The skeleton of _†Trigonias_ was, on the whole, very much like that of the succeeding genus, _†Cænopus_, of the middle substage of the White River, but with the important exception that the front foot had four digits instead of three. The pollex, or first of the original five, almost always the first to disappear, had been suppressed, the third or median digit was already the largest of the series, both in length and breadth; the second and fourth, somewhat shorter together made a symmetrical pair, while the fifth, though much the most slender of all, was still functional and had retained all of its parts. In the hind foot the digits had been reduced to three. This arrangement, four toes in the manus and three in the pes, is the same as is found in the existing tapirs and in the Eocene perissodactyls generally, with only two or three known exceptions. In the Oligocene, on the other hand, all the genera except the †titanotheres, tapirs, †lophiodonts and †amynodonts were tridactyl both before and behind. [Illustration: FIG. 179.—Left manus of _†Trigonias osborni_. (After Hatcher.)] With _†Trigonias_ the definitely known history of the true rhinoceroses breaks off abruptly, and it is possible that that genus was an immigrant, though it is perhaps more likely that its ancestors existed in the upper and middle Eocene (Uinta and Bridger stages) of North America. Some fragmentary specimens from the Uinta beds, too imperfect for any definitive identification, are an encouragement to hope that the forerunner and direct ancestor of _†Trigonias_ may yet be discovered in that formation. It is also quite possible that one of the larger species of the genus _†Hyrachyus_, so abundant in the Bridger and going back to the Wind River, may take its place in the same series. 6. _†Hyracodontidæ. †Cursorial and †Aquatic Rhinoceroses_ The luxuriant diversification of the rhinoceros-stem was not exhausted by the many phyla of what we have called the true rhinoceroses. Two other series, very distinctly marked and rather distantly connected with the first, are yet to be considered. These two series, the †hyracodonts (in the narrow sense) and the †amynodonts, ran courses which, in certain respects, were singularly alike; both were of North American origin and one, the †hyracodonts, was entirely confined to that continent, while the other sent out late migrants, which entered Europe, no doubt through Asia, and both ended their careers before the close of the White River time. Their history was thus a brief one when compared with that of the true rhinoceroses, three phyla of which persist to the present day, though their geographical range is greatly restricted in comparison with what it was in the Miocene and Pliocene, when they ranged over every continent except Australia and South America.