GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 389 
The following boreal genera of mammals disappear in the transition’ 
zone: 
Tamias. * Zapus. Vulpes.* Ursus. * 
Fiber.t Erethizon. Cervus. Neurotrichus. 
Evotomys. Arctomys. Ovis.* Concylura. 
The following Sonoran genera of mammals disappear in the transi- 
tion zone: 
Antilocapra. Geomys. Perognathus. Urocyon.} 
Cynomys. Thomomys.$ Bassariscus. t Scalops. 
Spilogale.+ 
As already stated, the only mammalian genus apparently restricted 
to the transition zone is Synaptomys, a lemming mouse. A number of 
species however seem to be nearly or quite confined to this zone. 
Among these are the following: 
Sciurus aberti. Spermophilus spilosoma pratensis. 
fossor. || grammurus. 
carolinensis leucotis. townsendi.|| 
Tamias merriami. Cynomys leucurus. 
minimus, Sitomys nebrascensis. 
pictus. boyli. 
striatus. michiganensis. 
Spermophilus elegans. Aryicola mogollonensis. 
richardsoni. austerus minof. 
obsoletus. curtatus. 
Arvicola pallidus. Perognathus fasciatus. 
Synaptomys cooperi. olivaceous. 
Lepus americanus virginianus. Putorius nigripes. || 
campestris. Vulpes velox. 
idahoensis. || Scapanus americanus. 
sylvaticus nuttalli.|| Vespertilio melanorhinus. 
Local elevations of the land in the Sonoran region are capped with 
isolated patches of transition or boreal species, according to the tem- 
perature to which their summits attain; and if the elevation is sufficient 
to secure a boreal fauna and flora, the latter is always separated from 
* Except one species, which inhabits a limited part of the Sonoran region. 
t Fiber ranges south beyond the normal limit of the transition zone, but it does so 
along the banks of cool streams that give it a much lower temperature than that of 
the surrounding atmosphere. It is probable that both Fiber and Castor should be 
classed with aquatic species, the limits of their distribution depending on the tem- 
perature of the water. The same is true in a less degree of the paludal sub-genera 
Neosorex and Atophyrax (of Sorex) and of the semi-amphibious members of the sub- 
genus Mynomes (of Arvicola). 
t These genera barely enter the transition zone at all except in a very small area 
in the far West. 
§ Except on high mountains in the Sonoran region. 
|| Range down into Upper Sonoran also. 
