape of the tympanic, the number of bones amongst which it is wedged, and the double buttresses extended on each side from the facial to the cranial part of the skull. In these characters the Ichthyosaurs resemble the Crocodiles ; but the upper or post- fronto-mastoid zygoma and the lower malo-zygomatic one are present in some extinct as well as existing Lizards, e.g. Rhi/nchosaurus^ and the Rlii/nchocephalia?
In the exclusion of the mid-frontal from the orbit Ichthyosaurus differs from the Crocodiles and from most Lizards, but it is in the Lacertian order only that exceptions occur of repetitions of this Ichthyosaurian structure.'' In the position, construction, and parial character of the external nostrils the Lizards repeat the Ichthyosaurian and Pleiso- saurian type, from which the Crocodiles have departed, but the lacrymal is excluded from the formation of the nostril in all Lizards. In the small relative size of the maxil- laries, especially as compared with the premaxillaries, Ichihi/osaurus differs from both Plesiosaurs and Crocodiles, and still more from Lizards : here we have in Fishes the nearest resemblance to the subjects of the present Chapter. Nevertheless, as in Lecertilia, the anterior boundary of the external nostril is formed by the pre- maxillary ; and, as the marine Reptilia, like the marine Mammalia, needed to have the nostrils at or near to the upper part of the head, so, agreeably with the Lacertian type, the premaxillaries, however they might be produced forward, retain in Ichthyosaurus, as in Plesiosaurns, the posterior relations with their antorbital nostrils.
Both Lacertians and Crocodilians differ from Ichthyosaurs in the connections of the nasal with the maxillary. The Crocodiles resemble them in the inter-pre- maxillary suture ; its presence is an exception in Lacertians, the Rhynchosaurians and Rhynchocephalians* again affording such examples; I have found it obliterated in a Plesiosaurus dolichodeirus} In the position and formation of the palatal nostrils the Lacertians agree with, whilst the Crocodiles widely depart from, the Ichthyosaurian type. The apertures are distinct or parial in the Plesiosaurs, but are placed far back."
In the structure of the mandible the dentary resembles that element in Lizards, and differs from that in Crocodiles, in being pierced externally by a longitudinal series of nervovascular foramina ; it differs, also, from the dentary in Crocodiles in its posterior termination being above instead of beneath the fore end of the surangular. In the aniphicoelian Crocodiles the vacuity between the angular and surangular is much reduced in size; it is still smaller in Rhynchocephalians ; it is absent in Ichthyosaurs, as in Plesiosaurs' and most Lizards.
In the conformation of the posterior angle and the robustness of the articular
1 'Trans. Cambridge Philos. Society,' vol. vii, 4to (1842), p. 350, pi. v.
- ' Catal. of Osteological Series in Mus. Coll. Surgeons,' 4to, 1853, p. 143, No. 663; and Guntheb. ' Phil. Trans.,' mdccclsvii, p. 32.
^ In CJiameleo parsoni, e.y. Ccvier, ' Oss. Foss.,' v, pt. ii, pi. xv, fig. 80.
* Pi. xvi, fig. 1, 23. "Oil the Affinities of Rhynchosaurus," 'Annals and Mag. of Nat. History,' iv, 1859, p. 237.
6 PI. iii, fig. 1. 6 PI. xvi, fig. 2, r, r. 7 pi, ji.
9
G6 BRITISH FOSSIL REPTILES.
extremity of the mandible tlie jaw of IclilliyomurHS more nearly resembles that of the Crocodiles than of the feebler Lizards, bnt in the mandibular structures indicative of affinity these latter existing Reptiles manifest their closer connection with the Ichthyo- saurus. This is conspicuously seen in the absence of distinct alveoli and the lodgment of the teeth of Ijoth upper and lower jaws in a continuous open channel, the inner wall of which, in the mandible, is in a large proportion contributed by the splenial element. But the cement-clad base or root of the tooth seems not to become anchylosed to the alveolar tract or groove in Ichthyosaurus, but to remain free, till shed, as in Crocodiles. Although a portion of the pul[)-cavity may persist in the fully developed tooth after the base or root becomes consolidated by a mass of interblended osteodentine and cement, into this mass the crown of the successional tooth presses, and occasions a cavity by absorption.' In no case have I found evidence of this successive supply of new teeth in the Triassic or Permian Theriodonts : herein differs their dentition from both Crocodilian, Dinosaurian, Lacertian, and Enaliosaurian Reptilia.
