406 University of California Publications in Zoology [Vol. 12 



As is well known, the appertaining faunas of greatest contrast 

 are those of the humid coast belt and the Sierra-Cascades. Twenty- 

 four Boreal species, or 17 percent of the Trinity mammals and birds, 

 occur also on both the Sierra Nevada and the Cascade Mountains, 

 but not regularly as elements of the humid coast fauna. Also there 

 are thirteen species, or 9 percent, of the Trinity boreal fauna which 

 are identical with, or show closest affinities to, representatives on 

 the Sierra Nevada, but not on the Cascades; and there are four 

 species, or 3 percent, of the Trinity Boreal fauna which are identi- 

 cal with, or show closest affinities to, representatives on the Cas- 

 cade Mountains, but not on the main Sierra Nevada. Mount Shasta 

 is here considered as properly included in the Sierran fauna. This 

 mountain, in spite of its relative isolation, certainly shows only 

 minor differences from the main central Sierras (see Merriam, 1899, 

 pp. 71-81). 



It is thus to be seen that a total of 41 mammals and birds, or 28 

 percent of its species, are shared by the Trinity fauna exclusively 

 with that of both the Sierras and Cascades together. On the other 

 hand, there are only eleven, or less than 8 percent, of the Trinity 

 species which are shared exclusively with the closely adjacent humid 

 coast fauna. As regards its Boreal elements, therefore, we may con- 

 elude that the Trinity fauna is much more nearly allied to the 

 Sierra-Cascades than to the humid coast belt. 



The fauna of the Trinity region is preponderantly Boreal; much 

 the larger part of the territory lies in the Transition and Canadian 

 life-zones. The strictly Sonoran (Upper Sonoran) elements occur 

 only in the valleys of low altitude. Of these there are three, of small 

 area, situated in the interior of the region : Scott River, south fork 

 of the Salmon River at Summerville, and Trinity River in the vicin- 

 ity of Helena ; and two on the borders : Shasta Valley, in which 

 Mayten is situated, and the upper extremity of the Sacramento 

 Valley in the vicinity of Tower House. Taking into account chiefly 

 the Sonoran "islands", we find that the Trinity region possesses 

 18 species in common with the Sacramento Valley. This is some- 

 what less than 12 percent of the total mammal and bird fauna of 

 the Trinity area. Three species, or little more than 2 percent, are 

 shared with the Modoc region of extreme northeastern California 

 (and which has its western confines in Shasta Valley) ; while only 

 four Sonoran species, not quite 3 percent, occur also in the humid 

 coast belt. 



