308 CLASSIFICATION OF VERTEBRATES. 



so that it can still further protect these parts. In the atheca 

 the body is covered by a thick leathery skin ; but in the others 

 it bears bony epidermal scales or plates, the arrangement of 

 which is of systematic importance. These plates in one species 

 (Eretmochelys imbricata) furnish the well-known ' tortoise 

 shell.' There is a median and a pair of lateral rows of plates 

 on the dorsal surface, while around the edge is a series of margi- 

 nal plates. Beneath these scales comes the dermal skeleton of 

 bony plates , which, however, do not correspond in position to the 

 epidermal coating. In the atheca this dermal skeleton is free 

 from the ribs and vertebrae, and consists of longitudinal rows 



of polygonal dermal ossicles. In all 

 others the dorsal portion of the armor, 

 the carapace, consists of a median row 

 of (usually eight) neural plates, each 

 being the expanded end of a neural 

 spine of a vertebra. In front of the 

 first neural is a nuchal plate, while be- 

 hind the last are two or three pygal 



FIG. 305. " Plastron of plates, these being unconnected with 

 Cheione midas, after Zittel. the vertebrae. On either side and cor- 



< clavicle; ,, episternum; ndi to the neumls are the CQstal 



hp, hypoplastron ; Ay, hyo- x & 



plastron; x, xiphopiastron. plates, each fused to a rib; around the 

 margin of the carapax is a series of 



marginal plates, the nuchal and posterior pygal forming parts 

 of the series. The ventral portion of the armor, the plastron, 

 usually consists of nine plates, in front a median episternum 

 (entoplastron) flanked on either side by a clavicle (epiplastron), 

 while behind, on either side, follow hyoplastron, hypoplastron, 

 and xiphiplastron. Occasionally the episterum is lacking. All 

 of these plates, except neurals and costals, are membrane bones. 

 Besides the characters quoted in the diagnosis, the absence 

 of ali-, pre-, or orbitosphenoid ossifications ; the distinct pro- and 

 opisthotic bones ; and the absence of an os transversum, are 

 distinctive. The epiotic is fused to the supraoccipital ; a tem- 

 poral fossa is usually present, but as in chelydosauria, it 

 may be absent, or again, as in Chelonc, it may be arched over 

 by an expansion of the parietal reaching to the squamosal. 



