196 



EEPTILIA 



CLASS III 



imier digits daioed. Marginal hones absent or forming an incomplete series, not 

 connected with the ribs. Nine plastral elements; epiplastra separated from the 

 hyoplastra by the \/-shaped eiitoplastron. 



The group of tliree-clawed mud-turtles, which appears first in the Upper 

 Cretaceous of the United States, and next in the lowest Tertiary strata of 

 both Europe and North America, exhibits the most generalised structui'e of 

 all Chelonians. The shell is incompletely ossified, and the plastral elements 

 remain separate throughout life. Vacuities persist in the carapace, and various 

 portions of the skeleton afford evidence of imperfect ossification. There are 

 no free nasals, and no parieto-squamosal arch ; the descending processes of 

 the prefrontal may or may not be connected with the vomer, the epipterygoids 

 are free, and the dentaries distinct. The stapes is entirely surrounded by 

 the quadrate. The pterygoid is broad, without lateral expansions, separating 

 the quadrate and basisphenoid. Only one family is recognised. 



ramily 1. Trionychidae. Gra}-. 



Skull depressed, the small orbits directed more or less upwards and approximated 



towards the nares ; temporal fossae 



^^^z^'y^C^ completely open, and squamosal anid 



supra-occipital with very long pos- 

 terior processes. Plastron totally 

 distinct from the carapace, with 

 large vacuities. Humerus much 

 curved. Eocene to Recent. 



The existing members of 

 this family, numbering in all 

 about twenty-five species, are of 

 fiuviatile habit, and distributed 

 in the tropical and temperate 

 zones of all the continents 

 except South America. Most 

 of the fossil forms belong to 

 the genus Trionyx, Gray (Fig. 

 300), which survived in Europe 

 throughout the Eocene and 

 Miocene, and still inhabits the 

 rivers of Asia, Africa, and 

 North America. Axestus and 



Plastomenus, Cope, from the Eocene of Wyoming and New Mexico, are closely 



related genera. Chitra, Gray, is Pliocene and Recent. 



Trionyx . it yriacns, Peters. Miocene lignites ; Ebiswald, Styria. 

 Imperfect carapace and aist of ribs. 1/4 (after Peters). 



Sub-Order B. CEYPTODIRA. Dumeril. 



Dorsal vertebrae and ribs fused and expanded into bony j^lates forming a carapace. 

 Nech bending by a sigmoid curve in a vertical plane. Cervical vertebrae without or with 

 mere indications of transverse processes. Posterior cervicals with two articular faces. 

 Skull with descending parieto-pterygoidal processes (except in the Dermochelyidae). 

 No free nasals; parieto-squamosal arch present or absent ; descending process of the 

 prefrontals connected with the vomer ; stapes in an open groove, entirely covered by the 



