In other words, Versluys believes that Hay’s and Wieland’s views of the primitive double layer of exoskeletal bones is essentially correct, but that _Dermochelys_ was derived from later forms in which some of them, only, as in _Archelon_, had remained. Baur’s contention that “_Dermochelys_ is not the least, but the most specialized marine turtle” seems to have been fully justified. RIVER TURTLES. TRIONYCHOIDEA No reptile is more familiar or more exasperating to the river fisherman than the turtle, variously known as the river, soft-shelled, or mud turtle. It lives, often in great numbers, in most of the rivers, ponds, and bayous of the interior east of the Rocky Mountains, and especially in those of sluggish current and muddy bottoms. It is voraciously carnivorous in habit, feeding upon the smaller fish, mussels, and such other living food as it can capture. With its long, sinuous neck and snake-like head, and soft, mottled skin, it is repulsive enough to most persons, but is especially annoying to the fisherman, since it devours with impunity his bait so long as he feeds it, and can seldom be caught on the hook because of its hard and bony mouth, in which only by good luck will the hook catch. And the luckless string of fish that the fisherman leaves in the water may be almost completely devoured in a few hours by these fiercely predaceous feeders. However, if so annoying while seeking for better game, it in part makes up for the annoyance it causes by furnishing in its own body a not unpalatable food for those who like to eat reptiles. [Illustration: FIG. 129.—_Trionyx_, river turtle. (By permission of the New York Zoölogical Society.)] The river turtles will be readily recognized from the accompanying illustration (Fig. 129). They are very flat, covered with a soft, smooth skin, with a long, sinuous neck and a small, snake-like and vicious-looking head which has a protuberant snout with the external nasal orifice at its end. Their feet are webbed and somewhat paddle-like, but always with three stout claws—whence comes the name of the group—on the anterior digits, which are used for burrowing in the mud and excavating holes for their eggs. These turtles burrow more or less in the mud, with the long neck free, lying in wait for their prey, and coming to the surface from time to time to breathe. As the shape of the body and the paddle-like feet would suggest, they are active swimmers and purely aquatic in habit, never leaving the water unless compelled to. They bury their hard-shelled eggs on the shores only a few feet from the water, and leave them to their fate. If the pools in which they live dry up, they burrow deeply in the mud and await the rains and floods. In captivity they feed upon all kinds of food, vegetable as well as animal, and are active and aggressive. [Illustration: FIG. 130.—_Aspideretes_, a trionychoid turtle from the Basal Eocene of New Mexico; skull from above. (From Hay.)]