384 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
MAMMALS OF THE BOREAL ZONE,—Continued. 
Lepus americanus. Sorex monticolus (a). 
bairdii (a). pacificus, 
washingtoni. richardsoni. 
Lynx canadensis. sphagnicolus. 
Ursus americanus. suckleyi. 
horribilis. trowbridgei. 
Putorius culbertsoni. vagrans. 
longicauda, sunilis (a), 
Mustela americana. albibarbis. 
caurina. palustris. 
pennanti. hodrodromus. 
Sorex belli. Condylura cristata. 
dobsoni (@). Vesperugo noctivagans. 
foster. Atalapha cinerea. 
idahoensis. 
The Sonoran region as a whole stretches across the continent from 
Atlantic to Pacific, covering nearly the whole country south of latitude 
43° and reaching northward on the Great Plains and Great Basin to 
about latitude 48°. It is invaded from the north by three principal 
intrusions of boreal forms along the three great mountain systems 
already mentioned; while to the southward it occupies the great inte- 
rior basin of Mexico and extends into the tropics along the highlands 
of the interior. It covers also the peninsula of Lower California, the 
southern part of which seems entitled to rank as an independent sub- 
division. 
The following genera belong exclusively to the Sonoran region (as 
distinguished from the boreal), none of them ranging north beyond the 
transition zone. Those preceded by the letter 7 are intrusions from the 
tropical region. 
T Didelphis. Geomys. Bassariscus. Euderma. 
T Tatusia. Dipodomys. T Nasua. Antrozous. 
T Dicotyles. Perodipus. * Conepatus. Nycticejus. 
Reithrodontomys.t Microdipodops. Spilogale. T Molossus. 
Onychomys. Perognathus. Notiosorex. T Nyctinomus. 
Oryzomys. Heteromys. Sealops. T Otopterus. 
Sigmodon. Urocyon. Corynorhinus. 
* The generic name Perodipus was proposed in 1867 by Fitzinger for the 5-toed 
kangaroo rats (Sitzwngsber. math. nat..Classe, K. Akad. Wiss. Wien, 1867, Lv1, p. 126), 
thus ante-dating by twenty-three years the name Dipodops proposed by the writer 
for the same type in 1890 (North Am. Fauna, No. 3, September, 1890, p. 72). Both 
generic names were based on Dipodomys agilis of Gambel, from Los Angeles, Cal. 
+ The generic name Leithrodontomys was proposed by Giglioli in 1873 (Richerche 
intorno alla Distribuzione Geagrafica Generale, Roma, 1873, p. 160), and ante-dates 
Ochetodon of Coues. 
