Horses they are abundantly coated with, cement. The shaft of the ulna is reduced to a mere style anchylosed throughout nearly its whole length with the radius, and appearing to be little more than a ridge on the surface of the latter bone until it is carefully examined. The front toes are still three, but the outer ones are more slender than in Anchitherium, and their hoofs smaller in proportion to that of the middle toe; they are, in fact, reduced to mere dew- claws, and do not touch the ground. In the leg, the distal end of the fibula is so completely united with the tibia that it appears to be a mere process of the latter bone, as in the Horses.
In Equus, finally, the crowns of the grinding-teeth become longer, and their patterns are slightly modified; the middle of the shaft of the ulna usually vanishes, and its proximal and distal ends anchylose with the radius. The phalanges of the two outer toes in each foot disappear, their metacarpal and metatarsal bones being left as the "splints."
The Hipparion has large depressions on the face in front of the orbits, like those for the "larmiers" of many ruminants; but traces of these are to be seen in some of the fossil horses from the Sewalik Hills; and, as Leidy's recent researches show, they are preserved in Anchitherium.
When we consider these facts, and the further circumstance that the Hipparions, the remains of which have been collected in immense numbers, were subject, as M. Gaudry and others have pointed out, to a great range of variation, it appears to me impossible to resist the conclusion that the types of the Anchitherium, of the Hipparion, and of the ancient Horses constitute the lineage of the modern Horses, the Hipparion being the intermediate stage between the other two, and answering to B in my former illustration.
The process by which the Anchitherium has been converted into Equus is one of specialization, or of more and more complete deviation from what might be called the average form of an ungulate mammal. In the Horses, the reduction of some parts of the limbs, together with the special modification of those which are left, is carried to a greater extent than in any other hoofed mammals. The reduction is less and the specialization is less in the Hipparion, and still less in the Anchitherium; but yet, as compared with other mammals, the reduction and specialization of parts in the Anchitherium remains great.
Is it not probable then, that, just as in the Miocene epoch, we find an ancestral equine form less modified than Equus, so, if we go back to the Eocene epoch, we shall find some quadruped related to the Anchitherium, as Hipparion is related to Equus, and consequently departing less from the average form?
I think that this desideratum is very nearly, if not quite, supplied by Plagiolophus, remains of which occur abundantly in some parts of the Upper and Middle Eocene formations. The patterns of the grinding-teeth of Plagiolophus are similar to those of Anchitherium, and their crowns are as thinly covered with cement; but the grinders diminish in size forwards, and the last lower molar has a
