REPTILIA 



227 



usually armed with a claw. As a rule the feet are webbed as in aquatic 

 birds. The skin of the head is usually smooth and scaleless, as is 

 also the neck in most species; but the rest of the body is usually 

 covered with scales, except the base of the thighs. The tail is as a 

 rule poorly developed, but in the more primitive types, as for example 

 the snapping turtles, it may retain its primitive reptilian proportions. 



THE ARMATURE 



The carapace and plastron (Fig. 129) are, in most of our modern 

 chelonians, somewhat stereotyped structures; they have settled down 



A B 



FIG. 129. A, Carapace; B, Plastron of tortoise, Graptemys. Capital letters 

 refer to chitinous scales or scutes, small letters to bony plates whether cartilag- 

 inous or dermal. A6, Abdominal scute; An, Anal scute; Cl~4, costal scutes, 

 cl-8, costal plates; e, epiplastral plate, en, endoplastral plate; F, femoral scute; 

 G, gular scute; H, humeral scute; ho, hyoplastral plate; hp. hypoplastral plate; 

 7, inguinal scute; M, marginal scutes; m, marginal plates; Nl-5, neural scutes; 

 nl-8, neural plates; NU, nuchal scute; pr, 1, 2, procaudal plates; X, axillary 

 scute; x, xiphiplastral plate. (From Newman.) 



upon a very definite arrangement of the principal units of structure. 

 The carapace (Fig. 129, A) is composed of two kinds of bony elements 

 (dermal and cartilaginous) and corneous scutes or shields. The main 

 part of the bony carapace is composed largely of the much broadened 



