FAM. 4. TITANOTHERIIDAE.--These Oligocene Ungulates, often attaining to large dimensions, are nearly peculiar, so far as is at present known, to the North American Continent, and are at least most abundant in it.[174] Many generic names, such as _Titanotherium_, _Brontotherium_, _Brontops_, _Titanops_, and _Menodus_, have been given to them; but a recent study of the entire material accessible for description or already described has led Professor Osborn to the opinion that there was but a single genus, to which the name _Titanotherium_ must be applied. Of this genus there are some thirty well-characterised species, of which the gradual evolution can be traced from the lowest strata of the White River beds where their remains occur. An entire skeleton of _T. robustum_ enables us to understand the osteology of these forms and to compare them with other Perissodactyles. This animal was more than 13 feet long, standing some 7 feet 7 inches in height. It seems to have presented during life the aspect of a Rhinoceros with perhaps a touch of Elephant. The skull is not unlike that of a Rhinoceros in general dimensions and shape; but there are a pair of apparent horn cores anteriorly, which are smaller in the more ancient forms and acquire a large size, a forward direction with a divergence of the two in the later forms. A glance at the accompanying figures of skulls (Fig. 137) of early and later Titanotheres will exhibit the changes in this particular which the skulls underwent in the lapse of time occupied by the deposition of these Oligocene beds. The nasals are short in the later, longer in the more early species, such as _T. heloceras_ and _T. coloradense_. The zygomatic arch projects much, and is "shelf-like" in the later forms, the skull thus getting an immense breadth, which, {266} together with the long and divergent horn cores, must have given to the living animal a most bizarre appearance. It is an interesting fact that this animal, though a Perissodactyle, agrees with the Artiodactyla in the nineteen dorso-lumbar vertebrae, of which seventeen bear ribs. [Illustration] FIG. 137.--Three figures showing the cranial evolution of _Titanotherium_. Upper figure, _T. trigonoceras_; middle figure, _T. elatum_; lower figure, _T. platyceras_. (After Osborn.)