The longest-known genus and the one containing the largest species is _Anchitherium_, typically from the Middle Miocene of Europe, but also represented by one species from the Upper Miocene of North America. The European _A. aurelianense_ was of the size of an ordinary donkey. The cheek-teeth are of the type shown in a of figs. 1 and 2; the premolars, with the exception of the small first one, being molar-like; and the lateral toes (fig. 3, c) were to some extent functional. The summits of the incisors were infolded to a small extent. Nearly allied is the American _Mesohippus_, ranging from the Lower Miocene to the Lower Oligocene of the United States, of which the earliest species stood only about 18 in. at the shoulder. The incisors were scarcely, if at all, infolded, and there is a rudiment of the fifth metacarpal (fig. 3, b). By some writers all the species of _Mesohippus_ are included in the genus _Miohippus_, but others consider that the two genera are distinct. _Mesohippus_ and _Miohippus_ are connected with the earliest and most primitive mammal which it is possible to include in the family _Equidae_ by means of _Epihippus_ of the Uinta or Upper Eocene of North America, and _Pachynolophus_, or _Orohippus_, of the Middle and Lower Eocene of both halves of the northern hemisphere. The final stage, or rather the initial stage, in the series is presented by _Hyracotherium_ (_Protorohippus_), a mammal no larger than a fox, common to the Lower Eocene of Europe and North America. The general characteristics of this progenitor of the horses are those given above as distinctive of the group. The cheek-teeth are, however, much simpler than those of _Anchitherium_; the transverse crests of the upper molars not being fully connected with the outer wall, while the premolars in the upper jaw are triangular, and thus unlike the molars. The incisors are small and the canines scarcely enlarged; the latter having a gap on each side in the lower, but only one on their hinder aspect in the upper jaw. The fore-feet have four complete toes (fig. 3, a), but there are only three hind-toes, with a rudiment of the fifth metatarsal. The vertebrae are simpler in structure than in _Equus_. From _Hyracotherium_, which is closely related to the Eocene representatives of the ancestral stocks of the other three branches of the Perissodactyla, the transition is easy to _Phenacodus_, the representative of the common ancestor of all the Ungulata. See also H.F. Osborn, "New Oligocene Horses," _Bull. Amer. Mus._ vol. xx. p. 167 (1904); J.W. Gidley, _Proper Generic Names of Miocene Horses_, p. 191; and the article PALAEONTOLOGY. (R. L.*)