University of Kansas or that were examined at other institutions. Most of these specimens are stuffed skins with skulls separate. Skulls without skins, skins without skulls, entire skeletons, and separately preserved bacula are included as a part of the total. Almost every specimen is accompanied by an attached label, which bears place and date of capture, name of collector, external measurements, and sex.

Specimens used in the study of geographic variation were arranged by season of capture and according to geographic location; then they were segregated as to sex, and, under each sex, by age. Next, individual variation was measured in comparable samples of like age, sex, season, and geographic origin. Finally, comparable materials were arranged geographically in order to determine variations of systematic significance.

The only external measurements used were total length, length of tail, and length of hind foot; these measurements were recorded by the collectors on the labels attached to the skins. Height of the ear was not used since it was not recorded by many of the collectors.

In order to determine which cranial structures showed the least individual variation but at the same time showed substantial geographic variation, a statistical analysis was made of the 30 measurements, of cranial structures, heretofore used in taxonomic work on Zapus. The following measurements of the skull showed the least individual variation but showed some geographic variation and therefore, were used in this study. See figs. 1-3 which show points between which measurements were taken:

Occipitonasal length.—From anteriormost projection of nasal bones to posteriormost projection of supraoccipital bone. a to a´

Condylobasal length.—Least distance from a line connecting posteriormost parts of exoccipital condyles to a line connecting anteriormost projections of premaxillary bones. b to n

Palatal length.—From anterior border of upper incisors to anteriormost point of postpalatal notch. b to b´

Incisive foramina, length.—From anteriormost point to posteriormost point of incisive foramina. c to c´

Incisive foramina, breadth.—Greatest distance across incisive foramina perpendicular to long axis of skull. f to f´

Zygomatic length.—From anteriormost point of zygomatic process of maxillary to posteriormost point of zygomatic process of squamosal. d to d´

Zygomatic breadth.—Greatest distance across zygomatic arches of cranium at right angles to long axis of skull. j to j´

Breadth of inferior ramus of zygomatic process of maxillary.—Greatest distance across inferior ramus of zygomatic process of maxillary taken parallel to long axis of skull. d to e

Palatal breadth at M3.—Greatest distance from inside margin of alveolus of right M3 to its opposite. g to g´

Palatal breadth at P4.—Same as above except taken at P4. g to g´

Mastoid breadth.—Greatest distance across mastoid bones perpendicular to long axis of skull. h to h´

Breadth of braincase.—Greatest distance across braincase taken perpendicular to long axis of skull. i to i´

Interorbital breadth.—Least distance across top of skull between orbits. k to k´

Length of maxillary tooth-row.—From anterior border of P4 to posterior border of M3. l to l´

Breadth of base zygomatic process of squamosal.—Greatest distance across base of zygomatic process of squamosal taken parallel to long axis of skull. m to m´

Figs. 1-3. Three views of the skull to show points between which measurements of the skull were taken. Based on Z. t. montanus, adult, female, No. 22165 KU, Cascade Divide, 6400 ft., Crater Lake Nat'l Park, Klamath County, Oregon. × 4.

The baculum has a characteristic size and shape according to the species, and the following significant measurements of the structure were taken:

Greatest length.—From posteriormost border of base to anteriormost point on tip.

Greatest breadth at base.—Greatest distance across base taken parallel to long axis of bone.

Greatest breadth at tip.—Greatest distance across tip taken parallel to long axis of bone.

In the descriptions of color the capitalized color terms refer to those in Ridgway (1912). Any color term that does not have the initial letter capitalized does not refer to any one standard.

In the description of the subspecies the two sexes are treated as one because no significant secondary sexual variation was found. Only fully adult specimens of age groups 3 to 5, as defined on pages 377 and 388, have been considered.

Unless otherwise indicated, specimens are in the University of Kansas Museum of Natural History. Those in other collections are identified by the following abbreviations: