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 populations which seem to represent intergrades between the two subspecies. Isolated areas of Canadian Life-zone, even though surrounded with Transition Life-zone, often harbor a population of obscurus, whereas the streams in the nearby dry valleys harbor populations of vagrans. Farther south in the Rocky Mountain chain, obscurus seemingly intergrades regularly with vagrans. This intergradation is seen in populations from several localities in Utah. There the lower elevations west of the Wasatch and Uinta mountains are inhabited by S. v. vagrans, the higher elevations by obscurus and where the ranges of the two abut intergrading populations occur. In these series of intergrades there are specimens which, using size as a subspecific criterion, would unhesitatingly be assigned, as individuals, to obscurus, and others would be assigned to vagrans, but these individuals represent extremes of a normally variable population. At Cuddy Mountain, Idaho, the two subspecies seemingly abut without intergradation; anyhow the available specimens from this locality are referable to one or the other subspecies and none is intermediate. The situation just described understandably has been the source of much anguish to students who sought to identify shrews from the Rocky Mountains. The reason for the relationship just described has been discussed at length in a previous section.
In the Rocky Mountains of Wyoming and Colorado the subspecies S. v. obscurus ranges almost uninterruptedly over relatively large areas, but southward in New Mexico and southwestward into Utah and Arizona, suitable boreal habitat becomes insular in nature and obscurus there is confined to the higher mountains. With one exception, once the shrew populations become 'insular' in this region they become smaller and show intergradation with Sorex vagrans monticola. The exception is the population in the Sacramento Mountains of southeastern New Mexico which is larger than obscurus and has been rightly recognized as a distinct subspecies, neomexicanus.
Almost without exception the range of typical Sorex vagrans obscurus is sympatric with that of Sorex cinereus, usually the subspecies S. c. cinereus. So close is this correspondence that the presence of S. cinereus comes near to being a useful aid in identifying S. v. obscurus. In areas where individuals of obscurus show intergradation with vagrans, Sorex cinereus is absent or rare. The implication is that as the species S. vagrans approaches the size of the species S. cinereus, competition between the two increases with resultant displacement of cinereus.
Specimens examined.—Total number, 982.
Alaska: Wahoo Lake, 69° 08' N, 146° 58' W, 2350 ft., 2 KU; Chandler Lake, 68° 12' N, 152° 45' W, 2900 ft., 1 KU; Bettles, 1 KU, 5 BS; Alatna, 1 BS; Yukon River, 20 mi. above Circle, 1 BS; Tanana, 1 BS; Mountains near Eagle, 18 BS; Richardson, 8 BS; head of Toklat River, 11 BS; Savage River, 8 BS.
Yukon: MacMillan Pass, Mile 282, Canol Road, 1 NMC; MacMillan River, Mile 249, Canol Road, 1 NMC; S. fork MacMillan River, Mile 249, Canol Road, 2 NMC; Sheldon Lake, Mile 222, Canol Road, 5 NMC; Rose River, Mile 95, Canol Road, 1 NMC; McIntyre Creek, 3 mi. NW Whitehorse, 2250 ft. 1 KU; Nisutlin River, Mile 40, Canol Road, 6 NMC; SW end Dezadeash Lake, 2 KU; 3 mi. E and 1½ mi. S Dalton Post, 2500 ft., 1 KU.
Mackenzie: Nahanni River Mtns., Mackenzie River, 1 BS; Fort Simpson, 3 BS; Fort Resolution, Mission Island, 1 BS.
British Columbia: W. side Mt. Glave, 4000 ft., 14 mi. S and 2 mi. E Kelsall Lake, 1 KU; Stonehouse Creek, 5½ mi. W jct. Stonehouse Creek and Kelsall River, 4 KU; Bennett City, 6 BS; Wilson Creek, Atlin, 1 PMBC; McDame Post, Dease River, 6 BS; McDame Creek, 3 BS; Hot Springs, 3 mi. WNW jct. Trout River and Liard River, 1 KU; NW side Muncho Lake, 1 KU; Little Tahtlan River, 1 AMNH; Junction (4 mi. N Telegraph Creek), 7 BS; Raspberry Creek, 16 AMNH; Klappan River Valley, 1 BS; Chapa-atan River, 4 BS; Fort Grahame, 3 BS; Kispiox Valley, 23 mi. N Hazleton, 1 BS; Bear Lake, site of Fort Connully, 2 BS; Tetana Lake, 1 PMBC; Hudson Hope, 2 BS; Charlie Lake, 3 PMBC; Babine Mts., 6 mi. N Babine Trail, 5200 ft., 1 BS; Big Salmon River (S branch near Canyon), 1 BS; Ootsa Lake, 2 PMBC; Indianpoint Lake, 4 PMBC; Barkerville, 7 BS; Yellowhead Lake, 2 NMC, 1 PMBC; N. fork Moose River, 1 BS; Moose Lake, 2 BS; Moose Pass, 1 BS; Glacier, 7 AMNH, 12 BS; Golden, 1 BS; Field, 2 BS; Caribou Lake, near Kamloops, 2 BS; Sicamous, 1 BS; Monashee Pass, 4 PMBC; Paradise Mine, 3 PMBC; Level Mtn., 4 AMNH; 6 mi. S Nelson, 6 BS; Morrissey, 5 NMC; Wall Lake, 1 BS.