156 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



are found, and has come to occupy a niche entirely different. 

 It has performed not only the usual geographic migration, 

 but also the comparatively rare ecological migration. 



F SUMMARY 



1. For 22 years subsequent to its discovery the micro- 

 tine rodent Phenacomys longicaudus True was represented in 

 natural history museums by but three specimens. There have 

 been recent accessions of notes and specimens which permit 

 of substantial contributions to knowledge of its systematic 

 and ecologic status. 



2. Habits and associational relations are just as much a 

 part of the animal as its physical characters. In the interest 

 of adequacy and comprehensiveness, emphasis upon study in 

 these fields should and probably will become more insistent 

 as time goes on. 



3. Phenacomys longicaudus is the type of Arhorwnis, a 

 new subgenus of Phenacomys. The most striking characters 

 of the type are its cinnamon reddish dorsal coloration, its 

 long tail, and its arboreal habitat. 



4. Phenacomys albipes Merriam is tentatively referred 

 to the new subgenus, though it is intermediate in certain 

 characters between Arborimns and Phenacomys, and differs 

 from P. longicaudus in several important particulars. 



5. The subgenus Arborimus is restricted to the humid 

 coast belt of western North America, specimens having been 

 taken in southwestern Oregon and northwestern California 

 only. 



6. All the clear evidence at hand indicates that the tree 

 mouse is dependent on the trees in which it lives for food, 

 drink and shelter. 



7. The tree mouse has been found nesting in the Sitka 

 spruce (Picea sitchensis), the grand fir or lowland fir (Abies 

 grandis), and the Douglas fir (Pseudotsuga taxifolia). 



8. Ordinarily the nests are found in groups, so that it is 

 proper to refer to the animal as loosely colonial. 



9. Our experience with Phenacomys longicaudus did not 

 show it to possess any extraordinary agility, quickness, or 

 aggressiveness. 



10. The tree mouse, from all evidence, feeds principally 



