Variation with age must be considered in order to assemble comparable samples of these shrews. Increased age results in wear on all teeth and in particularly striking changes in the size and shape of the first incisors. Skulls of older shrews develop sagittal and lambdoidal ridges, and further differ from skulls of young animals in being slightly broader and shorter, and in developing thicker bone, particularly on the rostrum which thus seems to be, but is not always in fact, more robust. Pruitt has recently (1954) noted these same cranial differences in specimens of _Sorex cinereus_ of different ages. Several students of American shrews, notably Pearson (1945) on _Blarina_, Hamilton (1940) on _Sorex fumeus_, and Conaway (1952) on _Sorex palustris_, have shown that young are born in spring and summer, usually reach sexual maturity the following spring, and rarely survive through, or even to, a second winter. The result is that collections made, as most of them are, in spring and summer, contain two age classes, first year and second year animals. These two age classes are readily separable on the basis of differences in the skull as well as on the decreased pubescence of the tail and the increased weight of second year animals. My own examination of hundreds of museum specimens confirms this for the _Sorex vagrans_ group. Separation of the two age classes in an August-taken series of _Sorex vagrans_ from coastal Washington is shown in figure 3, in which two tooth-measurements that are dependent upon wear are plotted against one another. First year animals are more abundant in collections than are second year animals. Within the first year, that is to say from spring to late fall, animals vary but little. Dental characters are best studied in first year shews. For this reason I have used them as the basis for the study of geographic variation, and descriptions are based on first year animals unless otherwise noted. CHARACTERS OF TAXONOMIC WORTH