In most Lizards the hyoid bones present modifications which relate to the size and uses of the thick, or long, and commonly bifurcate, tongue. In Ichthyosaurus the appa- ratus is reduced to the same number of pieces as in the Crocodile, in which it is less subservient to the support and movements of the tongue than to the mechanism for defending the larynx and pharynx from the entry of water during the struggles of a submerged prey, when the mouth of the air-breathing destroyer is necessarily exposed to the free ingress of the aquatic medium. The condition of the hyoid apparatus in the Ichthyosaurus indicates that its tongue may have been but little better developed than in the Crocodile, and, since the Ichthyosaur obtained its food under the same circumstances which necessitate the hyoid and lingual modifications in the Crocodile, it may be inferred that the hyoid arch was physiologically related to the working of a similar valvular apparatus for defending the orifice of the air-tube from the water admitted into the mouth during the capture of the fishes, the remains of which have been found in the region of the alimentary canal of the great Sea-lizards.
The modifications of cranial structure of the known kinds of Ichthyosaurus are chiefly presented by the upper and lower jaws, which become elongated and attenuated in degrees exemplified by the species next to be described. With these modifications are associated increase of number with decrease of size of the teeth, and their total dis- appearance, finally, as in the Ichthyosauroids of the upper Jurassic beds of Wyoming and some other American localities. For these edentulous Ichthysam's, their discoverer. Prof. Marsh, has proposed the generic name Sauranodon ; " it is probable that, as in the
1 ' Catalogue of the Fossil Reptilia and Pisces iu the Museum of the College of Surgeons,' -Ito, 1854, p. 40, Nos. 139, 140. In No. 141 I point out that in some of the teeth " the pulp-cavity has been obliterated in the crown as well as in the base of the tooth." See ' Quarterly Journal of the Geological Society' for May, 18/9, pp. 189 and 199.
2 ' American Journal of Science,' vol. xvii, p. 85, January, 1879 ; ib., vol. xi.\, p. 169, February, 1880.
LIASSIC ICHTHYOSAURS. 67
case of Cetacea, showing minor modifications than do the toothed Icldhijopteri/gia, other generic terms for some of these species may be proposed.
C. Species.
a. Ichthyosaurus breviceps, Ow., Tab. XXV.
In the skeleton of this species (Tab. XXV, fig. 2) the skull is almost equally divided between the antorbital part and that behind ; it is about one sixth the length of the entire body, as represented by the vertebral column. This includes, in the specimen figured, 125 vertebrae, of vphich 46 lie between the skull and pelvis. The neural spines of such trunk-region are lofty, equalling along its major part the vertical diameter of the rest of the vertebrae taken from the base of the spine. The intervals between the spines are very narrow. The centrums are largest at and near the pelvic region. The fore fin has five normal digital series, with smaller supplementary ossicles along both fore and hind borders ; it is twice as long and as broad as the hind one.
The specific characters are more fully exemplified in specimens of the skull of larger individuals, which show that the proportions of the rostrum to the rest of the skull in the smaller skeleton may be due to nonage, but the cranial conformation is the same.
The skull with the right side shown in profile (Tab. XXV, fig. 1) was discovered in the thick Liassic Limestone, called " Broad Ledge," at Lyme Regis ; and, as usual with such fossils from this locality, is somewhat compressed.
The length of the mandible is 3 feet 1\ inches ; that of the upper jaw from the fore part of the orbit is barely 2 feet ; and from the fore part of the nostril is 1 foot 4^ inches. These proportions indicate the character of the skull which suggested the specific name.
The species which Icldhyosaurus hreviceps is thus shown to have approached in size is Ich. platyodon, Conybeare; but, like Icli. triffonodon, Theod.,^ it had fewer and propor- tionately larger teeth.
In the portion of the iqiper jaw in advance of the nostril of a well-preserved skull of Ich. pJatijodon, the number of teeth is thirty ; whilst in Ich. breviceps they do not exceed eighteen. In a corresponding extent of the lower jaw of Ich. j^latyodon the number of teeth is thirty-two ; in that of Ich. hreviceps it is twenty-two. The length of the skull from the back of the orbit to the fore end of the upper jaw, in Ich. hreviceps, is three times and two thirds that of the long diameter of the orbit ; in Ich. plafyodon the length of the skull from the back of the orbit forwards is four times and one third that of the orbit in one specimen, and four times and a half that of the