56 NORTH AMERICAN FAUNA. [No. 16. 
perature still more. Higher parts of the same slope are not only very 
much drier, but are exposed to the direct rays of the afternoon sun, 
which, owing to the increasing steepness, strike the ground more nearly 
at aright angle than at lower elevations, the result being a material 
increase in the amount of heat received. It is obvious, therefore, that 
on the southwest and west slopes the middle part of the Transition 
zone is the hottest and driest, the part in which low Transition or even 
Upper Sonoran species are most apt to be found, while the upper and 
lower parts are coolest and dampest, the parts most likely to be invaded 
by Canadian zone species. Hence itis not surprising that Douglas and 
white firs, willows, alders, elder, snowberry, red spira, osier, and thim- 
bleberry flourish at the top and bottom of the Transition slope, but are 
absent from the middle part. At the extreme foot of the slope, along 
the cool streams near Sisson Tavern, two uormally boreal plants occur 
which were not found at all in the boreal belts on the mountain. These 
are Linnea borealis and Pachystima myrsinites. With them is associ- 
ated the mountain chinquapiu (Castunopsis sempervirens). There is 
something peculiar about the distribution of Pachystima. It is abund- 
ant in the neighborhood of Sisson Tavern and occurs here and there, 
apparently on dry soil, up to an altitude of 4,700 feet, but not above. 
Since all three of these species are boreal, it is possible that the forms 
here referred to are Transition zoue subspecies. 
MAMMALS OF TRANSITION ZONE. 
(1) Species known to occur in the Transition zone of Shasta. 
Antilocapra americana. Odocoileus colum)ianus, 
Lassariscus astutus raptor. Perognathus mollipilosus. 
Callospermophilus chrysodeirus. Peromyscus boyli. 
Canis ochropus. Peromyscus gambeli. 
Castor caniadensis.! Procyon psora pacitica. 
Cervus occidentilis. Scapanns cnlifornicus. 
Dipodomys californicus. Sciuropterus (klamathensis?). 
Erethizon epixanthus. Sciurus albolimbatus. 
Entamias ame-nus. Sciurus fossor. 
Eutamias senex. Sorex montereyensis. 
Felis oregonensis. Sorex vagrans amienus. 
Lutra hudsonica,. Spermophilus douglasi. 
Lutreola vison. Spilogale latifrons. 
Lynx fasciatus pallescens, Taxidea taxus. 
Mephitis occidentalis. Thomomys monticola pinetorum, 
Microtus montanus. Urocyon californicus townsendi. 
Microtus mordax.! Ursus americanus. 
Myotis evotis. Ursus horribilis. 
Myotis Incifugus longicrus. Vespertilio fuscus. 
Neotoma cinerea, Vulpes macrourus.? 
Neotoma fuscipes. Zapus paciticus. 
Neurotrichus gilbbsi miujor. 
' Restricted to culd streams which thoneh traversing the Transition zone atford 
Boreal temperatures. 
: Probably does not breed below Canadian. 
