CE NO ZOIC ERA AGE OF MAMMALS. 377 



have a common origin, it must be sought still lower, prob- 

 ably in the Laramie. 



Tertiary Lake-Deposits of the West. 



Nowhere in the world is there so complete a series of 

 Tertiary deposits and of Tertiary mammals as in the lake- 

 deposits of the Plateau and Plains and Basin regions al- 

 ready spoken of (page 365). We shall therefore take most 

 of our illustrations from these. 



Eocene Lake-Deposits. In the Lower Eocene de- 

 posits viz., Puerco beds and Wahsatch or Coryphodon 

 beds have been found nearly one hundred species of 

 mammals, including carnivores, herbivores, insectivores, 

 and monkeys. Perhaps the most remarkable and charac- 

 teristic animals of the lower Tertiary were the Corypho- 

 donts. These were huge animals with very small brains, 

 plantigrade feet, slow, awkward movements, and very 

 generalized structure. 



In the Middle Eocene Bridger beds, mammalian life 

 was even still more abundant. More than one hundred 

 species are known, and these are, of course, but a fraction 

 of what actually existed. Perhaps the most remarkable 

 animals of this time were those of the Dinoceras family. 

 The Dinoceras may be taken as a type of the family. 

 This was a heavy-built, sluggish-moving animal of ele- 

 phantine size, with a most singular conformation of head, 

 which was armed with three pairs of horns and a pair of 

 huge tusks, as shown in Fig. 340. Some are supposed 

 to have had a head five feet long. 



During the Eocene, also, Marsh finds the earliest pro- 

 genitors of the horse. In the early Eocene is found the 

 Eohippus, an animal which had three hoofed toes on the 

 hind-feet and four perfect hoofed toes and a rudimen- 

 tary fifth toe on the fore-feet. This was followed in the 

 Middle Eocene by the QroJiippus, similar to the other,, 



