For each of the series listed in Table 1 all adult mice having skulls that measured more than 24.0 mm. in condylobasilar length were studied. Total length, length of tail, and length of hind foot were taken from the collector's field labels. The measurements 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.