FIG. 111.—Carapace: _cp1_, _cp2_, etc., costal plates; _cs1_, _cs2_, etc., costal scutes, horny; _n1_, _n2_, etc., neural bones; _nup_, nuchal bone; _nus_, nuchal scute; _py_, pygal bone; _spy_, suprapygal; _spy 2_, second suprapygal, or postneural; _vs1_, _vs2_, etc., vertebral scutes; 1, 2, 3-12 on right side, marginal scutes; 1, 2, 3-12 on left side, peripheral bones. FIG. 112.—Plastron: _ab_, abdominal scutes; _an_, anal scutes; _ent_, entoplastron (interclavicle); _epi_, epiplastron (clavicle); _fem_, femoral scute; _g_, gular scute; _hum_, humeral scute; _hyo_, hypoplastron bone; _hypo_, hypoplastron; _in_, inguinal scute; _py_, pygal bone; _xiph_, xiphiplastron.] Were there no turtles living we should look upon the fossil forms as among the strangest of all vertebrate animals—animals 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 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 mid-line, 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 surrounding them and attached to them. All these bones compose what is called the carapace, which forms a complete roof in the more terrestrial types, more or less imperfect, with vacuities between the bones in the marine forms. On the under side, in addition to the clavicles and the interclavicle, there are three pairs of enlarged ventral ribs that go to form the plastron, solid and complete in land turtles, with openings in the water forms. And in the land forms the plastron is more or less firmly united with the carapace. [Illustration: FIG. 113.—_Toxochelys_; coracoid and scapula.]