CENOZOIC ERA: AGE OF MAMMALS 



599 



cestors of the hoofed mammals would be like when discovered. 

 This prophecy was fulfilled in the finding of Phenacodus, although 

 the animal has proved not to be directly ancestral to any form, but 

 rather to stand as a type. 



REFERENCE FOR PHENACODUS 

 Scott, W. B., — A History of Land Mammals in the Western Hemisphere, pp. 456-458. 



Divergence of the Even and Odd-toed Hoofed Mammals (Ungu- 

 lates). — The common herbivorous mammals 1 of the present are 

 separated into two great divisions, those with a cloven hoof (Fig. 

 540), the even-toed ungulates (Artiodactyla), such as the pig, deer, 



A BCD 



Fig. 540. — Evolution of the foot of even-toed mammals (artiodactyls) ; A, hog; 



B, roebuck; C, sheep; D, camel. 



and camel, and those with a large central toe, the odd-toed ungulates 

 (Perissidactyla) (Fig. 541), such as the horse with one toe, the 

 rhinoceros with three, and the tapir with four toes on the fore foot 

 and three on the hind foot. The five-toed ancestors of the earliest 

 Eocene had already developed feet that gave promise of odd-toed 

 and even-toed descendants ; even Phenacodus, the most generalized 

 of the early mammals, has a foot in which the central toe is rather 

 larger than the others, and should be placed in the division of odd- 

 toed ungulates (Perissidactyla). 



1 The Proboscidea (elephants) constitute a third great group of hoofed animals with five-toed 

 feet. 



