ORDER CHELONIA. 



iiiS 



In Argillochelys, of the London Clay, the skull (fig. 1020), which 

 is very short and has deeply emarginate pterygoids, differs from that 

 of the preceding genera by the presence of an oral ridge on either 

 side of the palate and in the middle of the mandibular symphysis, 

 although these are present in the Mexican Loggerhead. The 

 vomer, as in the young of Lytoloma, is very long, and extends 

 forwards to join the premaxillae ; and the pterygoids are charac- 

 terised by the forward position of their ectopterygoid processes. 

 The shell seems to have been much like that of Thalassochelys, 

 but had only four costal shields, and its plastron was perhaps 



Fig. 1020. — Frontal and palatal aspects of a young cranium of Argillochelys cuneiceps ; from 

 the London Clay. Owing to immaturity the palatal ridges are imperfectly developed, a, Oc- 

 cipital, b, Paroccipital, c, Parietal, and d, Frontal epidermal shields ; bo, Basioccipital ; fit, 

 Pterygoid ; ept, Ectopterygoid process of do. ; v, Vomer; mx, Maxilla; p7nx, Premaxilla. The 

 bones on the frontal aspect correspond with those of fig. 1022, in which letters are given. 



less ossified, although the xiphiplastrals united extensively in the 

 middle line. The mandibular symphysis is convex, and com- 

 paratively short. The skull of the type species was originally 

 described as Chelone cuneiceps ; while the shells figured as C. sub- 

 cristata and C. convexa are referable to this genus. 



In the typical genus Chelone the skull is much smaller in propor- 

 tion to the shell than in Thalassochelys. It is characterised by the 

 more or less nearly vertical position of the nares and orbits, and the 

 width of the bar between the latter ; by the tall alveolar walls and 

 the oral ridges of the palate and mandibular symphysis ; the long 

 pterygoids, with the ectopterygoid processes placed somewhat back- 

 wardly ; the forward position of the posterior nares ; and the more 

 or less marked shortness of the convex mandibular symphysis (fig. 

 1019, b). The skull, moreover, has no occipital epidermal shield, 

 which is generally present in the other forms (fig. 1020). In the 



