The Perissodactyla were the preponderant type of hoofed animals, and ancestral forms of most of the White River genera have already been identified. The †titanotheres (_†Diplacodon_, _†Protitanotherium_) were much smaller and lighter than those of the lower White River and had much shorter horns. The †hyracodonts, the lightly built, cursorial rhinoceroses, were represented by a genus (_†Triplopus_) which was smaller and more slender than the White River form (_†Hyracodon_) and its teeth were of distinctly more primitive character. The heavy, massive and presumably aquatic †amynodonts (_†Amynodon_) were likewise smaller and less specialized than their descendants of the Oligocene. No member of the true rhinoceros series has yet been identified in the Uinta, but there is some reason to think that they were nevertheless present. Tapirs are distinctly indicated by certain fossils, but they are still too incompletely known to make possible any statement as to their degree of development. The horses (_†Epihippus_), like the other families mentioned, were much smaller and distinctly more primitive than their successors in the Oligocene. The Artiodactyla were, for the first time in the history of North America, as numerous and as varied as the perissodactyls and, with the exception of the peccaries and †anthracotheres, representatives of all the White River families are known. The finding of the peccaries is merely a question of further exploration, but the †anthracotheres were migrants from the Old World, and there is no likelihood that they will be discovered in the Uinta at any future time. Fairly large, pig-like animals, probably referable to the †giant-pigs or †entelodonts, occurred, but nothing has yet been found which can be considered as the direct ancestor of the White River genus. As was true of the perissodactyls, the Uinta artiodactyls were nearly all much smaller and more primitive than their Oligocene descendants and the differences are most interesting from the evolutionary point of view. The ancestral camel (_†Protylopus_) was a little creature no bigger than a fox-terrier, though the †hypertragulids (_†Leptotragulus_) were as large as _†Leptomeryx_ and _†Hypertragulus_ of the White River. The most ancient known members of the †oreodonts (_†Protoreodon_) and the †agriochœrids (_†Protagriochœrus_) are found in the Uinta.