as follows: total length, 159(150-175); length of
tail, 37.6(31-46); hind foot, 18.6(17-20); condylobasilar length of
the skull, 25.3(24.2-26.7); zygomatic breadth, 15.3(14.1-16.7);
alveolar length of upper molar tooth-row, 6.2 (5.7-6.8);
prelambdoidal breadth, 8.7(8.3-9.4); lambdoidal breadth,
11.9(11.0-12.5). Average and extreme measurements of a series of 12
adult male topotypes are as follows: total length, 159(144-174);
length of tail, 36.4 (30-41); hind foot, 18.2(16-20);
condylobasilar length of skull, 25.8(24.7-26.7); alveolobasilar
length, 14.8(13.8-15.3); palatilar length, 13.8 (12.7-14.2);
alveolar length of upper molar tooth-row, 6.4(5.9-6.6); zygomatic
breadth, 15.9 (15.0-16.7); interorbital breadth, 3.6(3.4-3.7);
lambdoidal breadth, 12.1 (11.5-12.5); prelambdoidal breadth,
8.6(8.3-8.9); depth of braincase, 8.0 (7.6-8.3).

_Discussion._--This subspecies is separated from _M. m. codiensis_ to
the west by the Big Horn Basin. A series from along Buffalo Creek, 27
mi. N, 1 mi. E Powder River, 6075 ft., in Natrona County, Wyoming, is
intermediate between the topotypes of _zygomaticus_ and _nanus_ in the
characters cited above as distinguishing the two, but shows greater
resemblance to _zygomaticus_ in the shape of the zygomatic arch, in
color which is paler than in topotypes of _zygomaticus_, and in the
short hind foot. On these and on geographic grounds this population is
referred to _zygomaticus_. Unfortunately we cannot be certain in many
cases that an intermediate condition in a certain character indicates a
genetically intermediate population and therefore true intergradation
between the two subspecies to which the population is geographically
intermediate. The topotypes of this subspecies are the most distinct of
all the series which I have studied from the eastern Rocky Mountains, in
terms of the degree of morphological departure from the norm for the
species. After _zygomaticus_ the following populations are arranged
according to their degree of deviation from this norm (_codiensis_
deviates most): topotypes of _codiensis_, _fusus_ and a population from
southern Sweetwater County, Wyoming, and lastly the _nanus-caryi_
complex. Within the latter group, as I have mentioned, there are a
number of local variants most of which do not differ significantly and
do not conform to any geographic pattern.

_Specimens examined._--Total, 55, distr