the same side of the skull, size of the
foramen magnum, vertical height of the supraoccipital bone from the
dorsalmost point on the margin of the foramen magnum to the midpoint of
the lambdoidal crest, constriction posteriorly or narrowness of the
incisive foramen relative to its length, distance across the
premaxillary bone from the anteriormost point of the incisive foramen
to the posteriormost point of the margin of the alveolus of the upper
incisor, area of the maxillary septum (Howell 1926:112, or "zygomatic
plate" of Ellerman 1941:1), acuminateness of the anterior border of the
palatine opening (internal nares), size of auditory bullae, size of
foramen ovale, acuteness of the angle between the basioccipital and
basisphenoidal bones at the suture between them (degree to which the
area of the suture is raised between the bullae when viewed from the
ventral aspect), width of first upper molar tooth, least distance
between alveoli of first upper molars. Any differential feature present
in more than 75 per cent of the pairs of animals is reported in the
discussion of the subspecies concerned. The significance of each
difference reported was calculated by the Chi-square test and the
confidence limit is given in each case. The probability used in the
Chi-square formula is one-half of the percentage of all pairs compared
in which the skulls were different in regard to the character being
considered. For example, in 68 per cent of the total number of pairs of
skulls compared in this study a difference in the size of the auditory
bullae was noted. Therefore the probability that a specified skull of a
pair will have larger bullae than the other skull was taken as 34 per
cent. A different probability for each feature compared was derived in
like manner.

This study is concerned primarily with mice from Wyoming and
Colorado; I realize, however, that the physiographic and ecological
conditions important to the distribution and subspeciation of
_Microtus pennsylvanicus_ do not correspond to political
boundaries. Geographic variation within these two states can be
seen in proper perspective only when related to the neighboring
areas and to previous studies. I have attempted to do this in the
accounts of the subspecies.

Approximately five months in the field in Wyoming and Colorado in
the summers of 1950, 1951, 1952, and 1953 gave me a familiarity
with the region that has helped to c