rata of the United States under the name of
_Brontotheridoe_. These extraordinary animals, typified by
_Brontotherium_ (fig. 243) itself, agree with the existing Tapirs
of South America and the Indian Archipelago in having the fore-feet
four-toed, whilst the hind-feet are three-toed; and a further
point of resemblance is found in the fact (as shown by the form
of the nasal bones) that the nose was long and flexible, forming
a short movable proboscis or trunk, by means of which the animal
was enabled to browse on shrubs or trees. They differ, however,
from the Tapirs, not only in the apparent presence of a long tail,
but also in the possession of a pair of very large "horn-cores,"
carried upon the nasal bones, indicating that the animal possessed
horns of a similar structure to those of the "Hollow-horned"
Ruminants (_e.g._, Sheep and Oxen). _Brontotherium gigas_ is
said to be nearly as large as an Elephant, whilst _B. Ingens_
appears to have attained dimensions still more gigantic. The
well-known genus _Titanotherium_ of the American Miocene would
also appear to belong to this group.

[Illustration: Fig. 243.--Skull of _Brontotherium ingens_. Miocene
Tertiary, United States. (After Marsh.)]

The family of the Horses (_Equidoe_) appears under various forms
in the Miocene, but the most important and best known of these
is _Hipparion_. In this genus the general conformation of the
skeleton is extremely similar to that of the existing Horses,
and the external appearance of the animal must have been very
much the same. The foot of _Hipparion_, however, as has been
previously mentioned, differed from that of the Horse in the
fact that whilst both possess the middle toe greatly developed
and enclosed in a broad hoof, the former, in addition, possessed
two lateral toes, which were sufficiently developed to carry
hoofs, but were so far rudimentary that they hung idly by the
side of the central toe without touching the ground (see fig.
230). In the Horse, on the other hand, these lateral toes, though
present, are not only functionally useless, but are concealed
beneath the skin. Remains of the _Hipparion_ have been found
in various regions in Europe and in India; and from the immense
quantities of their bones found in certain localities, it may
be safely inferred that these Middle Tertiary ancestors of the
Horses lived, like their modern representatives, in great herds,
and in open grassy plains or prairies.

Amongst the Ev