396 AMERICAN TESTUDINATA. Part II. 



being very short in the females, and extending beyond the rim of the shield in 

 the males of all the species I know. In the Catalogue of the British Museum, 

 J. E. Gray restricts, in 1844, the name of Trionyx to the North American spe- 

 cies; separates Trionyx indicus, Gnuj, as a distinct genus under the name of Chitra; 

 changes Fitzinger's Amyda to Dogania, excluding however from it Tr. muticus, 

 which the Austrian herpetologist associated in that genus with Ti-. subplanus ; and 

 calls Tyrse a genus embracing Tr. gangeticus. Cm., javanicus, Geoffr., asgyptiacus, 

 Geoff r., and a few other less known species; and, finally, retains the name Einyda 

 for Wagler's Trionyx. To these. Dr. W. Peters ^ has added a new genus from 

 Mozambique, in which the absence of bony plates in the marginal rim is com- 

 bined with a broad hind lobe of the plastron, and which he calls Cycloderma. 

 Thus we have not less than thirteen generic names for about the same number 

 of species, some of which are still very imperfectly known. 



Under these circumstances a critical revision of the genera of Trionychidae 

 appears as a great desideratum in herpetology. But the materials for such a task 

 seem to exist nowhere, if I judge from the published catalogues of the great muse- 

 ums in Europe; and I possess myself large numbers of specimens only of the North 

 American species. Yet, from their careful examination I have gathered data which 

 may be of service to a future monographer of this type. Thus I have already 

 satisfied myself that the number of our species is much greater than is generally 

 supposed ; ^ and a careful study of their skeleton has taught me what constitutes 

 generic characters in this family, so that I feel prepared to express an opinion 

 respecting the value of the genera proposed by other writers.^ I hold that the 

 genus Trionyx, as limited by Wagler, is natural ; it embraces the species described 

 by Gray under the name of Emyda, and by Dumeril and Bibron under that of 

 Cryptopus. Next to it stands Cycloderma, Peters, also a natural genus. The 

 Indian genus Chitra, Gray, is no doubt well founded, and so also, probably, is 

 Dogania, Gray, for which the name Amyda, Fits., might have been adopted by 

 Gray, as this is older. But here ends the list of genera thus far proposed which 

 are at all circumscribed within natural limits, as I can show that Aspidonectes, 

 ^Vacjl., Gymnopus, Diun. and Bihr., Platypeltis, Fitz., Pelodiscus, Fiiz., Potamochelys, 

 Fitz., Trionyx, Gray, and Tyrse, Gray, either contain species which do not belong 



^ Monatl. Bericht der Akad. d. Wiss. in Berlin, Union, which he considered as distinct from the 



1855, p. 216. southern species, was correct. 



^ Dr. Holbrook reduces the North American Tri- ' In this connection I would remark, that it is 



onyx to two species, and so do Dumeril and Bibron, hardly possible to distinguish the Trionychidoe by 



and J. E. Gray. It will be seen hereafter, that the their external characters, and that nothing short of a 



supposition of LeSueur respecting the species occur- careful examination of the jaws, and especially of the 



rinpr in the North-western States of the American skull, will reveal their generic differences. 



