54 REPORT — 1839. 



as in the Crocodiles, an osseous ridge traversing longitudinally 

 the temporal fossa, like a second zygoma, and dividing it into 

 an upper and a lower cavity. 



Zygomatic. — The zygomatic element of the temporal bone, 

 instead of being extended obliquely, parallel with, and joined to 

 the tympanic bone, stretches horizontally from the malar and 

 post-frontal backwards to the lower end of the tympanic, and 

 is attached, as in the Lacertian Sauria, only by its two ex- 

 tremities. 



Tympanic. — The tympanic bone in its general form, and 

 especially its length, is intermediate to the Crocodilian and La- 

 certian types, but exceeds them both in its robustness. 



Facial bones. — When we come to examine the bones of the 

 face, the resemblance to the Lacertian Saiiria begins to dimi- 

 nish, and that to the Crocodiles to increase. This tendency to 

 the higher types of Saurian organization is shown in the strength 

 of the whole maxillary apparatus, in the great relative size of 

 the intermaxillaries, the rugged exterior surface of the bones, 

 and above all in the distinct alveolar cavities for the teeth. 



The external nostrils, however, form a striking exception to 

 this tendency ; and their size and position, combined with the 

 structure of the paddles, indicate the analogy of the extinct 

 Enaliosaurs to the existing Cetaceans, and offer a beautiful 

 example of the adaptation of structure to the peculiar exigencies 

 of a species. 



The apertures through which the air is respired are situated 

 a little anterior to the orbits near the highest part of the head. 

 In the Crocodiles they are situated, as is v/ell knov/n, near the 

 anterior extremity of the snovit, are blended together into a 

 single aperture, and nearly the whole of their circumference is 

 formed by the intermaxillary bones. In the Plesiosaurus the 

 intermaxillaries form an extremely small part of the boundary 

 of the nasal apertures, which chiefly consist on each side of an 

 interspace at the convergence of the anterior frontal, nasal, and 

 superior maxillary bones ; the nostrils are also separated from 

 each other by the nasal bones, as in the Lacertian Sauria. 



The intermaxillary suture extends from the anterior part of 

 the nostrils forwards to a little more than halfway between the 

 orbit and the anterior extremity of the cranium. One of the 

 strongest of the inferior teeth usually rises just in front of this 

 suture, and a slight notch at that part seems to correspond 

 with that tooth, presenting a resemblance to a very character- 

 istic structuse in the true Crocodiles. 



The lachrymal bone forms a great proportion of the anterior 

 part of the orbit : the superior maxillary enters next into the 



