Chap. I. SPECIAL CLASSIFICATION OF TESTUDINATA. 249 



they agree Avith the Chelydroids and Ilydraspids, while, in that respect, they differ 

 strikingly from the Cinosternoids, the margin of -which has a tendency to round 

 itself up and turn inwards, as is also the case in the genuine Testudos, which 

 constitute the last and highest family of the whole order. We shall presently 

 see that among our native Emydoids there are two species which have generally 

 been referred to the same genus, the Cistudo Carolina and the C. Blandingii, one of 

 which, however, is a genuine fresh-water species of the genus Emys, while the other 

 is entirely terrestrial. 



The family of Testudinina has always been circumscribed within its natural 

 limits, ever since it was first distinguished. 



Before we proceed to an analysis of the genera of the North American Testu- 

 dinata, we may now recapitulate the results at Avhich we have arrived respecting 

 the general classification of the whole order, as follows : — 



Order, Testuuinata, Klein. 



1st Suh-onlcr, Chelonii, 0/yj. With two families, Chelonioidae and Sphar- 



gididae. 

 2d Sub-order, Amvd.e, 0pp. With seven families, Trionychida?, Chelyoidas, 



Hydraspidida), Chelydroidaj, Cinostemoidae, Emydoidag, and 



Testudinina. 



It should further be remarked that, as in all larger divisions of the animal 

 kingdom, these families are not equally related to one another. The affmity of 

 the Trionychida3 to the other families is not so close as that which brings the 

 Cinosternoids near the Chelydroids, or certain Emydoids near the Testudinina, or 

 the Hydraspids near the Chelyoids; yet after testing all their characters as far as 

 my opportunities permitted, I have come to the conclusion that the seven groups 

 above enumerated as families under the head of the sub-order Amyda? are truly 

 natural families, characterized by different typical forms, which are defined bv 

 structural peculiarities, as we shall see more fully hereafter. The inequality among 

 these families, in the degree of their relationship, is a feature which will appear 

 objectionable, as long as the opinions respecting the supposed symmetry and equality 

 of the natural divisions of animals, entertained at present by many scientific men. 

 continue to prevail ; and imtil the inequality of endowment characteristic of all 

 organized beings is recognized as the law prevailing in the organic kingdoms, from 

 the humblest individual to the most comprehensive types. 



My opportunities of investigation do not justify me in attempting to charac- 

 terize all the genera of the order of Testudinata. I must limit myself, in this 

 part of my subject, to a general review of those which have representatives in 



32 



