The family of the Tapirs is represented, both in the Old and New Worlds, by species of the genus Lophiodon, some of which were quite diminutive in point of size, whilst others attained the dimensions of a horse.  Nearly allied to this family, also, is the singular group of quadrupeds which Marsh has described from the Miocene strata 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.