_Type._--Adult female, skin and skull; No. 23525/30943, U. S. Biol. Surv. Coll.; obtained on August 26, 1890, by Vernon Bailey and B. H. Dutcher from near Timber Creek, 8200 ft., Lemhi Mts., 10 mi. SSW Junction (now Leadore), Lemhi Co., Idaho. _Range._--Mountainous interior of western North America from central Alaska east across Yukon and southwestern Northwest Territories to northeastern Alberta, south in the mountains through north-central and western Washington, Idaho, western Montana, Wyoming, Utah, and Colorado, into northern New Mexico. _Diagnosis._--Size medium to small for the species; average and extreme measurements of 9 topotypes are: total length, 110.3 (105-117); tail, 46.4 (42-50); hind foot, 13.1 (12.5-13.5). Color grayish or brownish gray in summer, light grayish in winter. _Comparisons._--For comparisons with _S. v. setosus_, _S. v. longicauda_, _S. v. alascensis_ and _S. v. shumaginensis_ see accounts of those subspecies. Paler and slightly larger than S. v. soperi. Larger than the subspecies from central Montana herein described as new. Smaller than _S. v. neomexicanus_. Averaging larger in all dimensions than _S. v. monticola_ with which _obscurus_ intergrades in northern New Mexico and northern Arizona. Larger than _S. v. vagrans_ with more grayish rather than reddish fresh summer pelage and light gray rather than dark grayish-black fresh winter pelage. _Remarks._--Intergradation of _S. v. obscurus_ with _S. v. setosus_, _S. v. longicauda_, _S. v. alascensis_, and the new subspecies from Montana takes place in the usual way with specimens from intermediate localities being intermediate in size and color. However the relationship of _S. v. obscurus_ and _S. v. vagrans_ (as the latter subspecies is defined in this study) is rather complicated. In southern British Columbia where the two subspecies come together a situation of remarkable complexity prevails. Series from some localities seem to represent intergrades between _obscurus_ and _vagrans_; from other localities some specimens seem to be referable to one and some to the other subspecies; from other localities all specimens seem referable to one subspecies. A similar situation is seen in specimens from northeastern Washington, northern and central Idaho, and extreme western Montana. The region mentioned is one of extensive interfingering of life-zones. In southern British Columbia the main axes of the rivers, valleys and mountain ranges are north and south. Most of the valleys are in the Transition Life-zone; the forests are rather dry and of pine with more or less isolated hydrosere communities about streams and ponds. These hydrosere situations are the habitat of _Sorex vagrans_. Shrews from these situations are usually referable to _vagrans_. The high ridges and mountain ranges are usually in the Canadian Life-zone or higher and most of the shrews referable to _obscurus_ come from such places. Marginal localities with regard to life-zone produce most of the