It was about as large as a fox, and appears in the lower Eocene or at the base of the Tertiary formation. It was discovered since Professor
Huxley's lectures were given, and since the diagrams we follow were made, and we therefore have no figure of it. The Orohippus, in the next higher division of the Eocene, resembled its predecessor in size, but had only four toes in front, as the diagram shows. The Mesohippus came later, was about as large as a sheep, and had three usable toes, and the splint of another, on each fore-foot. In the later Miohippus, the splint-bone is reduced to a short remnant. In the Pliocene above, a three-toed horse (Protohippus), about as large as a donkey, was abundant; and, still higher up, a near ally of the modern horse (Pliohippus) makes his appearance. The series is completed in the subsequent appearance of a true Equus, as large as the existing horse.
The horse has thus advanced in his development by getting rid of superfluous toes or digits; but, under the principle of reversion to an early ancestral type, to which it is now well understood that animals are liable in various ways, these suppressed splints or digits break out as extra hoofs. Professor Marsh says: "In addition to each main digit of the ordinary horse, the anatomist finds concealed beneath the skin two slender metapodial 'splint-bones,' which are evidently the remnants of two other toes originally possessed by the ancestor of the horse. It is an interesting fact that these splint-bones are sometimes quite fully developed, and may even support extra digits which are much shorter and smaller than the main foot. As these small hooflets