TRIONYX. 45 



A small group, consisting of only eight species, two of 

 which are found in the central parts of America, and the 

 other six in the warm parts of the old world. 



Geoffroy, (Annal. Mus. xiv.) has given a monograph of this 

 group, in which he appears to have thought that the compa- 

 rative length of the free part of the ribs, compared to the 

 dilated part, was a good character ; but further examination 

 has proved that they vary with age, as was to have been 

 anticipated from the changes which similar parts undergo in 

 the other genera. 



Gen. 1. TRIONYX. [Triomjx.) 



Scutella marginalia cartilaginea fiexibiha, pedes liberi, ster- 

 num angustatum. 



The margin of the shell is destitute of any internal bony 

 pieces and quite flexible, except the first vertebra, which, in 

 this genus, is free and much dilated on the side, extending 

 nearly the breadth of the first pair of ribs, while in the other 

 families it only forms the central part of the front margin. 

 For the sake of distinction I have here called it the nuchal 

 bone [os nuchale.) The anterior bones of the sternum are 

 thin, and destitute of any callosities. The lobes of the 

 sternum are narrow, and leave the limbs quite free. The 

 Indian species are constantly seen eating the bodies of the 

 natives which are floating in the Ganges. 



1. Trionyx ferox, [Fierce Trionyx.) Testa subconvexa, 

 obtuse carinata, margine antice posticeque verrucoso, sterno 

 4-calloso. 



Test, ferox, Gmel. from Pen. Phil. Trans. Ixi. t. 1, f. 1, 

 2, 3. (v. Mus. Brit.) inov. Schoepf, t. 19, f. 1-3. inov. 

 Lacep t. 5, f. 1. cop. Daud. t. 18. f. 2, and Enc. M. t. 5, 

 f. 3. Fresh-water Turtle, Garden. Fierce Tortoise, Shaw. 

 Trionyx ferox, Merrem. Trionyx Georgicus, Geoff. 



Jun. Trionyx spiniferus, Lesueur, Mem. Mus. xv. t. 

 (v. Mus. Par.) Trion. carinatus, Geoff. Ann. Mus. xiv. t. 4. 

 and Trion. Brongniarti, Schw. (v. Mus. Par.) 



Hab. in America Boreali. 



The ribs are about one-sixth part free ; the sternum has 

 two lateral and two hinder callosities; they are very large, 

 and nearly unite together into a single disk. The anterior 



