The rest of the Ungulates continued to develop through the
Tertiary, and fortunately we are enabled to follow the
development of two of the most interesting of them, the horse and
the elephant, in considerable detail. As I said above, the
primitive Ungulate soon branches into three types which dimly
foreshadow the tapir, the horse, and the rhinoceros, the three
forms of the Perissodactyl. The second of these types is the
Hyracotherium. It has no distinct equine features, and is known
only from the skull, but the authorities regard it as the
progenitor (or representative of the progenitors) of the
horse-types. In size it must have been something like the rabbit
or the hyrax. Still early in the Eocene, however, we find the
remains of a small animal (Eohippus), about the size of a fox,
which is described as "undoubtedly horse-like." It had only three
toes on its hind feet, and four on its front feet; though it had
also a splint-bone, representing the shrunken and discarded fifth
toe, on its fore feet. Another form of the same period
(Protorohippus) shows the central of the three toes on the hind
foot much enlarged, and the lateral toes shrinking.
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