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_