SOME EARLY MAMMALS 253 



" basins " l in Western North America, where the strata have 

 been found to yield so many valuable relics of ancient Tertiary 

 life. 



American geologists tell us that a long time ago (during the 

 Eocene period) there was a great tropical lake in the Wyoming 

 territory, on the borders of which roamed, amidst luxuriant 



FIG. 93. Map of North America in Tertiary times, showing roughly the outline 

 of the coast, and sites of the principal fresh-water lakes. 



vegetation, a large number of strange and primitive quadrupeds, 

 together with many other forms of life. The most wonderful 

 group of animals that haunted the shores of this lake, or perhaps 

 river valley, was the Dinocerata so fully described by Professor 

 Marsh, in his exhaustive monograph. 2 The name implies that 



1 When strata have been bent downwards into a kind of trough going far 

 below the surface, they are said to lie in a " basin." Examples : the London 

 basin, the Paris basin. 



2 The Dinocerata, a monograph by 0. C. Marsh, United States Geological 

 Survey, vol. x. 



