Water Reptiles of the Past and Present

by Samuel Wendell Williston

Figs. 111 and 112.—Graptemys. (From Hay.)

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.

Fig. 113.—Toxochelys;
coracoid and scapula.

In the skeleton contained within the box thus formed is the very peculiar pectoral girdle, composed of scapula and coracoid, the scapula so peculiar that the controversy as to its homologies is not yet quite settled. Most authors, until recently, have believed that its peculiar shape ([Fig. 113]) is due to the co-ossification of the procoracoid with the scapula instead of as usual its loss or union with the true coracoid, so called. We are now pretty sure that this is not true, since in reality there is no such bone as the procoracoid, the bone so called being the real or true coracoid; and because, in the second place, the long anterior projection called the procoracoid is really only an outgrowth of the scapula itself and not a fused, separate bone. Hence the bone is properly called the scapula-proscapula, and not the scapula procoracoid, as it usually has been. The coracoids are elongate and flattened and without the usual supracoracoid foramen, so generally present in reptiles. The only other reptiles having a similar structure of the scapula are the plesiosaurs, and it has been because of this apparent resemblance that some good paleontologists have thought the turtles and plesiosaurs were allied. The sacrum is composed of two vertebrae only, and the pelvis of the usual three bones, the ilium, the ischium, and the pubis, all covered over by the shell.

Fig. 114.—Pelvis of Chelone, from below:
pu, pubis: is, ischium; il, ilium (in acetabulum).

In every known turtle the neck is composed invariably of eight vertebrae, but they are peculiar in many respects. In the earliest known turtles the neck vertebrae were, as would be supposed, biconcave, but they soon became very variable in all; in each neck some are biconcave, some biconvex, some opisthocoelous, and some procoelous. And Dr. Hay tells us that the neck has increased in length in the later forms.