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. 



