158 BRITISH FOSSIL REPTILES. 



them : tte widest diastema divides the fourth upper tooth from the fifth in the 

 Oxford specimen, not the fifth from the sixth : the maxillo-premaxillary suture 

 with the lateral compression at this interval, is as in the British Museum specimen. 

 If the pair of small anterior sockets and teeth are wanting, either through age or 

 accident, in the Oxford specimen the difference noted would be accounted for. It 

 may be remarked that the number of alveoli — twenty-six — on the least imperfect 

 side of the upper jaw is the same in both skulls, and in both a small part of the 

 series is wanting posteriorly. In both the premaxillary part of the jaw containing 

 four pairs of large teeth is slightly expanded. In the maxillary part of the 

 Oxford specimen the teeth increase in size to the sixth ; in the British Museum 

 specimen to the fifth ; beyond which they gradually diminish. The length of the 

 best-preserved alveolar series is 3 feet in the Oxford specimen, and 3 feet 7 inches 

 in that in the British Museum. 



In the mandible from Market-Raisin there are thirty- five sockets in each side ; 

 in that from Kimmeridge there are only twenty ; but as neither specimens have 

 the alveolar series quite complete, I do not feel that there is sufficient ground to 

 reject the hy^Dothesis of individual variety. In all the essential characters, 

 including length of symphysis mandibulte, the Market-Raisin skull agrees with 

 that in the Kimmeridge example of PUosaurus grandls, and differs from that of 

 Pliosmirus trochanterius, next to be described. If, however, the minor differ- 

 ences which have been noted between the Oxford specimen and that figured in PL 

 19, figs. 1 and 2, should prove to be constant, the specific name " brachydeirus," by 

 which I originally indicated Dr. Buckland's magnificent specimen^ from Market- 

 Raisin, might be retained for it. 



Species — PUosaurus troclianferms, Owen (Sauropterygia) Plate 19, figs. 3, 4, 5. 



Plesiosauetis TEOCHANTEEirs, Ow. Eeport on British Possil Eeptiles, Sto, p. 85, 



1839. 



In the work above cited the specific character of the fossil Reptile in question 

 was indicated by modifications of the femur ; but the chief distinction between 

 PUosaurus trochanterius and PL grandis is conspicuous in the greater relative 

 extent of the symphysis mandibuljs in the former, and in the greater proportion 

 of the dental series lodged in that part of the lower jaw. This character is 

 exemplified in the fourth admeasurement in the " Table," p. 156, and in PI. 19, 

 fig. 4, as compared with PI. 19, fig. 1. 



The surangular developes in PUosaurus trochanterius [fig. 29'] a low but well- 



1 ' Odontography,' p. 283. 



