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Show 40 DR. J. E. GRAY ON THE TRIONYCHIDcE. [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. CHITRADCE. Fam. T R I O N Y C H I D C E. 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 callosities. 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 callosities. 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 Cyclanosteus, between Chitra and the various genera of Trionychina ; indeed the skull of Callinia has all the thinness of the skull of the Chitradae, but retains the characters of tbe Trionychina. The animals of the Chitradae 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, sometimes 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 CHITRADCE, 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 without ang callosities ; front odd bone of the dorsal disk united to the ribs. |