GEOLOGICxiL SUEVEY OF THE TEREITOETES. 



649 



These genera had a common origin near the Jurassic predecessor of 

 Protostega. 



Trionyx 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-ossified carapace, 

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

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

 of the Cretaceous, which adds Ohelydrine and Pleurodire characters in 

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

 turn near to Anostira. 



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

 of carapace and, plastron as Protostega^ and is allied to the Ohelydrid 

 series, which existed cotemporaneously and during the Cretaceops. 

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

 ginal bones, and with affinities to Glielydra of a decided character. It 

 was probably its ancestor. Allied to it we have such forms as Adocus 

 and Bcena, which, while furnished with fully ossified shell, still present 

 the contracted form of plastron seen in Idiochelys and Glielydra, and 

 several points of affinity to the Pleurodire series. From some common 

 ancestor of these sprang also the true Pleuroderes of the Cretaceous, 

 as- Taplirospliys ; while, by the omission of most of the tendencies to- 

 ward that series, we have the genus of Emydidm near' to Adocus^ 

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

 the loss of a series of phalanges, to Testudo. From Taprosphys we pur- 

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



The accompanying table expresses the relations indicated, supposed 

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



Testuddjid^. 



Eniydiclce. 

 Dermatemys. 



Adoci 



Pelomedusidje. 



Hydeaspidid^. 



Bcenidce. 



Podocnemidictae. 



Anostira. Chelydrid^. Chelonides (?) P r o j) 1 e u r i d a 6 



Cheloniidcs. 



Plastomemis. Idiochelys 



Sphargidid^. Protostegidae. Trionyctidae. 



Aplacidffi. 



XoTE — Jnrasaic types in roman ; Cretaceous to recent, spaced; Eocene to recent, italics ; Miocene 

 to recent, small capitals. 



