The earliest traces of the Proboscidea in America have been reported from this formation, but the fragmentary specimens are inconclusive. The Perissodactyla are among the commonest fossils. The rhinoceroses belonged to native stocks, including both the horned and hornless forms. The horned genus (_†Diceratherium_) differed from all other rhinoceroses in having a transverse _pair_ of horns on the nose, and the species of the lower Miocene were quite small and light; the hornless genus (_†Cænopus_) was a larger and heavier animal. Tapirs are rare as fossils and consequently not well known. While there were several kinds of horses, they all agreed in having short-crowned and relatively simple grinding teeth and three-toed feet; they were smaller and of lighter, more slender build than those of the middle Miocene. The wonderful aberrant perissodactyls with clawed feet, the †chalicotheres (suborder †Ancylopoda), appear to have been more abundant in the Arikaree than at any other time in North America, though their history in this continent extends from the middle Eocene to the lower Pliocene. _†Moropus_, the lower Miocene genus, was as grotesque a creature as could well be imagined and, in advance of experience, no one ever did imagine such a beast. With rather small and somewhat horse-like head, long neck, long fore limbs and shorter hind limbs, these extraordinary animals united short, three-toed feet, which were armed with enormous claws. The long persistence (to the Pleistocene of Asia) and wide geographical range of the †chalicotheres are sufficient evidence that their very unusual structure must have been advantageous to them, but the problem of their habits and mode of life is still unsolved. From the character of the teeth, the long neck and fore limbs, it may, however, be inferred that they fed chiefly upon the leaves of trees. [Illustration: FIG. 129.—The small, †paired-horned rhinoceros (_†Diceratherium cooki_) of the lower Miocene. Restored from a skeleton in the Carnegie Museum, Pittsburgh.] [Illustration: FIG. 130.—A †chalicothere (_†Moropus elatus_) of the lower Miocene. Restored from a skeleton in the Carnegie Museum, Pittsburgh.]