320 PROCEEDINGS OF THE ACADEMY OF [1897. 



S. N. Rhoads, from Stuart Lake, B. C. ; total length 309 ; tail ver- 

 tebrae 143 ; hind foot 40. 



Average of seven adults of sabrinus ( jWeBangs, 1. c.) ; total length 

 278.6 ; tail vertebras 130.4 ; hind foot 37.6. Skull measurements of 

 the Stuart Lake specimen : occipito-nasal length 41.5 ; greatest 

 breadth 25 14 ; length of nasals 12.5 ; greatest length of mandible 23. 

 Skull measurements of an adult male sabrinus from Maine : occipito- 

 nasal length 38 ; greatest breadth 23; length of nasals 11 ; greatest 

 length of mandible 22. 



General remarks. — Making due allowance for the measurements 

 given by Richardson, Audubon and Bachman of their types of al- 

 pinus, and comparing these with the accurate field measurements 

 and complete skulls of the specimens of the alpinus group now avail- 

 able from numerous localities in the Rocky and Cascade Mountain 

 regions, it seems just to consider all the mountain forms of alpinus 

 as specifically distinct from sabrinus both in greater size and in the 

 relative proportions of the skull and extremities. The lowland 

 forms of alpinus closely agree in cranial characters with the type, 

 but in their diminished size approach sabrinus. 



A fine winter skin with skull and collector's measurements, from 

 Camp Davidson, on the Yukon River, near the eastern boundary of 

 Alaska, is larger than any other specimen of American Seiurop- 

 terus I have handled. Its measurements are given in the table. Its 

 color is somewhat brighter and redder than any Cascade specimens 

 of fuliginosus taken near the 49th parallel. Though not so dark as 

 the Stuart Lake specimen, its characters point it out to be the large 

 northern extreme in size of the alpinus group with color characters 

 intermediate between alpinus and fuliginosus, as its faunal position 

 would lead us to expect. The drab-gray shade of underparts, 

 smoke-gray of sides of head and blackish-slate of eyelids and ears, 

 ally it more closely to fuliginosus. 



Specimens examined. — British Columbia, Fort Liard,2 : Idaho, 3. 15 



14 This skull is relatively wider than the average of alpinus from other parts 

 of British Columbia, in this respect not being so typical of the narrow cranial 

 character of alpinus as contrasted with sabrinus. 



15 While the foregoing was in type, two winter skins, with skulls and meas- 

 urements, of the Rocky Mountain flying squirrel from Idaho County, Idaho, 

 were forwarded to me by Mr. O. Bangs. They are of great interest, their cran- 

 ial characters proving the specific difference between alpinus and sabrinus as al- 

 ready outlined, except that the typical alpinus form has relatively broader 

 parietals than fuliginosus, calijorniats and oregonensis. In this respect the 

 Idaho examples correspond to the Stuart Lake and Camp Davidson specimens 

 and agree with these in the large size of skull, the flattened fronto- 



