Chap. III. GENERA OF TRIONYCIIID/E. 397 



to the same genus, or ought also to embrace other species, which are referred 

 to different genera. Of Aspidonectes, Waf/l., Gymnopus, Dirm. and B/l/?:, and Tri- 

 onyx, Graf/, this will be self-evident, as soon as it is shown that the North Amer- 

 ican species, which have all been referred to these genera, belong in reaUty to three 

 different genera. Pelodiscus and Potamochelys, Fits., and Tyrse, Graf/, run together 

 in the same manner, on account of the heterogeneous species they contain. There- 

 fore, one question only remains, Which of these names are to be retained for the 

 North American species ? Of all the generic names not yet strictly applied, Aspi- 

 uoxKCTES, W(!//l., is the oldest; and as it was established for species, some of which, 

 as Tr. javanicus and cegyptiacus, agree with some of the American ones, as Tr. 

 spinifer, LcS., I shall retain that name for the genus to which our Tr. spinifer 

 belongs. Next stands the genus Platypeltis, Fit.i., which, though made to include 

 also Tr. spinifer, ZcS., is yet meant for Tr. ferox, Sc/iw., and may therefore be 

 retained for the genus of which Tr. ferox S'chio. must be considered as the type, 

 and which must also embrace Tr. gangeticus, Ciw. The adoption of these two 

 genera renders Gray's name Tyrse and Fitzinger's Potamochelys and Pelodiscus 

 entirely superfluous, as Tyrse includes Tr. javanicus, a^gyptiacus, and gangeticus, 

 and Potamochelys Fiiz. is founded upon Tr. javanicus, while Pelodiscus rests upon 

 Tr. sinensis, Wiegm., and labiatus. Bell. We have thus appropriated, for six natiu-al 

 genera, six of the names introduced among the Trionychidte, and shown that six 

 out of the remaining seven have no scientific value. But there is a third Amer- 

 ican genus, founded upon Tr. muticus, LeS. I am glad to have an opportunity 

 of honoring the memory of Schweigger by fixing upon this genus the name of 

 Amvda, first proposed by Schweigger for the whole type of Trionychidte, though 

 wantonly rejected by Geoffroy, and so vaguely applied by Fitzinger to one of his 

 genera. 



It has already been stated that the eggs of the Trionychidoj (PI. 7, fig. 20-23) 

 are spherical and very brittle.^ The young at the time of hatching (PL 6, fig. 

 1-7) exhibit fully their family character ; they are flat, discoid, and orbicular in 

 outline ; their he;vd only is comparatively shorter and rounder than in the adult, 

 and the neck thicker, but the proboscis is very prominent; the feet have already 

 their characteristic web, and the membranous fold which extends along the upper 

 edge of the four legs (PI. 6, fig. 2 and 5). The ossification of the shield is so 

 little advanced that there is no sign of a carapace or plastron visible externally 

 through the soft, scaleless skin. 



The Trionychidaj were for some time supposed to have existed upon our 

 globe as early as during the Devonian period. I have shown, however, that the 



» See Part II., Cliap. 2, Sect. 4, p. 334. 



