194 



EEPTILIA. 



Both the carapace and plastron are covered by a series of 

 horny plates (rarely wanting), which are developed in the 



Fig. 549.— Bones of the pla.stron of tlie Loggerhead Turtle (Chclonc caovamw). s, Entoster- 

 nal ; es, Episternal ; /ts, Hyosternal ; j«, Hyposternal ; xs, Xiphisternal. (After Owen.) 



epidermis, and which are perfectly distinct from the bones 

 which they cover. As encasing the upper surface of the 

 carapace, these plates have a general arrangement conforming 

 with that of the bony plates beneath, though there is no 

 numerical correspondence between the two. Thus the cara- 

 pace, as we have seen, consists of (1) a median series of 

 " neural " plates developed from the vertebra? ; (2), a lateral 

 series of " costal " plates on each side, corresponding with and 

 largely formed by the ribs ; and (3), a peripheral series of 

 "marginal" plates (see fig. 550). Similarly, the epidermic 

 plates (fig. 550) are arranged in (1) a median, "vertebral" 

 or " neural " series ; (2), a lateral series on each side of 

 "costal" scutes; and (3), a series of "marginal" scutes. 

 The " vertebral " scutes, however, are only five in number ; 

 and each series of " costal " scutes consists only oi four pieces, 

 so that the number of epidermic plates is much smaller than 

 that of the bony plates beneath. The " marginal " scutes, on 

 the other hand, correspond in number with the " marginal 



