roka and
Wind River ranges into the western part of the Big Horn Basin.

_Diagnosis._--A relatively large _Microtus montanus_; tail actually
and relatively long; hind foot actually but not relatively large;
skull large; zygomatic expanse actually and relatively large;
alveolobasilar length relatively large; upper molar tooth-row
relatively long; color relatively light, not reddish.

[Illustration: FIG. 1. Geographic range of _Microtus montanus_ in
Wyoming, Colorado, and adjacent areas. The solid circles represent
localities from which specimens have been examined; the hollow circles
represent type localities. The ranges of subspecies in Utah are after
Durrant, 1952.

Guide to subspecies

1. _M. m. nanus_
2. _M. m. codiensis_
3. _M. m. zygomaticus_
4. _M. m. fusus_
5. _M. m. micropus_
6. _M. m. nexus_
7. _M. m. amosus_
8. _M. m. rivularis_
]

_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