nct group of Hyracodonts, which
may be traced in successive modifications through the upper Eocene,
lower and middle Oligocene, then disappearing altogether. As yet, the
hyracodonts have been found only in North America, and the last genus of
the series, Hyracodon, was a cursorial animal. Very briefly stated,
the modifications consist in a gradual increase in size, with greater
slenderness of proportions, accompanied by elongation of the neck,
limbs, and feet, which become tridactyl and very narrow. The grinding
teeth have assumed the rhinoceros-like pattern and the premolars
resemble the molars in form; on the other hand, the front teeth,
incisors and canines, have become very small and are useless as weapons.
As the animal had no horns, it was quite defenceless and must have found
its safety in its swift running, for Hyracodon displays many superficial
resemblances to the contemporary Oligocene horses, and was evidently
adapted for speed. It may well have been the competition of the horses
which led to the extinction of these cursorial rhinoceroses.

The second sub-family, that of the Amynodonts, followed a totally
different course of development, becoming short-legged and short-footed,
massive animals, the proportions of which suggest aquatic habits; they
retained four digits in the front foot. The animal was well provided
with weapons in the large canine tusks, but was without horns. Some
members of this group extended their range to the Old World, but they
all died out in the middle Oligocene, leaving no successors.

The sub-family of the true rhinoceroses cannot yet be certainly traced
farther back than to the base of the middle Oligocene, though some
fragmentary remains found in the lower Oligocene are probably also
referable to it. The most ancient and most primitive member of
this series yet discovered, the genus Trigonias, is unmistakably a
rhinoceros, yet much less massive, having more the proportions of a
tapir; it had four toes in the front foot, three in the hind, and had a
full complement of teeth, except for the lower canines, though the upper
canines are about to disappear, and the peculiar modification of the
incisors, characteristic of the true rhinoceroses, is already apparent;
the skull is hornless. Representatives of this sub-family continue
through the Oligocene and Miocene of North America, becoming rare and
localised in the Pliocene and then disappearing altogether. In the Old
World, on the oth