Mammals of the Boreal Region. 23 



Alle;j;hanie.s, a central arm in the Rocky Mountains, and a west- 

 ern arm in the Cascades and Sierra Nevada. The latter at its 

 northern base occupies the entire breadth of the Pacific Coast 

 region from the eastern slope of the mountains to the sea, but in 

 passing southward bifurcates, the main fork following the lofty 

 Cascade and Sierra ranges to about latitude 36° ; the other fol- 

 lowing the coast, gradually losing its distinctive characters and 

 becoming invaded with Sonoran forms until it disappears a little 

 north of San Francisco. 



The following genera of mammals belong exclusively to the 

 Boreal Region, none of them ranging south beyond the Transi- 

 tion Zone : 



Cervus Cuniculus 



Rangifer Zapus 



Alee Erethizon 



Mazama Lagoiiiys 



Ovibos Tlialarctos 



Arctomys Latax 



Aplodontia Gnlo 



Evotomys INIustela 



Phenaconiys Neurotriclius (?) 



Myodes Condylura 



In addition to the above, the following genera are clearly of 

 Boreal origin, although reaching and in some cases penetrating 

 parts of the Sonoran Region : 



Ovis Vulpes 



Bison * Ursus 



Tamias Lutreola 



Castor Putorius 



Arvicola Sorex 

 Fiber 



Besides the senera here enumerated, the following subgenera 

 belong to the Boreal Region : Tnmifisriurus (containing the red or 

 spruce squirrels), Mijnomes and dt.ilotiis (field-mice or voles, of 

 which Myaomes reaches south a little beyond the Transition 

 Zone), Teonoma (the bushy-tailed wood-rats), and A^eosorex and 

 Atophyrax (subgenera of shrews). 



* The faunal position of the genus Bison is not so certain as in the case 

 of tlie other genera here mentioned, thoiigh both the American and the 

 European species seem to be of Boreal origin. 



