_Comparisons._--As compared with the specimens of _M. m. nanus_ from Idaho, the size is larger (see diagnosis and measurements). Certain proportions which differ from those of _nanus_ and which are not in close agreement with the observed differences with age in specimens of _nanus_ of a size comparable to _codiensis_ are relatively large alveolobasilar length, relatively long alveolar length of upper molar tooth-row, relatively wide-spreading zygomatic arches, and relatively long tail. The color in _codiensis_ is lighter than in _nanus_. As compared to the new subspecies named below from the Big Horn Mountains to the east, _codiensis_ is of similar size in head-body length, but has a relatively as well as actually longer tail; the hind foot averages longer; the upper molar tooth-row is relatively longer; the color is slightly paler and less grizzled; the bullae are larger and less flattened; the angle formed at the suture between the basioccipital and basisphenoid bones is less acute; and the region of the suture is less prominently elevated between the bullae when viewed from the ventral aspect. The pterygoid plates mesial and posterodorsal to the posterior end of the last upper molar are less fenestrated, and the incisive foramina are less constricted posteriorly. [Illustration: FIG. 2. Map showing the major barriers to _Microtus montanus_ in Wyoming and Colorado; the barriers are the low areas named on the map (the name "Black Hills" is on the map for another reason; these hills are not a barrier). The major mountainous areas higher than approximately 8000 feet in elevation in Wyoming, Colorado and Utah are stippled. These mountainous areas include the habitat that is most suitable for the montane meadow mouse. The Black Hills are unoccupied by this species but these hills seem to be ecologically suitable for the species.]