418 University of California Publications in Zoology [Vol.12 



recently described by Bangs (1913, p. 513), under the name of 

 Castor caecator. 



Two more subspecies of canadensis, presenting respectively a very 

 pale desert coloration, and a dark, rich coloration, have been described 

 by Bailey (1913, pp. 191-193). These are Castor canadensis mexi- 

 ca a us, type locality Ruidoso Creek, six miles below Ruidoso, New 

 Mexico, and Castor canadensis michiganensis, type locality Tahqua- 

 menaw River (five miles above falls), Luce County, Michigan. 



The beavers of the west coast recognized in this paper are as 

 follows : 



Castor canadensis belugae, new subspecies (see p. 429), Cook 

 Inlet region, base of Alaska Peninsula and probably Kenai Penin- 

 sula, and southward west of Rocky Mountains to central British 

 Columbia. 



Castor canadensis phaeus Heller, Admiralty Island, Alaska ; prob- 

 ably neighboring islands and mainland. 



Castor canadensis leucodonta Gray, Vancouver Island. 



Castor canadensis pacificus Rhoads, probably mainland of British 

 Columbia, Washington and Oregon ; precise limits of range unknown. 



Castor canadensis frondator Mearns, Colorado River drainage, 

 and probably of broad distribution in southern Great Basin region. 



Castor subauratus subauratus Taylor, Sacramento, Feather, 

 American, and San Joaquin rivers, California. 



Castor subauratus shastensis, new subspecies (see p. 433), east of 

 Sierra Nevada Mountains, California ; drainage of the Pit River. 



Change Due to Age in a Single Species (Castor canadensis leuco- 

 donta Gray) 



Beavers secured on Vancouver Island by the expedition of the 

 Museum of Vertebrate Zoology in 1910 represent three generations, 

 and so make possible an outline of the changes in certain character- 

 istics due to age. 



EXTERNAL CHARACTERS 



Measurements 

 (See table, p. 419) 



Difficulties are immediately apparent when one attempts to set 

 down laws of change of form with age, the most important of which 

 are that (a) weights and measurements of the youngest beavers are 



