SOME EARLY MAMMALS 253 
“basins”! 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 
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Fia. 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.?, 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 O. C. Marsh, United States Geological 
Survey, vol, x, 
