250 AMERICAN TESTUDINATA. Part II. 



North America, introducing only such comparisons with foreign ones as may be 

 imperatively required to appreciate their mutual relations. 



All the genera thus far established among the Chelonii have representatives 

 along the coast of the United States, and I am not aware that there are any 

 genera of this sub-order, except those which have already been recognized by 

 herpetologists : the flxmily of Sphargidida?, containing only one genus, the genus 

 Sphargis; and the family of Chelonioida3 proper, containing three genera, namely, 

 Chelonia, Thalassochelys, and Eretmochelys. But as some of the most prominent 

 herpetologists recognize only one genus in this family, I will give below my reasons 

 for believing that the genera Thalassochelys and Eretmochelys are as well founded 

 in nature as the genus Chelonia pi'oper. 



Of the sub-order Amyda^, the family of the Trionychidte has only four re2:)resenta- 

 tives in America, which however bear a very peculiar relation to the other mem- 

 bers of the family; for while all the Trionyx of the old world are inhabitants of 

 the tropical fresh waters, or at least occur only south of the twenty-first isothermal 

 line, those of America are all found to the north of that very line, neither Central 

 nor South America nourishing a single Trionyx, while in North America they range 

 over the whole continent east of the Rocky Mountains, as far noi'th as the great 

 Canadian lakes and the upper St. Lawrence. 



If we were to judge by the opinion prevailing about the Chelydroidoe a few 

 years ago, it would appear that we had only one species of that flimily ; and yet 

 Dr. Holbrook, in his North American Herpetology, long ago described a second 

 species, under the name of Chelonura Temminckii, which seems to have remained 

 unknown to European writers, for all their references to this animal are either 

 expressed with doubt, or are evidently mere compilations, or abstracts from the 

 North American Herpetology. I have now in my possession a number of speci- 

 mens of this species weighing between ten and fifty jjounds, preserved in alcohol, 

 and also several skeletons made from specimens presented to me by Prof Baird, 

 Prof Chilton, Dr. Gessner, and Winthrop Sargent, Esq. I had, besides, an oppor- 

 tunity of seeing two living specimens in their native waters, in the neighborhood 

 of Mobile, one of which weighed about two hundred pounds, and many others 

 which were sent to me alive by Mr. Sargent and which I preserved alive during 

 the whole of last summer. I have, in addition, examined several very young ones, 

 preserved in alcohol, which were forwarded to me by Prof Baird and Dr. Nott. 

 I can, therefore, not only vouch for the specific distinction of the two species, but 

 am prepared to show that they difter generically, as a fuller comparison below, 

 illustrated with many figures, Avill prove. (See also above, p. 248.) 



The family of the Chelyoidaj has no North American representatives, nor has that 

 of the Hydraspidida3 ; but of the family of the Cinosternoidte we have two genera, 



