As already mentioned, there was much variation in size among the species, but none was as large as those of the Miocene and Pliocene genera, not to mention the enormous animals of the Pleistocene and Recent epochs in the Old World. The commoner species of the middle White River substage (_†Cænopus occidentalis_) was an animal nearly equalling in size the American Tapir (_T. terrestris_) and quite like that species in its proportions, the limbs being relatively longer and less heavy and the feet narrower than in the rhinoceroses of the subsequent geological epochs. The skull, being hornless, had thin, pointed and nearly flat nasal bones, an almost straight and horizontal upper profile, and a short and low, but distinct, sagittal crest; the cranial bones were quite thin, there being no extensive development of sinuses within them. The articulation of the lower jaw with the skull was only beginning to take on the characteristic peculiarities seen in the later genera, and the hinder margin of the lower jaw was not much thickened. Thus, many of the features which distinguish the skull in all Recent and Pleistocene and most Pliocene, and upper and middle Miocene rhinoceroses were entirely lacking in _†Cænopus_, yet no anatomist could doubt that the White River animal was a genuine rhinoceros. The neck was short, but not very heavy, the trunk elongate, but not massive, the ribs not being inordinately long nor strongly arched, and the hip-bones so little expanded that they were tapiroid rather than rhinocerotic in appearance. The limb-bones were relatively much more slender than in any existing species, and, although every one of them was characteristically that of a rhinoceros, yet the comparative lightness of body and slenderness of limb gave to these bones a certain resemblance to those of tapirs. The feet, which were moderately elongate and rather narrow, were three-toed, as in all subsequent North American species and in all existing members of the family.