252 H. H. ROSS 



of cool-adapted phyletic lines during mid-Cretaceous, a similar 

 series of events occurring again during Paleocene, and a third 

 extensive set taking place in the Pleistocene. At each time the 

 details of the dispersals were different. Finding out more about 

 these details is proving a challenge to our investigations. Each 

 dispersal, however, crossed or crisscrossed the West and had a 

 tremendous effect on its cool-adapted fauna. In their total action 

 these three cool-dispersal periods, with long intervening periods of 

 chiefly local movement, were the most important events determining 

 the composition of the present northwestern fauna. 



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