a, _Hyracotherium_ (Eocene). b, _Mesohippus_ (Oligocene). c, _Anchitherium_ (Miocene). d, _Hipparion_ (Pliocene). e, _Equus_ (Pleistocene).] Be this as it may, the next North American representatives of the family constitute the genera _Protohippus_ and _Merychippus_ of the Miocene, in both of which the lateral digits are fully developed and terminate in small though perfect hoofs. In both the cheek-teeth have moderately tall crowns, and in the first named of the two those of the milk-series are nearly similar to their permanent successors. In _Merychippus_, on the other hand, the milk-molars have short crowns, without any cement in the hollows, thus resembling the permanent molars of the under-mentioned genus _Anchitherium_. From the well-known _Hipparion_, or _Hippotherium_, typically from the Lower Pliocene of Europe, but also occurring in the corresponding formation in North Africa, Persia, India and China, and represented in the Upper Miocene Loup Fork beds of the United States by species which it has been proposed to separate generically as _Neohipparion_, we reach small horses which are now generally regarded as a lateral offshoot from the _Merychippus_ type. The cheek-teeth, which have crowns of moderate height, differ from those of all the foregoing in that the postero-internal pillar (the projection on the right-hand top corner of c in fig. 2) is isolated in place of being attached by a narrow neck to the adjacent crescent. The skull, which is relatively short, has a large depression in front of the orbit, commonly supposed to have contained a gland, but this may be doubtful. In the typical, and also in the North American forms these were complete, although small, lateral toes in both feet (fig. 3, d), but it is possible that in _H. antilopinum_ of India the lateral toes had disappeared. If this be so, we have the development of a monodactyle foot in this genus independently of _Equus_. The foregoing genera constitute the subfamily _Equinae_, or the _Equidae_ as restricted by the older writers. In all the dentition is of the hypsodont type, with the hollows of the cheek-teeth filled by cement, the premolars molariform, and the first small and generally deciduous. The orbit is surrounded by a bony ring; the ulna and radius in the fore, and the tibia and fibula in the hind-limb are united, and the feet are of the types described above. Between this subfamily and the second subfamily, _Hyracotheriinae_, a partial connexion is formed by the North American Upper Miocene genera _Desmatippus_ and _Anchippus_ or _Parahippus_. The characteristics of the group will be gathered from the remarks on the leading genera; but it may be mentioned that the orbit is open behind, the cheek-teeth are short-crowned and without cement (fig. 1, a), the gap between the canine and the outermost incisor is short, the bones of the middle part of the leg are separate, and there are at least three toes to each foot.