94 MIGHTY ANIMALS 



now to be seen in the western part of the United 

 States. 



As the bottoms of the seas rose, the seas themselves 

 disappeared. But there were other seas, the bottoms 

 of which did not rise and form mountains. One of 

 these was in the southwestern part of Wyoming and, as 

 the mountains formed into the Wasatch range on the 

 west and the Rockies on the east, they shut off this 

 sea from the ocean. As thereafter it was fed only 

 by streams of fresh water, it became a fresh- water 

 lake. It was not until all these changes had occurred 

 that the Dinoceras appeared on the shores of this 

 lake, which was about one hundred miles long. The 

 climate was still mild, for the mountains had not then 

 risen high enough to shut off the warm winds from 

 the ocean. 



Here, in this luxurious land, the Dinoceras lived for 

 many generations. To-day he is famous, chiefly 

 because of his small brain and the fact that he was 

 one of the first gigantic mammals in the world. For 

 many ages before the time of the Dinoceras, the rep- 

 tiles had ruled the earth, but after the mammals 

 had once gained the mastery of the reptiles, they 



