[Illustration: FIG. 181.—Left manus of †cursorial rhinoceroses. _A_, _†Triplopus cubitalis_ (after Cope), upper Bridger. _B_, _†Hyracodon nebrascensis_, White River.] As in the case of so many other mammalian series, the †hyracodonts of the but partially explored Uinta formation are still very imperfectly known. Almost all that can be positively stated about them is that they were smaller than their White River successors and that the assumption of the molar-pattern by the premolars was incomplete. In the upper Bridger beds also not very much is known regarding the then representatives of the series, (_†Triplopus_). So much is clear, however, that they were still smaller and lighter animals, that the limbs were very light, and that the number of digits in the fore foot had already been reduced to three, the only known Bridger perissodactyl of which this is true, all the others having four digits in the manus and three in the pes. [Illustration: FIG. 182.—Primitive †cursorial rhinoceros (_†Hyrachyus eximius_), lower Bridger. Restored from a skeleton in the American Museum of Natural History.] In the middle and lower Bridger, and even in the Wind River, occurs a genus (_†Hyrachyus_) which contained a large number of species, ranging in size from a full-grown modern tapir to creatures no larger than foxes. It is among these smaller species that the most ancient member of the †hyracodont line is to be sought, though it is not yet practicable to select any particular one. _†Hyrachyus_, indeed, may very possibly have contained among its many species the ancestors of all three lines of the rhinoceroses and rhinoceros-like animals, and thus formed the starting point from which they developed in diverging series. It is always a very significant fact when two or more groups approach one another the more closely, the farther back in time they are traced, because that can only be interpreted to mean that ultimately they converged into a common term, even though that common ancestor should elude discovery.