32 Merriam Geographic Distribution of Life. 



The following Boreal genera of mammals disappear in the 

 Transition Zone 



Tamias * Yulpe.s * 



Fiber | Cervus 



Evotomys Ovis * 



Zapus U rsus * 



Erethizon Xeurotrichus 



Arctomys Conrlylura 



The following Sonoran genera of mammals disappear in the 

 Transition Zone : 



Antilocapra Perognathus 



Cynomys Bassariscus J 



Spilogale J Urocyon J 



Geomys Scalops 

 Thomomys \ 



As already stated, the only mammalian genus apparently re- 

 stricted to the Transition Zone is Synnptomys a lemming mouse. 

 A number of species, however, seem to be nearly or quite con- 

 fined to this zone. Among these are the following : 



Sciurus aberti Spermophilus spilosoma pratcnsis 

 fossor || grammurus 



carolinensis leucotis townsendi || 



Tamias merriami Cynomys leucurus 



minimus Sitomys nebrascensis 



pictus boylii 



striatus michiganensis 



Spermophilus elegans Arvicola mogollonensis 

 richardsoni austerus minor 



obsoletus curtatus 



* Except one species, which inhabits a limited part of the Sonoran 

 Region. 



f 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 temperature of the water. 

 The same is true in a less degree of the paludal subgenera Amso/vr and 

 Atophyrax (of Sorex) and of the semi-amphibious members of the submenus 

 Mynomes (of Arvicola). 



J 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 do\vn into Upper Sonoran also. 



