mens.

All measurements of the bacula were made by means of an eyepiece
micrometer.

A total of 194 bacula were seen. All of these are in the Museum
of Natural History of the University of Kansas, unless otherwise
indicated by the following symbols:

BS United States Biological Surveys Collection.
CN Chicago Natural History Museum.
LA Los Angeles County Museum.
MM University of Michigan, Museum of Zoology.
NM United States National Museum.
UU University of Utah, Museum of Zoology.

I am grateful to Professor E. Raymond Hall for guidance in my
study and thank Drs. Robert W. Wilson, Keith R. Kelson, and
Edwin C. Galbraith, as well as other friends and associates of
the Museum of Natural History, University of Kansas, for
encouragement and valuable suggestions. Dr. William L. Jellison,
United States Public Health Service, aided me in part of my
field work and kindly sent me some specimens of chipmunks. My
wife, Alice M. White, made the illustrations and helped me in
many ways.

For the loan of bacula I thank Dr. William H. Burt, University
of Michigan, Museum of Zoology. For permission to search for
bacula on study skins, and to process those that were found, I
thank Miss Viola S. Schantz, United States Fish and Wildlife
Service, Mr. Colin C. Sanborn, Chicago Natural History Museum,
Mr. Kenneth E. Stager, Los Angeles County Museum, Dr. David H.
Johnson, United States National Museum, and Dr. Stephen D.
Durrant, Museum of Zoology, University of Utah.

Assistance with field work is acknowledged from the Kansas
University Endowment Association, the National Science
Foundation and the United States Navy, Office of Naval Research,
through contract No. NR 161 791.

VARIATION

_Individual variation._--Individual variation is small. This is shown by
a coefficient of variability of only 3.85 in the length of the shaft in
a series of 12 specimens of _E. umbrinus umbrinus_ from Paradise Park,
21 mi. W and 15 mi. N Vernal, 10,050 ft., Uintah County, Utah.

_Variation with age._--In the chipmunks the baculum varies but little
with age. In the youngest specimens that I have taken, the M3 and m3
have not yet erupted and there is no wear on P4 and p4; nevertheless,
the baculum in these specimens has nearly an adult configuration and
size. In juvenal