The average differences in external measurements of the two sexes are: Total length, 37; length of tail, 13; length of hind foot, 7. General comparisons indicate that the Alberta-taken males may not attain so large a size as those from some other areas. Thus the differences in external measurements might be some greater elsewhere, say, in North Dakota. _Externals._--Longest facial vibrissae black, brown or white (often all three colors in same specimen) and extending beyond ear; carpal vibrissae same color as underparts and extending to apical pad of fifth digit; hairiness of foot-soles (in summer pelage) only slightly greater than shown in figure 20. _Color._--Winter pelage all white except tip of tail. Summer pelage with upper parts near (_h_) Clay Color or near tone 3 and 4 of Snuff Brown of Oberthür and Dauthenay, pl. 303. Chin and upper lips white. Remainder of underparts ranging from near (_a_) Olive Ocher to near (16´) Ochraceous Buff. Upper parts of uniform color except for occasional darkening of head in front of ears. Color of underparts extends distally on posterior sides of forelegs over toes onto antipalmar faces of feet and wrists, on medial sides of hind limbs to ankles over antiplantar faces of toes and distomedial third of each tarsus, and over proximal fourth to third of under side of tail. Least width of color of underparts averaging, in a series of 10 males from Alberta, 58 (45-60) per cent of greatest width of color of upper parts. Corresponding figures for 10 females from the same place are 57 (50-74). Black tip of tail in same series of males, most of which are in full summer pelage, averaging 43 (35-60) mm. long. Thus, averaging shorter than hind foot and 27 per cent of length of tail-vertebrae. As compared with _M. f. neomexicana_, _longicauda_ lacks the white facial markings, black ears, black forehead and nose, but otherwise is similarly colored. As compared with _M. f. nevadensis_, _M. f. oribasus_ and _M. f. spadix_, each of color pattern similar to _longicauda_, selected differences of _longicauda_ are its much lighter color, especially of the upper parts, with less conspicuous darkening on the nose. From _M. f. primulina_, _longicauda_ differs in lighter color of upper parts, reddish rather than yellowish underparts, and light rather than dark-colored hind feet.