_Comparisons._--For comparison with _M. m. nanus_, the subspecies to the northward, see the preceding account of that subspecies. For comparison with _M. m. amosus_ the subspecies to the west see Hall (1938) and Durrant (1952). I have not examined specimens of _amosus_. _Measurements._--Average and extreme measurements for 17 adults including both males and females from several localities in southern Colorado are as follows: total length, 160 (136-179); length of tail, 42 (35-55); hind foot, 19.2 (17-23); condylobasilar length of the skull, 25.2 (24.0-26.0); zygomatic breadth, 15.0 (14.1-15.5); alveolar length of upper molar tooth-row, 6.4 (6.0-6.7); prelambdoidal breadth, 8.7 (8.3-9.2); lambdoidal breadth, 11.7 (11.1-12.6). Average and extreme measurements of 4 adults (2 males and 2 females) from the type locality and 11 adults (4 males and 7 females) from other localities in southern Colorado are as follows: total length, 162 (157-168), 157 (137-169); length of tail (means only), 44.5, 40.5; hind foot, 18.8 (18-19), 18.6 (18-23); condylobasilar length of skull, 24.5 (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.