of the larger mammals now living; and the group dates back at least to the lowest Eocene. Of the two well-marked divisions of this order, the Bunodonts and the Selenodonts, as happily defined by Kowalevsky, the former is the older type, which must have separated from the Perissodactyle line after the latter had become differentiated from the primitive Ungulate. In the Coryphodon beds of New Mexico occurs the oldest Artiodactyle yet found, but it is at present known only from fragmentary specimens. These remains are clearly Suilline in character, and belong to the genus Eohyus. In the beds above, and possibly even in the same horizon, the genus Helohyus is not uncommon, and several species are known. The molar teeth of this genus are very similar to those of the Eocene Hyracotherium, of Europe, which is supposed to be a Perissodactyle, while Helohyus certainly is not, but apparently a true lineal ancestor of the existing pigs. In every vigorous primitive type which was destined to survive many geological changes, there seems to have been a tendency to throw off lateral branches, which became highly specialized and soon died out, because they are unable to adapt themselves to new conditions. The narrow path of the persistent Suilline type, throughout the whole Tertiary, is strewed with the remains of such ambitious offshoots; while the typical pig, with an obstinacy never lost, has held on in spite of catastrophes and evolution, and still lives in America to-day. In the lower Eocene, we have in the genus Parahyus apparently one of these short-lived, specialized branches. It attained a much larger size than the true lineal forms, and the number of its teeth was reduced. In the Dinoceras beds, or middle Eocene, we have still, on or near the true line, Helohyus, which is the last of the series known from the American Eocene. All these early Suillines, with the possible exception of Parahyus, appear to have had at least four toes of usable size.
In the lower Miocene, we find the genus Perchœrus, seemingly a true Suilline, and with it remains of a larger form, Elotherium, are abundant. The latter genus occurs in Europe in nearly the same horizon, and the specimens known from each continent agree closely in general characters. The name Pelonax has been applied erroneously to some of the American forms; but the specimens on which it was based clearly belong to Elotherium. This genus affords another example of the aberrant Suilline offshoots, already mentioned. Some of the species were nearly as large as a rhinoceros, and in all there were but two serviceable toes; the outer digits, seen in living animals of this group, being represented only by small rudiments concealed beneath the skin. In the upper Miocene of Oregon, Suillines are abundant, and almost all belong to the genus Thinohyus, a near ally of
- Bunodont (hill-tooth); hence, teeth the crowns of which are composed of rounded tubercles.
- Selenodont (moon-tooth); teeth which have the crowns marked by crescents.