ents of
the skulls listed below were taken by means of dial calipers
reading to one-tenth of a millimeter, and in the same fashion as
described previously (Anderson, 1954:492). Measurements of
specimens in each series were averaged (the arithmetic means were
computed). If the averages differed noticeably the significance of
the difference was tested statistically. Averages referred to in
the text as significantly different differ by as much as, or more
than, the sum of two times the standard error of each of the two
averages. Linear measurements are in millimeters; color values are
in the arbitrary units described in a preceding paragraph.
Measurements taken of the skulls are: condylobasilar length,
zygomatic breadth, interorbital breadth, lambdoidal breadth,
prelambdoidal breadth, depth of braincase, and alveolar length of
upper molar tooth-row.

Secondary sexual variation was not detected in the material
studied. Variation with age is important to the taxonomist even
among specimens designated as "adults", because growth and changes
in various proportions continue throughout the life of the mice.
The possibility that differences detected in the statistical
treatment or observed directly could be the result of differences
in average age within the samples of "adults" was considered in
each case.

In order to study certain variations, the following "method of pairs"
was used. Skulls of two series to be compared were matched in pairs so
that they corresponded in size and ontogenetic stage of development.
Then the two skulls of each pair were examined for differences in each
of the following features: size of circle inscribed by the upper
incisor teeth, width of nasal bones relative to their length, curvature
of the zygomatic arch, elongation of the braincase relative to its
width when viewed from the dorsal aspect, degree of indentation in the
anterior edge of the zygomatic arch near the rostrum, degree of
depression of the nasal bones when viewed from the side, width in the
vertical plane of the zygomatic arch at the suture between the
maxillary and jugal bones, length relative to width of the prominent
fenestra in the posterodorsal part of the squamosal bone, size of the
meatus of the auditory canal, distance between the internal margin of
an occipital condyle at its posteriormost point and the tip of the
paraoccipital process of