iii8 



CLASS REPTILIA. 



fossil state. In the second subfamily, or Trionychince, the hyo- and 

 hypoplastrals remain distinct throughout life; the pterygoid does 

 not join the opisthotic behind the labyrinth ; the sculpture usually 

 consists of sinuous ridges or pits ; and marginal bones are never 

 developed in the carapace. The type genus Trionyx is widely dis- 

 tributed at the present day, and 

 extends downwards into the Lower 

 Eocene of both Europe and the 

 United States, -and is also repre- 

 sented in the Upper Cretaceous of 

 the latter area. Many species are 

 remarkable for showing dimorphic 

 types of skull, which in one modi- 

 fication has a broad palate adapted 

 for crushing, while in the other the 

 palate is very narrow. The Indian 

 species, in which the skulls are not 

 subject to this variation, have two 

 short neural bones between the first 

 costals ; while in the recent Ameri- 

 can forms there are but seven pairs 

 of costals. All the European fossil 

 forms agree with the majority of 

 species in having only one long 

 neural bone between the first cos- 

 tals. In Europe this genus appears 

 to be unknown above the Upper 

 Miocene of (Eningen, and it is ex- 

 tremely abundant in the Upper 

 Eocene (Lower Oligocene) of Hord- 

 well. The existing Indian species 

 occur in the Pleistocene and Plio- 

 cene of that country. Of the American Tertiary species at least 

 some have eight costals, and in some cases only six neurals. The 

 Lower Eocene and Upper Cretaceous forms described by Professor 

 Cope as Plastomenus may be included in the type genus, since the 

 thickening of the plastron can scarcely be regarded as a generic 

 character. The same remark applies to the Eocene Axestus, of the 

 United States, in which the plastron is smooth, as in some existing 

 forms. An apparently distinct type, in which the outer margin of 

 the costals is deeply grooved, occurs, however, in the English 

 Upper Eocene, which has been named Aulacochelys. Finally, the 

 exclusively Oriental genus Chitra is represented in the Pliocene 

 and Pleistocene of India by remains of the one existing species C. 

 indica, the largest member of the family. 



Fig. 1022. — Frontal aspect of the cranium 

 of Trionyx gangeticus ; from the Pleisto- 

 cene of India. Reduced, sup, Supraocci- 

 pital ; par, Parietal ; pt.f, Postfrontal ; fr, 

 Frontal ; pr.f, Prefrontal + nasal ; mx, 

 Maxilla; ju, Jugal ; q.j, Quadratojugal ; 

 or, Orbit. The processes on either side of 

 the supraoccipital are formed externally by 

 the squamosal, and internally by the opis- 

 thotic, of which the inner border articulates 

 with the supraoccipital ; externally to the 

 parietal is the prootic. 



