430 AMERICAN TESTUDINATA. Part II. 



Platytiiyra flavescens, Af/. I have examined several specimens of this species, 

 sent to me by the Smithsonian Institution. Some of them were obtained in 

 Texas, near San Antonio, and upon the Lower Rio Grande ; others on tlie Red 

 River, Arkansas ; and others at Camp Yuma, on the Gila River, by Dr. R. 0. 

 Abbott. It is of a yellowish green color; the scales are imbricated, and edged 

 with black. The young are represented PI. 5, fig. 12-15. 



SECTION VIII 



THE GENERA OF EMYDOIDiE. 



From want of sufficient materials, I cannot attempt to characterize all the 

 genera of this numerous fiimily, and shall have to limit myself to the North 

 American types. Fortunately these are numerous enough to enable me to show 

 upon what features the genera are founded ; even though I do not intend to 

 enter here into such minute details of their characteristics as I have presented 

 for the genera of the preceding families,^ excepting where this becomes necessary 

 to establish the validity of the new genera which I have recognized. The Che- 

 lydroids and Cinosternoids being excluded from the Emydoids, this family appears 

 here circumscribed within narrower limits than those assigned to it by previous 

 writers. All its American representatives are included by most modern herpe- 

 tologists in two genera, Emys and Cistudo,^ to which J. E. Gray has added the 

 genus Malaclemys, and two sub-genera, Chrysemys and Lutremys.^ They all lay 

 oblong eggs, and the young when hatched are circular in outline in all of them ; * 

 but, even at that time, they vary in various ways in different genera and 

 sub-families. The difierences between the males and females are not so constant 

 as in some other families. It is, however, generally the case that the males are 

 flatter and more elongated. It will not be possible to determine accurately the 

 period of the first appearance of this family in past geological ages, until the 



' ]My object, in tbis second part of my work, is done satisfactorily, without enlarging too much the 



chiefly to show in what manner the principles advo- bulk of this volume. As to the species, I have lim- 



cated in the first part may be applied in illustrating ited myself to mere hints, because I intend to give 



any special group of animals. Having done this in elsewhere full descriptions with figures of the new 



the preceding sections as far as I am prepared to do ones. 



it now, it would be superfluous to extend ftirther this ^ Compare p. 251 and 252. 



analysis of the Testudinata. Moreover, the genera * Cat. Brit. Mus., 1844, p. 27, 28, 31. 



of Emydoida; are too numerous to allow this to be * See p. 292 and 386. 



