128 



THE AGE OF MAMMALS 



different exposures of the Wasatch, all animals of the size of small foxes, 

 graceful, light-limbed, and like their European contemporaries {Hyraco- 

 therumi), large-brained. Close comparison (cf. p. UG) of these hyracotheres 

 with those of the Lower Ypresian (London Clay) of the northern coast 

 of Europe shows that the grinding teeth are in a very similar stage 

 of evolution. The species Eohippus validus, on the whole the most ancient 

 in type of the American hyracotheres, is in exactly the same state of evolu- 

 tion as the H. vulpiceps of the London Clay; but nothing in America is known 

 quite so ancient as the H. leporinum of the London Clay in respect to the 

 simplicity of the anterior premolar teeth. 



A modern aspect is also given to this fauna by the appearance of the 

 Rodentia {Paramys, Sciuravus) of the family Ischyromyidae, embracing a 



number of species sug- 

 gesting in their tooth 

 structure the sciuro- 

 morphs, or squirrel 

 group of rodents. It 

 appears probable that 

 some of these ischy- 

 romyids were destined 

 to give rise to the true 

 sciurids or squirrels. 



Wind River Life of 

 Wyoming and Colorado 



Geological conditions 

 and distribidion. — In 

 their grand sequence 

 the Wind River sedi- 

 ments and their geologic 

 parallels in Wyoming 

 and Colorado take up 

 the life story of the 

 Wasatch and continue 

 it into the beginning of 

 the Bridger deposition, 

 which we regard as 

 true Middle Eocene. 



The typical forma- 

 tion lies at the head 

 waters of the Big Horn River (Fig. 41), northeast of the Wind River 

 Mountains, 400 to 500 feet in thickness, irregularly disposed with an east 

 and west extent of 100 miles and north and south extent of forty miles. 



By permission of the U.S. Geological Survey. 



FiQ. 41. — Map showing location of the Wasatch (oblique 

 lines) and Wind River (horizontal lines) of the Big Horn region 

 of Wyoming. 



