292 FOSSIL HORSES IN AMERICA. 
Hipparion, Protohippus and Pliohippus, of the Pliocene; and — 
Equus, Quaternary and recent. 
The most marked changes undergone by the successive equine — 
genera are as follows: 1st, increase in size; 2d, increase in speed, ; 
through concentration of limb bones; 3d, elongation of head and — 
neck, and modifications of skull. The increase in size is remark- — 
able. The Eocene Orohippus was about the size of a fox. Mio — 
hippus and Anchitherium, from the Miocene, were about as large — 
as a sheep. Hipparion and Pliohippus, of the Pliocene, equalled — 
the ass in height: while the size of the Quaternary Equus was fully — 
up to that of the modern horse. 2 
The increase of speed was equally marked, and was a direct 
result of the gradual modification of the limbs. The latter were — 
p Fis: 7. 2 
Xx a at 4 
a, Orohippus (Eocene); b, Miohippus (Miocene); c, Hipparion (Pliocene); E 
d, Equus (Quaternary). ee 
slowly concentrated, by the reduction of their lateral elements : 
and enlargement of the axial one, until the force exerted by 6% 
limb came to act directly through its axis, in the line of motion. 
This concentration is well seen, e. g., in the fore limb. *® 
was, Ist, a change in the scapula and humerus, especially 12 the : 
latter, which facilitated motion in one line only; 2d, m “~ — 
pansion of the radius, and reduction of the ulna, until the formet 
alone remained entire, and effective; 3d, a shortening of all the 
carpal bones, and enlargement of the median ones, enst 
firmer wrist; 4th, an increase in size of the third digit, at the or 
pense of those on each side, until the former alone supported : 
limb. The latter change is clearly shown in the above diagram 
(Fig. 75), which represents the fore feet of four typical genera 
