40 DR. J. E. GRAY ON THE TRIONYCHID^E. [Jail. 7, 



to Trionyx, which have each different jaws, but common external 

 characters. 



I believe that, for the division of the Trionychidce into tribes or 

 groups, the formation of the dorsal disk affords the best character, 

 and one which can be observed in the animal after it has passed its 

 most juvenile state. 



The following Table exhibits the affinities of the genera of the two 

 families to each other : — 



Fam. Chitrad^;. Fam. Trionychid^:. 



The skull very thin, light ; al- The skull solid, thick ; alveolar 



veolar surface narrow. surface more or less wide. 



I. The hinder lateral edge of the sternum narrow, exposing the hind 



feet. The front pair of bones of the sternum without any cal- 

 losities. The front odd bone of the dorsal disk without any 

 bone before it in the margin. 



Chitraina. Trionychina. 



II. The hinder lateral edge of the sternum with flaps to cover the 



hind feet. The front pair of bones of the sternum with callo- 

 sities. The front odd bone united to the ribs of the dorsal disk, 

 with an oblong free bone in the margin before it. 



Heptathyrina. Emydina. 



u. Dorsal disk without any posterior marginal bones. 



Heptathyra. Cyclanosteus &c. 



b. Dorsal disk with posterior marginal bones. 



Emyda. 



There is great general affinity between Heptathyra and Cycla- 

 nosteus, between Chitra and the various genera of Trionychina ; 

 indeed the skull of Callinia has all the thinness of the skull of the 

 ChitradcB, but retains the characters of the Trionychina. The animals 

 of the Chitradcc must have very different habits and food (as proved 

 by the form and lightness of the skull, and the weakness of the lower 

 jaws) from the Trionychidce, where the skull is generally solid, some- 

 times very much so, and the lower jaw very strong. 



The latter must differ considerably in their food ; for many have 

 a broad expanded alveolar surface for chewing, and others, like Tyrse, 

 have a sharp edge for cutting their food. 



Those with the broad alveolar surface live on dead animals ; the 

 Gangetic species are said to be found often feeding on the dead 

 Hindoos that are thrown into the river. 



Family Chitrad;e, Gray, Suppl. Cat. Sh. Rept. p. 89. 



I. Chitraina. Hinder lateral edge of sternum narrow, hind feet 

 exposed ; front pair of bones of the sternum tvithout any 

 callosities ; front odd bone of the dorsal disk united to the 

 ribs. 



