_Comparisons._--From _M. p. modestus_, _M. p. uligocola_ differs as follows: averages paler; prelambdoidal breadth and alveolar length of molar tooth-row significantly greater. Six pairs of skulls were compared. Of the features listed above under the "method of pairs" only two features differed in more than 75 per cent of the pairs; in 5 of 6 pairs _uligocola_ had a less distinctly indented anterior margin of the zygomatic arch (Confidence Limit .95) and a more elongate posterodorsal squamosal fenestra (C. L. .85). Seven pairs of skulls from Boulder, Colorado, representing _uligocola_ and from Colfax County, New Mexico, representing _modestus_ differed in more than 75 per cent of the pairs in three features. Only one of these differences, the elongation of the posterodorsal squamosal fenestra, was the same as a difference noted above between topotypes of _uligocola_ and _modestus_. A comparison of ten pairs of skulls of _uligocola_ from Boulder, Colorado, and topotypes of _uligocola_ revealed no significant differences. These observations are indicative of 1) the differences between samples and populations which may be assigned to a single subspecies, and 2) the fact that in general these local differences are less than the differences between subspecies. From _insperatus_, the subspecies to the north, _uligocola_ differs as follows: darker in both summer and winter pelage; averaging larger in most measurements of the skull; significantly longer molar tooth-row; hind foot averaging longer. For comparisons with the subspecies to the east and the northwest see the accounts of those below. _Remarks._--_M. p. uligocola_ is more closely restricted to wet situations than _M. ochrogaster haydeni_ (Baird) whose general range lies to the eastward. The numerous lakes, the continuous supply of water from the mountains, and the irrigation systems at lower altitudes along the eastern base of the mountains provide the conditions to which _uligocola_ is suited. It is named for its predilection for water. The variability in color is relatively greater in topotypes of both _uligocola_ and of _modestus_ than in many of the other series studied. Specimens from Denver and Colorado Springs taken in late autumn and winter (October to February) are paler, more reddish and less blackish, than specimens taken in June and July at Loveland. This reddishness results from longer, and more intensely reddish tips of the hair. The entire hairs also are longer. The average weight of 16 adults (12 males and 4 non-pregnant females) from near Loveland is 49.7 gms. The average length of the ear is 13.4.