[Illustration: FIG. 218.—Skull of _†Leptomeryx evansi_, White River. (After Matthew.)] The third phylum, that of _†Leptomeryx_, had about the same range in time as the preceding one, though it has not yet been found in the John Day, and the genus is assuredly known only from the White River beds, in which it is not uncommon. _†Leptomeryx_ comprised a number of species, all very small animals, and none larger than a jack-rabbit. (See Fig. 277, p. 563.) In size, proportions and appearance, these dainty little creatures must have been very like the existing chevrotains or “mouse-deer” of Asia and the Malay islands, and by many writers they have been classed in the same suborder, the Tragulina. The upper incisors had been suppressed and the upper canine reduced to very small size, while the lower canine had become functionally one of the incisors. The skull had a very long and slender facial region, but had a less llama-like appearance than in _†Hypertragulus_. The neck was short and the fore limbs much shorter than the hind, so that the back sloped downward from the rump to the shoulders, as in the chevrotains. There was a remarkable, indeed quite unparalleled, difference between the fore and hind limbs and feet, the hinder extremity being not only much longer, but also much more specialized, while the anterior one retained in very large degree its primitive characteristics. Thus, in the fore-arm the ulna was complete and separate from the radius, but in the lower leg the fibula was reduced to its minimum. In the manus there were four entire and functional digits, in the pes only two, which were joined in a cannon-bone. Finally, there was a fourth phylum, that of _†Hypisodus_, which was confined to the White River stage and is still incompletely known. This was a tiny creature, much smaller than any of the preceding ones, and is the only known White River ungulate with fully hypsodont grinding teeth. Another very exceptional peculiarity of its dentition was that in the lower jaw it had ten incisor-like teeth; not only the canine, but the first premolar as well, had assumed the character of the incisors. This same peculiarity is found in the lower Miocene †gazelle-camel, _†Stenomylus_ (see p. 394), but in no other mammal. A considerable assemblage of genera belonging to this family occurs in the upper Eocene, but the material yet obtained is too fragmentary to permit the assignment of these forms to the different phyla, though it is very probable that among them are to be found ancestors of all the White River and subsequent genera.