TBSTUDINATA. 



115 



The Jurassic genus Aplax Myr., is nearly as deficient in ossification of 

 carapace and plastron as Protostega, and is allied to the Chelydrid series, 

 which existed cotemporaneously and during the Cretaceous. Idiochelys 

 represents a rather more advanced form, with distinct marginal bones, and 

 with affinities to Chelydra of a decided character. It was probably its 

 ancestor. Allied to it we have such forms as Adocus and Baiina, which, 

 while furnished with fully ossified shell, still approach Chelydra in the 

 contracted form of plastron, and have several points of afiinity to the 

 Pleurodire series. From some common ancestor of these, sprang also the 

 true Pleuroderes of the Cretaceous, as Taphrosphys, while, by the omission 

 of most of the tendencies towards that series, we have Dermatemys, the 

 genus of EmydidfB neai-est to Adocus. From this point we pass to true 

 Emydidce, and thence, by the loss of a series of phalanges, to Testudo. From 

 Taphrosphys we pursue the Pleurodire series to the similarly modified type 

 Pelomedusa. 



The accompanying table expresses the relations indicated, supposed 

 to b§ genetic, and in accordance with the theory of evolution. (Types 

 beginning in the Jurassic in italics; Cretaceous to recent, small capitals); 

 Eocene to recent, spaced; recent only, Roman. The apparent reversal 

 of the order of time displayed by the Jurassic and Cretaceous families is 

 an indication of our ignorance of the Jurassic Testudinata. 

 Cistudinidae. Cmosternidse. Pelomedusidse. Chelydidae. PoDOCNEMiDn>.aE. 



udinidae. Hydraspididse. 



Chelydridce. 



\ 



EmydidcB. 



Baenidte. 



Cheloniid^. Pbopleukid^. 



ADOCrD.^E. 



Sphargididce. Pbotostegid^. Trionychidte. 



/ 



