CiiAP. m. GENERA OF EMYDOID.E. 431 



remains of this order have been compared anew to ascertain wliich arc genuine 



Emydoids, and which Iljdraspides. The modifications noticed in the form have 



suggested their subdivision into several tribes or sub-families. (Comjjare p. ooo.) 



GENERA OF THE SUB-F.\MILY OF NECTEMYDOID.E. 



I. Ptvciiemys, Aff. Horizontal alveolar surface of tlie upper as well as the 

 lower jaw very broad, and divided by a ridge, the crest of which is tuberculate, 

 and parallel to the cutting edge of the jaws. This edge is either smooth or 

 serrate. The front of the alveolar margin of the upper jaw is either emargi- 

 nated or more or less deeply notched, with or without a projecting tooth on 

 either side (PI. 27, fig. 5). Lower jaw very flat, with a hook or sharp point in 

 front, behind which a keel extends along the symphysis, on each side of which 

 there is a deep pit; alveolar surface spreadiug inward beyond the vertical bi-anches 

 of the jaw. Horny sheath of the lower jaw rough externally. A row of large 

 scales, in the shape of a fold, along the outer edge of the forefeet (PI. 27, fig. 

 1-3). Tessellation of the epidermis, amounting to scales upon the neck, but not 

 upon the loose skin between the legs. The clawless fifth toe of the hind foot 

 forms an angular projection on the posterior edge of the foot (PL 27, fig. 1-3). 

 The color varies greatly with age, and even in different specimens of the same 

 age. When young, the whole surface has more or less confluent ocellated and 

 crescent or lozenge-shaped figures, which become more transvei'se afterwards, and 

 may be resolved into simple blotches in old age. The claws also vary greatly 

 in length and strength ; sometimes, especially in half grown specimens, those of 

 the three middle toes exceed the length of the whole foot. In the young, the 

 median row of scales forms a blunt keel along the back, which fades entirely in 

 the adult. The scales are at first smooth, or rather finely granulated ; afterwards 

 radiating rugosities appear upon their periphery, while in old age ' they are lon- 

 gitudinally rugos'e. 



Ptychemys rugos.v, Aff? Its most prominent specific character consists in the 



' This shows Iiow unsatisfactory specific cliarac- Sc. Pliil., 1854, p. 189,) it had been described before, 



tcrs must be which are derived from the direction, or by Shaw, as Testudo nigosa. Merrem and Schlegel 



even the presence, of tlicsc rugosities. consider it as a variety of Emys scrrata, while Say 



' Tliis species is well known to the American and Ilarlan liavc actually confounded it with Emys 



naturalists, under the name of Eniys ruhriventris, serrata, from which it differs, even generically. Gray 



(Ilullirook, N. Amer. Ilerp., vol. 1, p. 5;"), pi. G.) also describes it as Emys serrata, (Emys irrigata, 



first applied to it by Major LeContc ; but, as this able Bell). Emys rivulata. Gray, h not specifically dis- 



observer has himself acknowledged, (Proc. Ac. Nat. tinct. Dumeril and Bibron describe it under three 



