Available specimens were arranged according to geographic origin. These
were segregated as to sex and then under each sex by age. Individual
variation was next measured in each of several samples in which
individuals were of like geographic origin, sex, age and season.
Finally, comparable materials were arranged geographically for detection
of variations of systematic worth. Following preliminary studies of
material thus arranged, additional specimens were collected from
critical areas.
When fully adult animals (see next paragraph) were segregated as to sex,
and then measured, the degree of secondary sexual variation was found to
be less than the degree of individual variation; therefore in the tables
of indices, no distinction as to sex has been made.
The only external measurements of the animals used were those recorded
by the collectors on the labels attached to the skins. These
measurements were total length, length of tail and length of hind foot.
Measurements of the ear have not been used since they were not in all
instances recorded by collectors and since measurements of dry ears
proved to be unsatisfactory. Only measurements of fully adult specimens
have been used. The term fully adult is applied only to those specimens
in which the auditory bulla is shiny and translucent, the permanent P4
is fully erupted and worn, and the tail is fully striped and
penicillate. No one of these characters alone was accepted as proof of
adulthood but only the three in combination.
The following measurements of the skull have been used in the tables:
_Greatest length._--From the most anterior tip of the nasals to
the most posterior projection of the auditory bullae.
_Greatest breadth across bullae._--From the most lateral
projection of the auditory bulla on one side to the corresponding
position on the other bulla.
_Breadth across maxillary arches._--Greatest breadth across arches
in a plane perpendicular to the long axis of the skull.
_Width of rostrum._--Width of the premaxillae and the nasals taken
immediately anterior to the upper incisors (not greatest width of
nasals which is attained farther anteriorly).
_Length of nasals._--Maximum length of a nasal bone.
_Least interorbital breadth._--Least width between the orbits
immediately posterior to the lacrimal processes.
_Basilar length._--From the anterior margin of the foramen magnum
to the posterior border of the alveolus of one of the upper
incisors.
Capitalized color terms are from Ridgway, "Color Standards and Color
Nomenclature," Washington, D. C., 1912. Color determinations were made
by comparing a masked area of pure color on the side of the animal with
a masked rectangle of named color on Ridgway's plates in natural light
always from the same angle.
Abbreviations used for specimens examined from the various collections
are as follows:
AMNH--American Museum of Natural History.
BYU--Brigham Young University.
CNHM--Chicago Natural History Museum.
CM--Carnegie Museum.
CMNH--Colorado Museum of Natural History.
DJC--Dixie Junior College.
DRD--Donald R. Dickey Collection.
KU--Museum of Natural History, University of Kansas.
LACM--Los Angeles County Museum.
MHS--Collection of Myron H. Swenk.
MVZ--Museum of Vertebrate Zoology, University of California.
OU--Museum of Zoology, University of Oklahoma.
RH--Collection of Ross Hardy.
UM--Museum of Zoology, University of Michigan.
UN--Museum of Natural History, University of Nebraska.
USAC--Utah State Agricultural College.
USBS--United States Biological Surveys Collection.
USNM--United States National Museum.
UU--Museum of Zoology, University of Utah.
TCWC--Texas Coöperative Wildlife Collection.
This study is based on 3,732 specimens which were assembled at the
Museum of Natural History, University of Kansas, or studied at other
institutions. For the loan of this material and for the opportunity
afforded for its study, I am extremely grateful to the authorities of
each of these institutions and to the owners of the private collections.
Acknowledgement is made to the Office of Research and Inventions of the
United States Navy for assistance with the field work which permitted
the acquisition of essential specimens from several of the critical
geographic areas while the author was research assistant on a larger
over-all project (N6 ori-164-T02) of which the determination of the
geographic range of this rodent species, a potential host of tularemia,
was one facet. Tularemia was not detected in this genus.
I extend my thanks also to Professor Stephen D. Durrant, of the
University of Utah, for helpful corrections in the preparation of the
manuscript; to Mrs. Virginia Cassell Unruh for the preparation of the
drawings; to Professor E. Raymond Hall, of the University of Kansas, for
guidance in the study and critical assistance with the manuscript; to
Professors H. H. Lane and Worthie H. Horr for valued suggestions; to Mr.
J. R. Alcorn for providing specimens for dissection when he was working
under the University of Kansas endowment fund; and to the other friends
and associates who have given of their time and criticism.