MATERIALS, METHODS, AND ACKNOWLEDGMENTS
The present report is based on a study of approximately 3,600 specimens that were assembled at the Museum of Natural History of the 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´