1872.] 



DR. J. E. GRAY ON THE GENUS CHELYMYS. 



505 



thinner skulls of the American genus Platemys. They may be 

 divided thus : — 



I. Head covered with a skin, which is reticulated over the temples ; 

 nuchal plate distinct ; neck smooth, reticulated. 



1. Chelymys. Chin not bearded; nuchal plate generally broad; 



occiput like the crown. (Fig. 1, p. 505, skull.) 



2. Euchelymys. Chin two-bearded ; nuchal plate narrower ; sides 



of occiput with two oblong, subtriangular, diverging, hard 

 plates. 



II. Head and temporal muscles covered with a hard bony sheath; 

 nape spinose ; chin two-bearded ; nuchal plate none (or, rarely, 

 very small and narrow). 



3. Elseya. (Fig. 3, p. 507, skull). 



1. Chelymys. 

 Head covered with a smooth skin, reticulated and divided into 



Fig. 1. 



Skull of Chelymys victoria. 



