1887.] 



Ceratochelys sthenurus. 



235 



Prima facie, the skull found in the same block might also be expected 

 to be that of a Chelydroid ; and, in fact, it is so. I do not base this 

 interpretation on the Chelonian character of the upper jaw, as there 

 are various extinct Saurian reptiles which closely approximate 

 Chelonia in this part of their structure. The diagnostic characters 

 lie in the back part of the skull ; and especially in the auditory region, 

 which is altogether Chelonian. Not only so, but when this fragmentary 

 skull is compared with that of Chelydra, the correspondence between 

 the two is singularly exact (figs. 3 and 4). In two respects, however, 

 the fossil differs from Chelydra and Gypochelys. 



Fig. 3. 



Skulls of Ceratochelys (fig. 3) and Chelydra (fig. 4) ; the latter of the natura 

 size, the former much reduced. The portion of the skull of Chelydra which 

 corresponds with the fossil is shaded. 



1. The roof over the temporal fossa formed by the parietal, post- 

 frontal, and other bones, which leaves the auditory region uncovered 

 in the recent genera,* extends back, beyond the occiput, in the fossil, 

 and sends down a broad vertical rim from its margin. 



* The ' roof ' extends much further back in Platysternum. 



