ITS ANCESTORS AND RELATIONS. 27 



In the ankle or tarsus the cuboid articulates with 

 the calcaneum only. The astragalus presents a uni- 

 formly convex distal articular surface, as in Carniv- 

 ora, but it has a trochlea or pulley-like proximal end, 

 which the still more generalized Periptychus has not. 

 The toes are five in number, much resembling those 

 of the fore-foot. The animal was apparently not 

 plantigrade, or walking with the whole of the sole of 

 the foot, from the heel or hock to the toes, on the 

 ground, as the bears do, nor did it walk on the tips 

 of the toes only, as the horse does, but probably ha- 

 bitually stood in an intermediate position, with the 

 heel raised more or less from the ground. 



The remains of animals referable to this genus 

 already discovered in the Wasatch Eocene are re- 

 markably numerous, and differences in size and de- 

 tails of conformation have enabled Cope to describe 

 and name nine species considered to be distinct from 

 each other. They vary in size from that of a bull- 

 dog to a leopard or sheep. The structure of the bones 

 of the nasal region has led to the suggestion that the 

 head may have carried a short proboscis like that 

 of the tapir. 



As mentioned above, Phenacodus is not an iso- 

 lated form, and allied but less perfectly known spe- 

 cies appear to bridge over the interval between it and 

 the next that will be spoken of. 



