_Description._--_Size._--Male: Fifteen subadult topotypes yield average and extreme measurements as follows: Total length, 400 (357-437); length of tail, 149 (122-171); length of hind foot, 47.6 (42-59). Tail averages 59 per cent as long as head and body. Length of hind foot averaging more than basal length. Corresponding measurements of one adult and 3 young from Mount Rainier are: 415 (405-423); 155 (145-164); 51 (50-53). Female: Five adult topotypes yield average and extreme measurements as follows: Total length, 349 (330-393); length of tail, 124 (114-133); length of hind foot, 38 (36-39). Tail averages 55 per cent as long as head and body. Length of hind foot averaging about same as basal length. Corresponding measurements of two adults and 6 young from Mount Rainier are: 338 (320-360); 121 (115-132); 36 (34-40). The average differences in external measurements of the two sexes, from Mount Adams, are: Total length, 51; length of tail, 25; length of hind foot, 9.6. Corresponding differences between the specimens from Mount Rainier are: 77; 34; 15. _Externals._--Longest facial vibrissae black or brown (often both colors in same specimen) and extending beyond ear; carpal vibrissae same color as underparts and extending to or beyond apical pad of fifth digit; hairiness of foot-soles slightly less than shown in figure 19. _Color._--Upper parts in summer near (14 _n_) Argus Brown or tone 4 of Burnt Umber of Oberthür and Dauthenay, pl. 304; one topotype Buckthorn Brown or tone 3 to 4 of Snuff Brown of Oberthür and Dauthenay, pl. 303. Dark spot at each angle of mouth present or absent, and when present, often fused with color of upper parts, which rarely covers lower lips. Chin, and usually lower lips, white. Remainder of underparts Buff-Yellow to Naples Yellow. In winter, all white except tip of tail which is at all times black, or upper parts near (14) Brussels Brown to near (_j_) Snuff Brown with smoked effect and underparts white, rarely with trace of yellowish. Color of underparts extends distally on posterior sides of forelegs over toes onto antipalmar faces of feet and usually all of wrists, on medial sides of hind legs anywhere from knee to tips of toes. Least width of color of underparts averaging in ten topotypes, 24 (10-37) per cent of greatest width of color of upper parts. Black tip of tail in same series averaging 55 (45-60) mm. long, thus longer than hind foot and averaging 37 per cent of length of tail-vertebrae. The color of the underparts is not so narrow in the specimens from Mount Rainier and it is believed that the slender bodies used in stuffing the topotypes has accentuated in them the appearance of narrowness of the light-colored underparts.