192 REPTILIA. 



are so excavated in front and behind as to leave apertures 

 for the head, tail, and fore and hind limbs. The limbs and 

 tail can almost always be withdrawn at will under the 



Fig. 547. — Skeleton of Tortoise (-Ehi^/s Enropaa), the plastron lieing removed, ca, Carapace ; 

 V, Ribs, greatly expanded, and united by their edges ; a-. Scapular arch, placed within the 

 <arapaee, and carrying the fore-limbs ; p, Pelvic arch, also placed vt-ithin the carapace, and 

 carrying the hind-limbs. 



shelter of the thoracico-abdominal case formed in this way 

 l)y the carapace and plastron, and the head is also generally 

 retractile. 



The carapace or dorsal shield (fig. 548) is composed of 

 the following elements : — 



1. The spinous processes of the dorsal vertebrae, which 

 are much flattened out laterally and form a series of broad 

 plates, which are eight in number, and are termed the 

 " neural plates " {n). 2. The ribs (r, r) are united wdtli 

 broad and flattened plates of bone (c\ c'), which are con- 

 nected with one another by lateral sutures, and are known 

 as the " costal plates." In some cases, however, the costal 

 plates, instead of being united by the whole of their lateral 



