CuAP. I. SPECIAL CLASSIFICATION OF TESTUDINATA. 247 



early as 1825, Gray had distinguished them as a family, which was adopted by 

 Bell, by Fitzinger, by Canino, and by Dunieril and Bibron, the latter only chang- 

 ing the name of TrionychidfB into that of Potamides. This group constitutes one 

 of the most natural liunilies among Turtles, at once recognized by the flat, thin 

 shield of an elegant oval form, by the long neck, the pointed head, and project- 

 ing nose. But the question is farther, whether this family can be associated in 

 one sub-order with Einys and Testudo, or not. If we consider the total absence 

 of scales, the imjierfect ossification of the shield, the absence of ossification of 

 the margin, or the limited extent to which it is ossified, the slight protection of 

 the jaw by a small, horny sheath, we cannot fail to recognize characters of 

 inferiority in these features, when comparing them with those of the Emyds and 

 Testudos ; and I would not hesitate to consider that family, though exhibiting alone 

 such characters, as forming a sub-order of the same organic value as that of the 

 Chelonii, did we not observe similar differences between the Sphargidida; and the 

 true Chelonioidje, and had we not learned long ago that any amount of difference 

 existing between two groups never constitutes a difference of kind. The question 

 might even be raised, whether the very imperfect ossification of Aspidonectes, and 

 especially the total absence of marginal scutes, do not place them below the Che- 

 lonioida?. But when it is remembered that among Chelonii the ossification is still 

 more imperfect, at least in Sphargis, and that the skin is as destitute of scales 

 in this genus as in Trionyx, there can be little doubt left that all the peculiarities 

 of Trionyx are only family characters. The structure of their limbs is almost as 

 perfect as in Emys, and, as we shall see hereafter, their whole organization brings 

 them close to the Emydoids, Chelys and Chelydra forming the intermediate links. 

 The remaining two types, Emys and Testudo, evidently stand, in every respect, 

 highest among the Amydoe or Digitata, and close the series of Testudinata. 



I greatly question the propriety of separating Trionyx, Chelys, Emys, and Tes- 

 tudo as groups coequal with Chelonia, as so many herpetologists do. There are 

 many modifications in the degree of separation of the fingers among them, which 

 alone do not establish differences of the same kind nor of the same desrree as 

 between these on one side and Chelonia on the other, even though as to ossification, 

 development of scales, and armature of jaws, Trionyx differs somewhat from Emys 

 and Testudo, while the two latter agree as closely as possible with one another. 

 I would, therefore, consider Testudo, Emys, Chelys, and Trionyx together as one sub- 

 order, showing the whole number of sub-orders among Testudinata to be only two, 

 Chelonii and Amyd.k, — the latter, however, including a nund)er of distinct families, 

 as I shall demonstrate presently. 



The same argument which has led us to consider Sphargis and Chelonia as 

 distinct families, leads naturally to the separation of a number of families among 



