A third phylum of the †oreodonts, which appeared for the last time in the middle Miocene (genus _†Cyclopidius_), was a series of small and very small species, of which the skull was almost as peculiar as that of _†Pronomotherium_, but in a different fashion. The face was very much shortened and on each side a great vacuity reduced the nasal bones to mere splints; the elevated position of the eye-sockets, which projected above the forehead, and of the tubular entrance to the ear is an evidence of an aquatic or amphibious mode of life, such as is illustrated by the hippopotamuses, which can float almost completely submerged, with only the ears, eyes and nostrils above the surface of the water. The tympanic bullæ (see p. 66) or bony chambers into which the ear-tubes opened, were of relatively enormous size and added much to the unusual appearance of the skull. The incisors were very small and the grinding teeth narrow and completely hypsodont, this and the _†Merychyus_ series being the only two phyla of the family in which the hypsodont molar was fully acquired. The remainder of the skeleton differed but little from the type common to the whole family, except for a somewhat shorter tail. [Illustration: FIG. 200.—Skull of _†Leptauchenia nitida_, upper White River.] The animals of this series were common in the middle and lower Miocene and in the upper substage of the White River, but have not been found in the intermediate John Day. This may have been a matter of geographical distribution, these creatures not extending west of the main ranges of the Rocky Mountains. In the upper White River the genus _†Leptauchenia_ is extremely common, but below that level they suddenly and completely vanish and, as in the case of the _†Merychyus_ phylum, it is not yet practicable to determine the point in time or space of their branching off from the main stem of the family. Were the †oreodonts not entirely confined to North America, we should, as a matter of course, explain the seemingly sudden appearance of _†Leptauchenia_ as due to immigration, and it is entirely possible that the series did actually originate in some part of North America which has left no record of its Eocene or Oligocene terrestrial life. On the other hand, no one can imagine that everything that can be known of the mammals of the middle and lower White River has already been learned, and at any time the sought-for ancestor of _†Leptauchenia_ may be found in those beds. [Illustration: FIG. 201.—_Leptauchenia nitida_, upper White River. Restored from a skeleton in the Museum of Princeton University.]