E. L. Troxell — Early Pliocene One-Toed Horse. 341 



habitat. Slenderness and lightness in general are cursorial 

 adaptations to the needs of the individual; for instance, where 

 food and water are scarce the animal has to go farther to 

 satisfy its wants. Moreover, life in the open country was 

 imperilled by preying animals and the little horses had less 

 opportunity to hide and dodge about among bushes or over 

 hilly country ; to offset this disadvantage they had to be able 

 to outdistance any pursuers. 



Aside from the actual rate of speed, the long-limbed animals 

 seem to be better suited to long continued effort. The slender- 

 ness of the cannon bone sometimes goes beyond the point of 

 fitness for speed ; the Arab, which is probably our swiftest 

 horse, has only a moderate speed index : 7*26 ; on the other 

 hand the ass, with a higher index : 8 # 68, is noted for its endur- 

 ance but not for its actual speed. It is equally true that where 

 food is scarce nature has to economise, hence the slender, small- 

 boned type is better suited to the environment where less 

 material is available for building up the frame. 



As a matter of fact the limbs of Pliohippus lullianus are 

 not out of harmony with the general buiid ; it stands three 

 feet and ten inches, while an ordinary horse may be five feet 

 high. The linear dimension of the former is £ that of the 

 latter ; if all the proportions are the same the relative weights 

 would be as the cubes of 3 and 4, that is as 27 and 64 ; the 

 larger horse would weigh over twice as much and would there- 

 fore need greater strength of limb. However, the present 

 specimen, being a colt, does not show the development of the 

 adult, so it is quite probable that later the bones would have 

 increased considerably in diameter, while changing only 

 slightly in length. 



Radius and ulna. — A unique feature in this one-toed horse 

 is the complete ulna, separate throughout from the radius. 

 As in the modern horse, the proximal and distal portions serve 

 as part of the articulations ; unlike it, the middle portion is 

 not fused to the radius but lies along its posterior surface, a 

 mere remnant of a former functional member. 



The shaft of the ulna in its smallest part is 2 mm wide and a 

 little more than half as thick. The distal epiphysis is a long 

 segment (3-4 mm ) which joins the shaft well above the epiphysis 

 of the radius and conforms to the contour of the latter. The 

 radius is quite slender through its middle portion, but the ends 

 are large ; the junction planes of the epiphyses mark the largest 

 parts. The form would change somewhat with age as the 

 shaft fills out to correspond to the heavier joints. 



Metacarpals. — The especial character which distinguishes 

 this specimen is its monodactyly. It has commonly been pre- 

 dicted that one-toed specimens of the Protohippinae would be 



