284 MR. E. LTDEKKEK ON A CROCODILIAN JAW 



] G. On a Crocodilian Jaw from the Oxford Clay of Peterborough. 

 By R. Lydekkek, Esq., B.A., F.G.S., &c. (Eead March 12, 

 1890.) 



The symphysis of the mandible of a Thecodont Eeptile recently 

 obtained from the Oxford Clay near Peterborough, by Mr. A. N. 

 Leeds, and entrusted to me for description, affords evidence of a 

 species new to the English fauna, and apparently belonging to a 

 genus not hitherto recorded from the Oxfordian. 



The specimen, which is rejDresented of one third of the natural size in 

 the diagram (fig. 2), belonged to a comparatively large reptile, and 

 comprises the anterior portion of the splenial and dentary elements ; 

 that part of the dentary which still remains including the whole of 

 the alveolar region. The symphysial portion of the jaw is very 

 broad and flat, with a rugose inferior surface ; but the original flat- 

 ness has been abnormally increased by pressure, which has crushed 

 the oral floor into the dental alveoli, and has thus destroyed the 

 original circular contour of the latter. The splenials enter exten- 

 sively into the formation of the symphysis, reaching as far forward 

 as the seventh tooth on the left side. INo traces of teeth remain, 

 and the empty dental alveoli are thirteen in number on the left, and 

 twelve on the right side. The alveoli, in their original condition, 

 must have been nearly circular ; and none of them are markedly 

 enlarged, or separated from one another by spaces in the neighbour- 

 hood of the muzzle. On the inferior aspect the dentaries and 

 splenials are completely anchylosed together, this feature, taken 

 together with the rugose surface of the bones themselves, indicating 

 that the specimen belonged to a fully adult animal. The length of 

 the symphysis on the oral aspect is close upon 8 inches, while the 

 length of the alveolar portion of the dentary is some 9 inches. 



The thecodont character of this jaw indicates that its owner be- 

 longed either to the Sauropterygia or the Crocodilia, and it is neces- 

 sary, in the first place, to point out why I refer it to the latter Order. 

 I do not, indeed, know what characters there are by which it is pos- 

 sible to say at once whether a given thecodont mandible is Sauro- 

 pterygian or Crocodilian ; and it is therefore necessary to rely upon 

 empirical means of distinguishing between the two. Now it is 

 quite clear that the specimen under consideration does not belong 

 cither to Pliosaurus or to Peloneustes ; while in the other Sauro- 

 pterygians of the Oxford Clay, which I have provisionally included 

 in the genus Cimoliosaurus, the mandibular symphysis is very short, 

 and the whole jaw very much smaller than in the present specimen. 

 Compared with Crocodilians, the mandible before us agrees in the 

 rugose character of its outer surface, and also in the relations of the 

 splenials to the symphysis ; and since, as will be shown below, it 

 agrees in all essential features with an undoubtedly Crocodilian jaw, 

 I take it for granted that the specimen really belongs to a Croco- 

 dilian genus. 



