THE BONES IN THE ENALIOSA.URIA. 317 



near an approximation to such an arrangement as to show that 

 that condition is quite consistent with the mammal plan. The 

 squamosal is extended outward to cover the quadrate ; and so all 

 the outer and backward part of the skull is modified on a plan 

 unlike that of the mammal. 



The occipital condyle would appear sometimes to consist only 

 of the basioccipital bone, though in all the specimens in the 

 Woodwardian Museum at Cambridge the exoccipital bones also 

 contribute to it. 



The teeth are all in sockets, as in adult Porpoises ; about half 

 appear to be in the premaxillary, and half in the maxillary bone. I 

 have seen no evidence of their replacement by successional teeth 

 in Plesiosaurus. The closed flat palate, which seems to have two 

 perforations behind for the posterior nares, between the palatine 

 and pterygoid bones, and two perforations external to these, mar- 

 gined outwardly by the transverse bones, finds a general parallel 

 in the Porpoises, though the posterior nares in those animals 

 are not ovoid perforations, and the external foramina have no 

 existence. 



The vertebrae have a mammalian aspect. The neck -vertebrae are 

 shortened in Pliosaurus, as in Balsenidae and Elephants ; but in most 

 Plesiosaurs the centrum slightly elongates in the neck, as in many 

 land mammals, though the vertebrae diff'er remarkably in number, 

 sometimes counting as many as forty-five and never so few as 

 seven. The atlas and axis are usually anchylosed together ; and the 

 cervical vertebrae all carry ribs, some of which sometimes have a di- 

 vided articular head, and all of which articulate with the centrum : 

 in these characters the vertebrae diff'er from mammals'. The dorsal 

 vertebrae have much the proportion and characters seen in Por- 

 poises, except that the epiphyses are not separable from the cen- 

 trum, that the neural arch is often separable from the centrum, 

 that the centrum is usually somewhat cupped, that the neural 

 canal is smaller, and the transverse process rounder, longer, and 

 stronger, and given off" from the neural arch throughout the whole 

 of the dorsal region. But in the cardinal character of having 

 the ribs attached by single heads to transverse processes of the 

 ?ieural arch, a number of dorsal vertebrae of Porpoises off'er a close 

 resemblance to those of Plesiosaurus. And when only the centrum 

 is preserved, it would often be impossible to distinguish between 

 the Cetacean and the Plesiosaur. The neural arch in its general 

 features is very similar in the two. In neither group is there 



