dicate
that topotypes of _pullatus_ are darker, longer-tailed, slightly
larger-skulled and perhaps longer over all.

_Remarks._--In this subspecies there is a cline in color from dark in
extreme southwestern Wyoming to pale in north-central Wyoming and
Montana as the range of _M. p. insperatus_ is approached. There is thus
a broad zone of intergradation in color and the line separating the
subspecies must be drawn somewhat arbitrarily. In Wyoming the most
distinct break in this cline is in the Big Horn Basin and if a detailed
study of the species were made in Montana the break would probably be
found where the mountains meet the plains, roughly as shown in Figure
1. There is a similar cline in western Montana in color. The mice are
paler farther north as one approaches the Canadian border although they
do not become so pale as _insperatus_. Darkness is a characteristic of
several non-adjacent subspecies of _Microtus pennsylvanicus_, for
example _M. p._ _kincaidi_ Dalquest in central Washington (Dalquest,
1948:347), _M. p. finitus_, and _M. p. nigrans_ Rhoads in eastern
Virginia, but these subspecies presumably differ in other characters.

Some morphological features of the same kind and degree that
differentiate subspecies in one place may not vary geographically in
another place. Furthermore the geographic variation in one feature may
be only partly correlated with the variation in another feature. The
variation in _M. p. pullatus_ is an example: Specimens from near
Pocatello, Idaho, are darker than topotypes of _modestus_ but specimens
from Fremont County, Idaho, are indistinguishable from topotypes of
_modestus_ (Davis, 1939:315). I have examined a number of mice from the
Bitterroot Valley in western Montana and the color value for 12 adults
is 2.7. They are slightly but not significantly paler than topotypes of
_modestus_. This is a result of the cline mentioned above and does not
indicate relationship with _modestus_. Some average measurements of 10
skulls from this series are as follows: condylobasilar length, 24.9;
zygomatic breadth, 14.4; interorbital breadth, 3.4; lambdoidal breadth,
11.5; prelambdoidal breadth, 9.1; alveolar length of upper molar teeth,
6.5; and depth of braincase, 7.6. Average external measurements of 9
specimens are as follows: total length, 157; length of tail, 36; length
of hind feet, 19.4. Mice from the Bitterroot Valley were compared with
topotypes of _modestus_ by the "method o