(2) The molar teeth, originally made up of conical cusps, changed to a highly complex pattern of crests and ridges, and the premolars, one by one, assumed the size and pattern of the molars; the low-crowned, rooted and cement-free teeth, fitted only for browsing, became very high-crowned, prismatic and cement-covered, admirably adapted to grazing. Beginning in the upper incisors of the White River _†Mesohippus_, the “mark” became established as an enamel-lined pit, growing in depth as the teeth increased their length. (3) The face grew relatively longer, the eye-socket being shifted behind the teeth and becoming completely encircled in bone, and the jaws were greatly increased in depth to accommodate the very long teeth. (4) The short neck was greatly elongated and the individual vertebræ modified so as to give flexibility with no loss of strength. The primitive peg-like odontoid process of the axis became first semicylindrical and then spout-shaped. (5) The arched back was straightened and the neural spines, especially of the anterior dorsals, elongated. (6) The limbs grew relatively much longer; the bones of the fore-arm and lower leg were fused together, the one on the inner side (radius and tibia) enlarging to carry the entire weight and the external one (ulna and fibula) becoming more or less atrophied. [Illustration: FIG. 154.—Series of horse skulls in ascending geological order. _A._, _†Eohippus_, lower Eocene (after Cope). _B._, _†Mesohippus_, lower and middle Oligocene. _C._, _†Protohippus_, upper Miocene (after Cope). _D._, _Equus_.] [Illustration: FIG. 155.—Right manus and left pes of _Equus_.] [Illustration: FIG. 156.—Right manus and left pes of _†Protohippus_.] (7) The feet were much elongated and the median (3d) digit of each gradually enlarged until it carried the whole weight, at the same time modifying the shape of the hoof so as to fit it to be the sole support of the body. The other toes gradually dwindled and became functionless, though often retained as splints. The first digit (pollex and hallux) was first lost, then the fifth, then the second and fourth were reduced to dew-claws and finally to splints. Thus the pentadactyl horses of the lower Eocene were transformed into the monodactyl species of the Pliocene and Pleistocene.