392 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
Geographic distribution of North American genera of nonpelagic mammals occurring | 
north of Mexico. 
BOREAL GENERA. 
Cervus. Arectomys. Cuniculus. Gulo. 
Rangifer. Aplodontia. Zapus. Mustela. 
Alce. Castor.* Erethizon. Lutreola.* 
Ovis.* Arvicola.* Lagomys. Putorius. * 
Mazama. Fiber.* Vulpes.* Sorex.* 
Bison. (?) Evotomys. Ursus.* Neurotrichus. (7?) 
Ovibos. Phenacomys. Thalarctos. Condylura. 
Tamias.* Myodes. Latax. 
SONORAN GENERA. 
Cariacus.t Geomys. Bassartscus. Corynorhinus. 
Antilocapra. Thomomys. Taxidea. Kuderma. 
Cynomys. Dipodomys. Conepatus. Antrozous. 
Reithrodontomys. Perodipus. Mephitis.t _Nycticejus. 
Onychomys. Microdipodops. Spilogale. Vesperugo.t 
Sitomys.t Perognathus. Motiosorex. Atalapha.t 
Oryzomys. Heteromys. Blarina.t Vespertilio.t 
Sigmodon. Lynx.t Scapanus. 
Neotoma.t Urocyon. Scalops. 
TROPICAL GENERA. 
Didelphis. Felis.t Nasua. Nyctinomus. 
Tatusia. Procyon.t Molossus. Otopterus. 
Dicotyles. 
TRANSITION ZONE GENERA. 
Synaptomys. 
GENERA INHABITING BOTH BOREAL AND SONORAN ZONES. 
Sciuropterus. Spermophilus. Lutra. Lepus. 
Sciurus. Canis. 
DISTINCTNESS OF THE TROPICAL REGION FROM THE SONORAN. 
It has been shown that the fauna and flora of tropical America reach 
the United States, though in a somewhat dilute condition, along the 
Lower Rio Grande in Texas and in southern Florida, and that in the 
vast majority of cases their genera and species differ widely from those 
of other parts of America. Except for the presence, chiefly in the south- 
ern United States, of a comparatively few forms derived from the 
tropical region, the fauna and flora of North America are as distinctive 
and independent of the existence of this area as if separated from it by 
the broad ocean. Among the eighty-one genera of non-pelagic Mam- 
malia inhabiting North America north of Mexico the number of these 
* Having one species in Sonoran zone or reaching Sonoran. 
t Having one species in Boreal zone or reaching southern edge of Boreal. 
