2i8 WATER REPTILES OF THE PAST AND PRESENT 



which had developed the strange habit of concealing themselves 

 inside of their ribs, for that is literally what the turtles do. The 

 box or shell of an ordinary turtle is composed of the backbones and 

 ribs, to which are soldered a shell of bony skin plates above, with 

 the clavicles, interclavicle, and ventral ribs below. Except in 



Fig. Ill Fig. 112 



Figs, hi and 112. — Graptemys. (From Hay) 



Fig. III. — Carapace: cpi, cp2, etc., costal plates; csi, CS2, etc., costal scutes, 

 homy; ni, ni, etc., neural bones; mtp, nuchal bone; nus, nuchal scute; py, pygal 

 bone; spy, suprapygal; spy 2, second suprapygal, or postneural; roi, vs2, etc., verte- 

 bral scutes; 1, 2, 3-12 on right side, marginal scutes; i, 2, 3-12 on left side, peripheral 

 bones. 



Fig. 112. — Plastron: aft, abdominal scutes; an, anal scutes; ent, entoplastron 

 (interclavicle); epi, epiplastron (clavicle); fern, femoral scute; g, gular scute; hum, 

 humeral scute; hyo, hs^joplastron bone; hypo, hypoplastron; in, inguinal scute; 

 py, pygal bone; xiph, xiphiplastron. 



the strange leather-back turtle described farther on, these plates 

 form definite series. Ten of them cover the spines of the dorsal 

 vertebrae, in the midline, one over each, of which the turtles have 

 the smallest number of any known reptiles. There are eight on 

 each side over the ribs, united by suture with each other and with 

 the middle series; and, in addition, there are twenty-six bones 



