196 



EEPTILIA 



CLASS III 



inner digits clawed. Marginal hones absent or forming an incomplete series, not 

 connected wüh the ribs. Nim ylastral Clements; epiplastra separated from the 

 hyoplastra by the y-shaped entoplastron. 



The group of three-clawed inud-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 structure of 

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

 remain separate throughout life. Yacuities 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. 



Family 1. Trionychidae. Gray 



Skull depressed, the small orbits directed 



more or less upwards and appvximated 

 towards the nares ; temporal fossae 

 completely open, and squamosal and 

 supra-occipital with very long pos- 

 terior processes. Plastron totally 

 distinct from the carapace, with 

 large vacuities. Humerus much 

 cnrved. Eocene to Eecent. 



The existing members of 

 this family, numbering in all 

 about twenty-five species, are of 

 fluviatile 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 

 Fig. 300. Miocene, and still inhabits the 



rivers of Asia, Africa, and 

 North America. Axestus and 



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



related genera. Chitra, G-ray, is Pliocene and Recent. 



Trionyx styriactis, Peters. Miocene lignites ; Ebiswald, Styria. 

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



Sub-Order B. CRYPTODIRA. Dumeril. 



Dorsal vertebrae and ribs fused and expanded into bony plates forming a carapace. 

 Neck bending by a sigmoid ciirve in a vertical plane. Gervical 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 01^ absent ; descending process of the 

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



