n. A genus of Eocene horses, representing the oldest known type of the family Equidæ, founded by Marsh (1876) upon remains from the coryphodon-beds of the Lower Eocene of New Mexico, indicating a kind of horse about as large as a fox, with four toes and a half on each fore foot, all incased in horn and forming hoofs, and three hoofed toes on each hind foot.