Because of certain peculiarities, they are usually classed in a separate suborder all their own, the Trionychoidea, especially distinguished from the Cryptodira, which in general they resemble in most respects, aside from the absence of the usual horny dermal plates, in the lack of a marginal row of bony plates around the carapace—not a very important distinction. Less than thirty living species are known, all of them exclusively, or chiefly of fresh-water habit. Six species are known from North America; the remainder inhabit Africa, south of the Sahara Desert, southern India, and most of the East Indian islands; none is known from Australia. No species lives in South America and none is known to have lived there in past times. During Eocene, Oligocene, and Miocene times these fresh-water turtles lived in the region of Europe in great numbers, but for some inexplicable reason they became extinct there and never returned. Nearly seventy species of the Trionychoidea, belonging in two families, are described by Dr. Hay from the Tertiary rocks of North America, more than twice the number now living throughout the world. Some of these were of relatively large size, measuring fully two feet in the length of the shell. And in some places they must have been very abundant. The writer has seen, in the Bad Lands of the Continental Divide, their weathered-out remains so numerous that they might be raked into windrows miles in length along the sloping bluffs, all in small fragments, for their bones, like those of most turtles, are only loosely united by sutures and readily drop apart before fossilization. Their shells may be readily distinguished from those of all other turtles by the granulated, pitted, or sculptured exterior surface, that was covered by the skin in life; other turtles have the surface smooth below the horny shields, the margins of which are marked on the bones by grooves or sulci; the few marine turtles of the past that were probably covered with a soft skin instead o£ horny shields had the shell smooth and much less completely ossified. [Illustration: FIG. 131.—_Aspideretes_, a trionychoid turtle from the Eocene of New Mexico; front leg. (From Hay.)]