separate American genera towards the European horned genera. A genus sometimes united with Aceratherium, but still differing from it in some points, is Aphelops (Teleoceras).[[164]] This animal is more nearly approximated to "the modern standard" of Rhinoceroses than is its possible ancestor Aceratherium. The skeleton in general is more robust, even surpassing that of modern forms, and approaching the Hippopotamus. There is a reduction in the upper incisors, which are limited to two pairs, and the lower molars
are reduced to five. The lower incisors are only two. The sagittal crest is less marked; the fifth digit is reduced to a tiny nodule representing the metacarpus. It had a small nasal horn. There are numerous other details of likeness to modern Rhinoceroses in this creature, which has only community of descent with them from the older hornless forms, such as Aceratherium and Caenopus. In the genus Peraceras the upper incisors are as completely gone as in the living African Rhinoceroses.
Fig. 135.—Skeleton of Aphelops (Teleoceras) fossiger. × 1⁄15. (After Osborn.)
The most ancient rhinocerotine types[[165]] are the Hyracodonts and the Amynodonts. They both date from the Eocene, and became extinct in the succeeding Oligocene. Hyracodon[[166]] (Fig. 134) was "an agile, light-chested, and rather long-necked" type, resembling a Horse in build. There were no horns present, but the hoofs were more like those of the Horses than of the existing Rhinoceroses. These animals were apparently plain dwellers and defenceless, which is held to account for their compact hoofs and outward similarity to a Horse. The genus is Oligocene. The dental formula is I 3/3 C 1/1 Pm 4/3 M 3/3.
It is surmised by Professor Scott that the number of dorso-lumbar vertebrae was twenty-three or twenty-four. The radius and ulna are complete and separate bones, but the latter is somewhat reduced. There are four metacarpal bones, of which, however, the fifth is much reduced. The animal is only three-fingered. The tibia and the fibula are distinct, and show no tendencies towards fusion; but the fibula is much reduced. There are only three metatarsals and three toes. Had this line, which is to be regarded as a side branch of the Rhinoceros stem, not died out, it would probably have resulted, thinks Professor Scott, in monodactyle—very Horse-like types. It is later than the next genus to be described, Hyrachyus, of which it is possibly a descendant. An intermediate type, Triplopus, appears to bind together Hyracodon and Hyrachyus.
In Hyrachyus agrarius the skull is long and narrow, the facial region being markedly longer than in existing Rhinoceroses. The mastoid portion of the periotic bone is widely exposed upon the outer face of the skull, which is, as has been said, not the case with the existing genus Rhinoceros. The dentition is the complete Eutherian dentition of forty-four teeth. The upper
molar teeth are strikingly like those of the genus Rhinoceros. The fore-feet are pentadactyle, but functionally tetradactyle; the hind-feet tridactyle. The ulna is less reduced than in Hyracodon, and the dorso-lumbar vertebrae are twenty-five.
Fig. 136.—Skeleton of Metamynodon planifrons. × 1⁄22. (After Osborn and Wortman.)