1 882.] The Reptiles of the American Eocene. 987 



from the Bridger and Washakie. Of these, six are common to 

 the two formations, as indicated by imperfect material, leaving a 

 total of forty-two. Three genera, linns, Tii'ouyx, and t Plasto- 

 menus hold over from the Cretaceous period, while six appear for 

 the first time. Of these, five genera are not known to continue 

 later than the Eocene period. In order to understand their rela- 

 tion to members of the order which lived in other periods, I give 

 a general sketch of the classification of the Testndinata. 



Three primary divisions of this order are generally recognized. 

 The first of these, the Athccce, includes one living and one extinct 

 genus. It is characterized by the absence of the combined coos- 

 sification of ribs and skin, which form the carapace of other tor- 

 toises, and by the annular shape of the inferior shell or plastron, 

 which has no connection with any other part of the skeleton. In 

 the recent genus Sphargis (the leather-back turtle), the skin is 

 filled with small osseous plates, which form by their union a dor- 

 sal shield. 



The other two suborders have the usual carapace and plastron, 

 but they differ in some curious particulars. The greater number 

 of the tortoises of the southern hemisphere cannot draw their 

 heads into their shells, but throw them round sideways when they 

 wish to protect themselves. As if to compensate for this defect, 

 they have the pelvis united by suture below to the plastron, which 

 insures strength but not elasticity. Then they have a peculiar 

 frontal bone, and an additional scutum of the front of the plastron. 

 This group is called the Pleurodira. In North America its species 

 are only known as fossils of the Cretaceous period, and will there- 

 fore not be further mentioned here. The group which has char- 

 acterized the Northern Hemisphere since the beginning of Ter- 

 tiary time, although some of its members appeared earlier, is the 

 third division of tortoises, the Cryptodira of Dumeril and Bibron. 

 They draw the head within the shell by a sigmoid flexure of the 

 cervical vertebra: ; the pelvis is not coossified with the plastron ; 

 the frontal bone reaches the palatine below, and there is no addi- 

 tional scutum of the plastron. 



Three prominent divisions or tribes may be recognized among 

 the Cryptodira, by the various modes of articulation of the 

 plastron with the carapace. In the first, the breast-plate sends 

 out a few digitations to the edge of the dorsal shield on each side, 



