During the Miocene period the horse passed through the most interesting phases of his evolution, his elaborate dental battery was almost brought to perfection, and the second and fourth toes were gradually dwarfed and hidden out of sight, not even a trace of the hoofs being left, as in sheep, to suggest polydactylous ancestors. As already hinted, though Europe was the birthplace of the remote ancestor of the Equidæ, it is in the Miocene deposits of North America that we have a record of the most important phases of their evolution. The remarkable progress made in Miocene times was doubtless necessary to enable horses and other grass-eaters to keep abreast of the profound changes in the environment—the great increase of prairies in some areas, and the upheavals which resulted in the appearance of extensive mountain ranges in others. The Oligocene species which proved sufficiently plastic to respond to the new conditions varied in different directions, and gave rise to, amongst other types, one well adapted for a forest life, and one highly specialised for ranging far and wide over boundless prairies. In _Hypohippus_ we have an example of a “forest” horse, in the American _Hipparion_ (_Neohipparion_) we have a horse more specialised for a desert life than the fleetest Arab, while in _Merychippus_ we have a link with Oligocene species deserving attention, because, on the one hand, it gave rise through _Neohipparion_ to the _Hipparions_, now extinct, but once common in Europe and Asia; and because, on the other hand, through _Protohippus_ it seems to be the ancestor of the slender-limbed species of the “desert” or plateau type, now best represented by Celtic and Mexican ponies. In _Merychippus_ the orbit is complete, and the crowns of the permanent molars are cemented as in recent horses, but the hoof bone has a cleft at the apex (fig. 19), and in some cases there is a minute vestige of the “splint” bone of the fifth or outer digit of the fore-foot. In fig. 11_a_, the fore-foot of _Neohipparion_, the Miocene race-horse, is represented. Fig. 19 gives the fore-foot, and fig. 16 is a restoration of _Merychippus_. Hitherto _Merychippus_, through _Protohippus_ and _Pliohippus_, has been by many regarded as the progenitor of all the modern horses, as well as of the extinct _Hipparions_. That slender-limbed horses with short-pillared molars are mainly descended from one or more varieties of _Merychippus_ is possible, but it seems to me that modern breeds with short broad cannon-bones and long-pillared molars are probably mainly descended from browsing ancestors with limbs of the _Hypohippus_ type.