256 0. C. Marsh—Notice of New Tertiary Mammals. 
The increase of speed was equally marked, and was a direct 
result of the gradual modification of the limbs. The latter 
were slowly concentrated, by the reduction of their lateral ele- 
ments and enlargement of the axial one, until the force exerted 
by each limb came to act directly through its axis, in the line 
of motion. This concentration is well seen, e. g., in the fore 
imb. There was, 1st, a change in the scapula and humerus, 
especially in the latter, which facilitated motion in one line 
only; 2nd, an expansion of the radius, and reduction of the 
ulna, until the former alone remained entire, and effective; 
3rd, a shortening of all the carpal bones, and enlargement of 
the median ones, ensuring a firmer wrist; 4th, an increase in 
size of the third digit, at the expense of those on each’ side, 
until the former alone supported the limb. The latter change 
is clearly shown in the following diagram, which represents the 
fore feet of four typical genera in the equine series, taken in 
succession from each of the geological periods in which this 
group of mammals is known to have lived: 
a. b. é 
a, Orohippus (Eocene); b. Miohippus (Miocene); c. Hipparion (Pliocene); 4. Eqwis 
(Quaternary). 
* The modern horse occasionally has one of the ancestral hoofiets developed, 
usually on the fore foot, : 
