In order to find any American members of this family, it is necessary to go back to the lower Pliocene, where a great abundance of them is encountered, representing, according to Osborn’s view, four or five phyla; and just as in the case of the horses of the same formation, they were an assemblage curiously made up of progressive and old-fashioned, conservative genera,—some were persistent native stocks, others the descendants of immigrants from the Old World, which reached America in the middle Miocene. There was great variety of form, size and proportions among these animals, North America at that time having a larger number of genera and species than Africa and Asia combined have now. Some were quite small, some large, though none equalled the larger modern species. Some of the genera had relatively long legs, but in one genus, _†Teleoceras_ (Fig. 125, p. 230), an Old World type, they were most grotesquely short, the belly almost touching the ground, as in a hippopotamus. Most of these rhinoceroses were hornless, but _†Teleoceras_ had a small horn on the very tip of the nose. In consequence of the lack of horns, the nasal bones were thin and weak, in marked contrast to the massive, convex nasals of the modern species, and, for the same reason, the upper profile of the skull was nearly straight. Except for minor details, the dentition was in very nearly the modern stage of development; there was a single trenchant upper incisor on each side, a procumbent lower tusk and between the tusks a pair of small incisors; the other incisors and the canines were already lost. One genus (_†Peraceras_) had lost all the upper front teeth. The grinding teeth had the same character as in the existing species, but were somewhat simpler, owing to less development of the accessory spurs. In the more progressive types the teeth were rather high-crowned, though in none were they actually hypsodont; while the persistent ancient genera had teeth with much lower crowns. Aside from the differences in the skull, which are obviously to be correlated with the absence or very small size of the horn, the skeleton in these Pliocene genera differed but little from the type common to the existing rhinoceroses, and in all the species the feet were three-toed. In short, the dentition and skeleton, except the skull, had already attained to substantially the modern conditions. While the Old World at that time had both horned and hornless rhinoceroses in abundance, none of the genera with large and fully developed horns ever migrated to the western hemisphere. This is the more remarkable in that the great †Woolly Rhinoceros (_Opsiceros †antiquitatis_) of the Pleistocene, which had two very large horns, inhabited Siberia with the †Mammoth (_Elephas †primigenius_). The latter extended its range through Alaska and the northern United States, but the rhinoceros, for some unknown reason, did not accompany it in its eastward wanderings.