382 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. | 
and a western 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, buf in pass- 
ing southward bifurcates, the main fork following the lofty Cascade 
and Sierra ranges to about latitude 36°; the other following 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 excluslvely to the Boreal 
region, none of them ranging south beyond the Transition zone: 
Cervus. Arctomys. Cuniculus. Latax. 
Rangifer. Aplodontia. Zapus. Gulo. 
Alce. Evotomys. Erethizon, Mustela. 
Mazama. Phenacomys. Lagomys. Neurotrichus(?). 
Ovibos. Myodes. Thalarctos. 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. Castor. Vulpes. Putorius. 
Bison. * Arvicola. Ursus. Sorex. 
Tamais. Fiber. Lutreola. 
Besides the genera here enumerated, the following sub-genera belong 
to the Boreal region: Tamiasciurus (containing the red or spruce 
squirrels), Mynomes and Chilotus (field-mice or voles, of which Mynomes 
reaches south a little beyond the Transition zone), Teonoma (the bushy- 
tailed wood-rats), and Neosorex and Atophyrax (subgenera of shrews). 
The Boreal region is made up of two principal divisions, both cireum- 
polar: (1) An Arctic division, above the limit of tree growth; and (2) 
a Boreal Coniferous Forest division. 
ARCTIC MAMMALS. 
(Found above the limit of trees and all circum-polar.) 
A. Exclusively Arctic. 
ID OHNO SE aoossaoaaconsoootnSssobbrscssSce cues Homo. 
Polar beat 4.24 252 c aes eee ee eee eee eee Thalarctos maritimus. 
Barkenyonound beats eee eee ee Ursus richardsoni. 
Musk 0x5). . 022. 2eis2 232525 es eee eee Ovibos moschatus. 
Barrens sround caribou. eee e eee Rangifer grenlandicus, 
Arétic 10x: <2 52 -est Clee ee ee eee ae Vulpes lagopus. 
Areticdhare.s2 sce 426 eo eee ee eee eee Lepus glacialis. 
Remmin oe 2.5 .2365220 5 see eee eee eee Myodes obensis. 
eOMMING. .o2e ste ee ee Cuniculus torquatus. 
Arctiemed-backedimouses= 525+ eeee see eee eee Evotomus rutilus. 
Parny sispermophile, 22. .2o 5 eee ee eee Spermophilus empetra. 
*The faunal position of the genus Bison is not so certain as in the case of the 
other genera here mentioned, though both the American and the European species 
seem to be of Boreal origin. 
— 
