3916] GrinneU: Analysis of Trinity Fauna 407 



It would thus appear that, as regards its Sonoran fauna, the 

 Trinity region allies itself strongly with the Sacramento fauna 

 rather than with either of the other two named. 



As far as specimens have become available and carefully studied 

 (Kellogg, 1916), only five endemic forms have been found, all 

 mammals. Four of these are Boreal: Thomomys monticola pine- 

 torum Merriam, Aplodontia chryseola Kellogg, CaUospermophilus 

 chrysodeirus trinitatis Merriam, and Glaucomys sabrinus flaviventris 

 Howell; one is Sonoran: Dipodomys californicus trinitatis Kellogg. 

 Only one of the five is well-marked, the Aplodontia, the other four 

 being but faintly characterized. This indicates a differentiation 

 center of weak power. 



It is apparent that the Trinity region shows but weak faunal 

 individuality. It is not sharply set off, except on the west, nor does 

 it contain notably distinct forms. This relative unimportance as a 

 differentiation center is doubtless due to its small area and to the lack 

 of efficient barriers which would prevent intercrossing with repre- 

 sentatives in adjacent faunas. No very low zone is interposed, the 

 Transition, merely, being continuous between the Boreal areas. The 

 climatic features, too, are clearly not so pronouncedly different from 

 those of the Sierra Nevada as to make up for the very short distance 

 between the main Trinity mass and the Sierras. 



There must be a very abrupt line of demarcation between the 

 Trinity fauna and the humid coast fauna, for we have very few forms 

 venturing from the latter into the former, in spite of the very short 

 distance. It is true that with a few birds we find intergrades in the 

 Trinity region between typical Sierran and typical coast-belt forms ; 

 for example, in the genera Dendragapus, Dryobates and Cyanocitta. 

 But in by far the greater number of appropriate cases, in both birds 

 and mammals, the Trinity representatives are unequivocally of in- 

 terior forms, and this in spite of the fact that practically all of the 

 Trinity drainage is west directly to the Pacific and lies within a 

 maximum distance of ninety miles of the coast. The climatic barrier, 

 of abruptly and greatly increased humidity on the west, is evidently 

 much more efficient in cheeking the spread of species than the various 

 Transition-zone "gaps" between the mountain masses of the interior 

 or the Transition-zone divides between the "islands" of Upper 

 Sonoran. 



The whole idea of basing the efficacy of zonal "gaps" on the 

 summation of the species occurring on either side (see Merriam, 



