THE TERTIARY PERIOD 319 



nevertheless they were not then differentiated into the more or 

 less clearly defined groups of today such as the Carnivores, or flesh 

 eaters (e.g. Dogs, Bears, Tigers, etc.); Perissodactyls, or hoofed 

 Mammals with an odd number of toes (e.g. Horses, Rhinoceroses, 

 etc.) ; Artiodactyls, or hoofed Mammals with an even number of 

 toes (e.g. Camels, Deer, Pigs, etc.) ; Proboscidians, or trunk-bearing 

 hoofed Mammals (e.g. Elephants); Rodents, or gnawers (e.g. 

 Rats, Squirrels, etc.); Insectivores (e.g. Moles, Hedgehogs, etc.); 

 Cetaceans, or exclusively swimmers (e.g. Whales, Dolphins, etc.); 

 Primates, or the very highest of all Mammals (e.g. Monkeys, 

 Man, etc.); and many others. 

 These groups, traced back to- 

 ward the early Tertiary, grad- 

 ually become less and less dis- 

 tinct until, in the Eocene, they 

 cannot be at all distinguished 

 as separate groups, but rather 

 we find ancestral or generalized 

 forms which show combinations Fig- 200 



of features of the later groups. A nearly perfect skeleton of the 



One of the most character- f^Tc Ph Q T aCodm vrimaevus ' 

 istic of these generalized types er ° 



of the early Eocene was Phenacodus (see Fig. 200). The various 

 species of this genus showed about the same range in size as modern 

 Dogs. Each foot had five toes which were supplied with nails 

 rather between true claws and true hoofs in structure. The simple 

 (primitive) teeth indicate that the animal was omnivorous, that 

 is both plant and flesh eating. In harmony with other early Ter- 

 tiary Mammals, the brain was relatively small and almost devoid 

 of convolutions, thus pointing to a low grade of mental devel- 

 opment. 



Perissodactyls (e.g. Horse). As an example of the history of 

 the odd-toed, hoofed, Mammals, we shall consider the well-known 

 evolution of the Horse family. At least forty species of this family, 

 ranging from early Eocene to the present, have been described, 

 and practically every connecting link in the evolution of the 

 family is known. Only a few of the most important changes can 

 be noted in our brief description, which is, in fact, not much more 

 than an explanation of the excellent chart shown in Fig. 201. The 

 earliest form, called Eohippus, occurring in the lower Eocene, was 



