416 University of California Publications in Zoology [Vol.12 



amount of critical material representative of western beavers than 

 has probably ever before been available to any one worker. 



Ridgway's Color Standards and Color Nomenclature (1912) has 

 been used as a guide to color names. 



Overhair, as used in the following pages, refers to the long hairs 

 making up the contour pelage, those which would be removed in the 

 furrier's process of plucking. The under fur is the short, soft hair 

 which covers the skin closely and which remains in the plucked skin. 



For manner of taking cranial measurements see Taylor, 1911, 

 p. 206. Special or exceptional methods of measuring are explained 

 in the course of the paper. 



Material and Acknowledgments 



For the loan of material grateful acknowledgment is made to the 

 following institutions: The United States National Museum through 

 Mr. Richard Rathbun, Assistant Secretary, and Mr. Gerrit S. Miller, 

 Jr., Curator, Division of Mammals ; the Field Museum of Natural 

 History through Mr. Wilfred H. Osgood, Assistant Curator of Mam- 

 malogy and Ornithology ; and the United States Department of 

 Agriculture through Mr. H. W. Henshaw, Chief of the Bureau of 

 Biological Survey. 



Considerable material representative of western beavers is con- 

 tained in the collection of the Museum of Vertebrate Zoology of the 

 University of California. The gathering of this material has been 

 largely due to the interest of Miss Annie M. Alexander in the par- 

 ticular problem. The specimens from the San Joaquin River, Cali- 

 fornia, were obtained directly by her from a local trapper, and those 

 from Vancouver Island and southeastern Alaska were collected on 

 three expeditions from the Museum made possible through means 

 furnished by her. Altogether 86 specimens of beavers, some repre- 

 sented by skins and skulls, others by skulls alone or skins alone, and 

 one by jaws only, have been available for study. 



The writer is also indebted to the following persons, who have 

 very generously given of their time and interest in assisting through 

 helpful criticism and suggestion : Professor Charles A. Kofoid, 

 Professor Samuel J. Holmes, Professor J. Frank Daniel, Professor 

 John C. Merriam, Dr. Harold C. Bryant, Mr. F. H. Holden, and 

 especially Dr. Joseph Grinnell. 



