GEOLOGICAL SURVEY OF THE TERRITORIES. 



649 



These genera had a common origin near the Jurassic predecessor of 

 Protostega. 



TrioM?/j? appears to represent another point of departure. Its plastron 

 presents a grade of development near to that of Propleura., and its nine 

 costal bones have a similar significance. Its half-ossitied carapace, 

 wanting the marginals, is inferior. Its peculiar sculpture is seen iu the 

 Eocene Anostira, (which is much like Cliehjdra in form,) and in Adocus 

 of the Cretaceous,' which adds Ohelydrine and Pleurodiro charactei^ in 

 a remarkable manner. It is closely joined by Plastomenu.s, which is in 

 turn near to Anostira. 



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

 of carapace and ])lastron as Protostega, and is allied to the Chelydrid 

 series, which existed cotemporaneously and during the Cretaceous. 

 Idiochclys represents a rather more advanced form, with distinct mar- 

 ginal 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 Bcena, Avhich, while furnished with fully ossified shell, still present 

 the contracted form of plastron seen in Idiochelijs and Chelydra, and 

 several points of afliuity 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 to- 

 ward that series, we have the genus of Emydkkc near to Adocus, 

 Dermatemys. From this point we pass to true Einydidce, and thence, by 

 the loss of a series of phalanges, to Testndo. Froai Taprosphys we pur- 

 sue the Pleurodire series to the similarly modified type, Petomedusa. 



The accompanying table expresses the relations indicated, supposed 

 to be genetic, and iu accordance with the theory of evolution : 



TESTUDINID.S. 



Hmydidce. 



Dermatemys. 



Bccnidic. 



PELOMEDUSIDiE. 



Hydraspididje. 



Podocuemididae. 



Cheloniidce. 



Anostira. Ciielydkid.e. Chelouides (?) P r o p 1 e u r i d a e 



Plastomenus. Idiochelvs 



Sphakgidid^. Prot OS te gidae. Tr ion y oL- i d a c. 



Aplacidaa. 



Note — Jurassic types in roman ; Cretaceous to recent, spaced: Eocene to recent, italics ; Mincerio 

 to recent, small capitals. 



