Page:Popular Science Monthly Volume 12.djvu/697

Rh The Sirenians, which appear first in the Eocene of the Old World, occur in the Miocene of our eastern coast, and throughout the later Tertiary. The specimens described have all been referred to the genus Manatus, and seem closely related to our living species. In the Tertiary of Jamaica, a skull has been found which indicates a new genus, Prorastomus, also allied to the existing manatee. The genus Rhytina, once abundant on our Northwest coast, has recently become extinct.

The Ungulates are the most abundant mammals in the Tertiary, and the most important, since they include a great variety of types, some of which we can trace through their various changes down to the modified forms that represent them to-day. Of the various divisions in this comprehensive group, the Perissodactyle, or odd-toed Ungulates, are evidently the oldest, and throughout the Eocene are the prevailing forms. Although all of the Perissodactyles of the earlier Tertiary are more or less generalized, they are still quite distinct from the Artiodactyles, even at the base of the Eocene. One family, however, the Coryphodontidæ, which is well represented at this horizon, both in America and Europe, although essentially Perissodactyle, possesses some characters which point to a primitive Ungulate type from which the present orders have been evolved. Among these characters are the diminutive brain, which in size and form approaches that of the reptiles, and also the five-toed feet, from which all the various forms of the mammalian foot have been derived. Of this family, only a single genus, Coryphodon (Bathmodon), is known, but there were several distinct species. They were the largest mammals of the lower Eocene, some exceeding in size the existing tapirs.

In the middle Eocene, west of the Rocky Mountains, a remarkable group of Ungulates makes its appearance. These animals nearly equaled the elephant in size, but had shorter limbs. The skull was armed with two or three pairs of horn-cores, and with enormous canine tusks. The brain was proportionally smaller than in any other land mammal. The feet had five toes, and resembled in their general structure those of Coryphydon, thus indicating some affinity with that genus. These mammals resemble in some respects the Perissodactyles, and in others the Proboscidians, yet differ so widely from any known Ungulates, recent or fossil, that they must be regarded as forming a distinct order, the Dinocerata. Only three genera are known, Dinoceras, Tinoceras, and Uintatherium, but quite a number of species have been described. During the later part of the middle Eocene these animals were very abundant for a short time, and then