(24.0-24.7), 25.2 (24.3-26.1);
alveolobasilar length, 14.2 (13.9-14.5), 14.6 (14.1-15.1);
palatilar length, 13.2 (13.0-13.4), 13.5 (13.1-14.2); alveolar
length of upper molar tooth-row, 6.3 (6.0-6.6), 6.4 (6.3-6.7);
zygomatic breadth, 15.0 (14.3-15.5), 14.9 (14.1-15.5); interorbital
breadth, 3.5 (3.3-3.6), 3.5 (3.3-3.7); lambdoidal breadth, 11.8
(11.1-12.6), 11.7 (11.2-12.3); prelambdoidal breadth, 8.6
(8.3-9.2), 8.8 (8.3-9.0); depth of braincase, 7.5 (7.2-7.8), 7.6
(7.1-7.9).

_Discussion._--There is no sharp boundary between _M. m. fusus_ of
southern Colorado and the subspecies to the north, _M. m. nanus_.
Although the line separating these two subspecies is drawn somewhat
arbitrarily, on the whole the samples from north of this line more
closely resemble _nanus_. All of the means for total length given above
are larger than the maximum given in Hall's description of _fusus_. The
caudal index (38 and 35% in two series) is slightly larger than that
cited by Hall (33.3%) and is not significantly different from that in
_nanus_ (35.2%). The color in both young and old mice is variable, but
in general is more yellowish, and less grayish, than in any other series
studied.

There is a large area in western Colorado and eastern Utah, between the
known ranges of _M. m. fusus_ and _M. m. amosus_ from which there are no
specimens. Probably the species occurs only at certain places in this
arid region which seems to be a partial barrier to the species.

Specimens of _M. montanus_ from northern New Mexico have been referred
previously to _M. m. arizonensis_. When he named _M. m. fusus_, Hall
mentioned its resemblance to _arizonensis_ in reddish coloration, but
pointed out that _fusus_ is less reddish. Of six specimens from Valle
Santa Rosa, Jemez Mountains (USBS), 8500 ft., Rio Arriba County, New
Mexico, three are immature, and the skulls of the remaining specimens
are damaged. In reddish color and relatively large size these few
specimens resemble _arizonensis_ more than _fusus_ although the locality
of occurrence is closer to the geographic range of the northern _fusus_
than to that of _arizonensis_. The identification of these specimens as
_arizonensis_ is provisional; additional specimens are needed from the
area, 200 miles wide, which separates the ranges as now known of
_arizonensis_ in Arizona from the occurrence in New Mexico. There is a
single specimen from this area, the damage