31.4, and breadth of rostrum amounting to less, instead of more, than 30 per cent of basilar length; from _M. e. alascensis_ in males in that black tip of tail more than 43, total length more than 320, tympanic bullae more than 14 and longer than tooth-row rather than less than 14 mm. and sometimes shorter than tooth-row, females not individually distinguishable. _Description._--_Size._--Male: Four adults (Fort Franklin, Fort Simpson, Mts. W Fort Nelson, and Govt. Hay Camp, Wood Buffalo Park) yield average and respective measurements as follows: Total length, 331 average (340, 325, 330, 328); length of tail, 93 (102, 91, 93, 87); length of hind foot, 45 (48, 43, 45, 44). Weight of 4 adults from the Belcher Islands is 175 (135-180) grams. Of 10 subadults from Belcher Islands it is 119 (92-137) grams. Female: Three adults from Great Slave Lake (Willow River, Fairchild Point, and Fort Resolution) yield average and respective measurements as follows: Total length, 252 (237, 238, 282); length of tail, 69 (63, 60, 85); length of hind foot, 32 (31, 32, 34). Corresponding, average measurements for three adults from Glacier Lake are 240, 60, 32 and for 3 adults from the Athabasca Delta, 243, 65, 30. Weight of 8 subadults from the Belcher Islands is 69 (64-78) grams. Weight of adults would be more. _Color._--Winter pelage all white except tip of tail. Summer pelage with upper parts uniform in color and darker (16_n_) than Raw Umber, and about tones 3 to 4 of Chocolate of Oberthür and Dauthenay, pl. 343. Underparts Sulphur Yellow, Colonial Buff, or Primrose Yellow, often nearly white on chin and insides of forelegs; color of underparts extends narrowly over upper lips, distally on posterior sides of forelegs onto antipalmar faces of toes and sometimes over most of antipalmar surfaces of forefeet, on medial sides of hind legs to a point between knee and ankle but reappears on antiplantar faces of toes and in some individuals is narrowly continuous onto toes. Least width of color of underparts averaging, in a series of 12 males from the Athabasca Lake Region, 40 (25-54) per cent of greatest width of color of upper parts. Black tip of tail averaging 56 (45-63) mm. in 5 adult males from same region and thus 60 (48-70) per cent of length of tail-vertebrae.