_Eohippus_, the dawn horse, is the name given a skeleton found in 1880 in the lower Eocene strata in Wyoming. This specimen lay buried in a rock formation ages older than that in which the oldest known skeleton of this family had been found. Its discovery made a great sensation among scientists. This little animal, the skeleton of which is no larger than that of a fox, had four perfect toes, and a fifth splint on the forefoot, and three toes on the hind foot. The teeth are herbivorous. _Orohippus_, with a larger skeleton, was found in the middle Eocene strata of Wyoming. Its feet are like those of its predecessor, except that the splint is gone. The teeth as well as the feet are more like those of the modern horse. _Mesohippus_, the three-toed horse, found in the Miocene, shows the fourth toe reduced to splints, and the skeleton as big as that of a sheep. In this the horse family becomes fairly established. _Hypohippus_, the three-toed forest horse, found in the middle Miocene strata of Colorado, is a related species, but not a direct ancestor of the modern horse. _Neohipparion_, the three-toed desert horse, from the upper Miocene strata, shows the three toes still present. But the Pliocene rocks contain fossils showing gradual reduction of the two side toes, modification of the teeth, and increase in size of the skeleton. _Protohippus_ and _Pliohippus_, the one-toed species from the Pliocene strata, illustrate these changes. They were about the size of small ponies. _Equus_, the modern horse, was represented in the Pliocene strata by a species, now extinct, called _Equus Scotti_. This we may regard as the true wild horse of America, for it was as large as the domesticated horse, and much like it, though more like a zebra in some respects. No one can tell why these animals, once abundant in this country, became extinct at the end of the Tertiary Period. But this is undoubtedly true. The types described form a series showing how the ancestors of the modern horse, grazing on the marshy borders of ancient ponds, lived and died, generation after generation, through a period covering thousands, possibly millions, of years. Along the sides of the crumbling buttes these ancient burying-grounds are being uncovered. Within a dozen years several expeditions, fitted out by the American Museum of Natural History, have searched the out-cropping strata in Dakota and Wyoming for bones of mammals known to have lived at the time the strata were forming in the muddy shallows along the margins of lake and marsh. Duplicate skeletons of the primitive horse types above have been found, and vast numbers of their scattered bones. Each summer geological excursions will add to the wealth of fossils of this family collected in museums.