While there were many small fishes in the Niobrara seas which the Protostegas inhabited, the most striking thing in the fauna is the great abundance of molluscal shells, especially _Ostrea congesta_. And with them were great hordes of larger pelycypod mollusks, some of them of enormous size. Some of the largest reach a diameter of nearly four feet, with shells so thin that one can hardly understand how they could have supported such large, oyster-like creatures. One can imagine that such shell-fish might have afforded an almost inexhaustible source of food for the large turtles; and several times the writer has found remains of _Protostega_ associated with such shells. From all of which evidence it seems very probable indeed that Dr. Wieland is right in imputing to these gigantic turtles a shell-feeding habit, a habit which required neither speed nor great prowess; and perhaps the formidable beak was used more in social quarrels than for food-getting. That these marine turtles departed from the usual reptilian habit of laying their eggs upon land is improbable. The tortoise shell turtles of the Bahamas lay three or four hundred eggs in a hollow scooped out in the sand and then leave the young to their own devices; certainly many a one is gobbled up by birds of prey or other enemies on their way to the water. Perhaps the young _Archelon_ lost its hind leg in some such mishap. LEATHER-BACK MARINE TURTLES The most remarkable member of the Chelonia now living is _Dermochelys coriacea_ (Fig. 128), the great leathery or leather-back turtle of the warmer parts of the Atlantic, Indian, and Pacific oceans, the sole member of the family Dermochelydidae. It is the largest of all living turtles and the most thoroughly aquatic of all, whether living or extinct. It sometimes reaches a length of six feet, or half that of the largest known extinct forms, and weighs a thousand or more pounds. Agassiz saw a specimen that he said weighed a ton. Unlike other turtles, it has a carapace quite peculiar to itself, composed of a layer of thin, irregularly polygonal bones forming a mosaic, completely hidden in the thick skin, and entirely free from the skeletal bones beneath them. The larger of these skin bones form seven rows above, which appear in the living animal as sharp keels running the whole length of the shell. On the under side there are five rows of smaller-sized bones, under which there are vestiges of bones representing the normal plastron of turtles. The limbs are powerful, flattened paddles, not unlike those of _Eretmochelys_, but wholly destitute of claws. The front paddles are much larger than the hind ones; the humerus is long and flattened, and the digits are elongated. The leather-back is a powerful and effective swimmer, going long distances. Its habits are not well known; its food is chiefly fish, crustaceans, and mollusks. [Illustration: FIG. 128.—_Dermochelys coriacea._ (From Brehm.)]