EXOSKELETON 



43 



minal ribs" (cf. p. 42) are present in Hatteria in the rectus 

 abdominis muscle, each consisting of a median and of a paired 

 lateral bar, and being considerably more numerous than the body 

 metameres. In Crocodiles similar bars are present, their number 

 corresponding with that of the ribs: they no longer reach the 

 middle line, and with the exception of the first, each consists, on 

 either side, of two firmly united portions. Evidently these structures 

 have here begun to undergo reduction. 



Crocodiles, many Lizards (Anguis, Cyclodus, Scincus), and more 

 especially Chelonians, exhibit a well-developed dermal skeleton, 

 the scutes composing which cover the body more or less completely. 

 In the last-mentioned group there is a dorsal and ventral shield 

 (carapace and plastron} consisting of numerous more or less closely 

 united pieces, and completely encircling the body (Fig. 33). The 



FIG. 33. A, CARAPACE, and B, PLASTRON OF A YOUNG Testudo c/rceca; 



C, PLASTRON OF Chelone midas. 



G, costal plates ; E, entoplastron ; Ep, epiplastron ; Hp, hypoplastron ; Hy, 

 hyoplastron ; M, marginal plates ; N, neural plates ; Np, nuchal plate ; 

 Py, pygal plates ; R, ribs ; Xi, xiphiplastron. ( V indicates the anterior, 

 and If the posterior margin.) 



plastron, the larger posterior portion of which probably corresp 

 to greatly modified abdominal ribs, arises entirely by ossification 

 of the derm ; while parts of the carapace have a close relation to the 

 endoskeleton (neural arches and ribs), which early in development 

 became broadened out into plates. At the same time, the inter- 

 costal muscles disappear completely, and the muscles of the back 

 undergo partial reduction, while the articulations of the vertebrae 

 and ribs disappear. The nuchal, pygal, and marginal plates (cf. 

 Fig. 33) are entirely independent of the endoskeleton that is, are 

 purely exoskeletal bones ; while the costal and neural plates 

 correspond to much thickened periosteal bones developed around 

 the cartilaginous ribs and neural spines respectively : though they 

 are subcutaneous in position, they have nothing to do with the 

 skin genetically. 



