the ulna is greatly reduced, while the fibula,
though still complete, is hardly more than a thread of bone. The skull
has a longer face and a nearly enclosed orbit, and the brain-case is
fuller and more capacious, the internal cast of which shows that the
brain was richly convoluted. The teeth are still very short-crowned,
but the upper incisors plainly show the beginning of the "mark"; the
premolars have assumed the molar form, and the upper molars, though
plainly derived from those of Eohippus, have made a long stride toward
the horse pattern, in that the separate cusps have united to form a
continuous outer wall and two transverse crests.

In the lower Miocene the interesting genus Desmatippus shows a further
advance in the development of the teeth, which are beginning to assume
the long-crowned shape, delaying the formation of roots; a thin layer
of cement covers the crowns, and the transverse crests of the upper
grinding teeth display an incipient degree of their modern complexity.
This tooth-pattern is strictly intermediate between the recent type
and the ancient type seen in Mesohippus and its predecessors. The
upper Miocene genera, Protohippus and Hipparion are, to all intents and
purposes, modern in character, but their smaller size, tridactyl feet
and somewhat shorter-crowned teeth are reminiscences of their ancestry.

From time to time, when a land-connection between North America and
Eurasia was established, some of the successive equine genera migrated
to the Old World, but they do not seem to have gained a permanent
footing there until the end of the Miocene or beginning of the Pliocene,
eventually diversifying into the horses, asses, and zebras of Africa,
Asia and Europe. At about the same period, the family extended its range
to South America and there gave rise to a number of species and genera,
some of them extremely peculiar. For some unknown reason, all the horse
tribe had become extinct in the western hemisphere before the European
discovery, but not until after the native race of man had peopled the
continents.

In addition to the main stem of equine descent, briefly considered
in the foregoing paragraphs, several side-branches were given off at
successive levels of the stem. Most of these branches were short-lived,
but some of them flourished for a considerable period and ramified into
many species.

Apparently related to the horses and derived from the same root-stock is
the family of the Pal