iform tooth-row are longer; nasals truncate versus emarginate, and
consistently shorter; basilar length consistently less in specimens of
equal age; mastoidal breadth less in 16 of 17 specimens of _rostralis_;
temporal ridges parallel instead of divergent posteriorly; exposed parts
of upper incisors shorter; tympanic bullae more angular
antero-laterally.

From _Thomomys talpoides fossor_ (specimens from Rico, Silverton, Hermit
and Pagosa Springs, all in Colorado), the subspecies to the southward,
_T. t. rostralis_ differs in: Longer body; lighter color of upper parts;
nasals truncate rather than rounded posteriorly; temporal ridges more
nearly parallel (less divergent posteriorly); rostrum longer (averaging
longer and broader); skull wider across zygomatic arches in 11 of 12
specimens of _rostralis._

_Remarks._--Geographic variation is evident in the material examined. In
the initial study, one of us, Montague, separated the material from the
Medicine Bow Range in Wyoming as a subspecies different from that at
Laramie and the adjoining mountains to the eastward because of the
darker color of the western animals and the smaller size of males.
Acquisition of more material from still farther west (Sierra Madre) in
Wyoming and the examination of material in the United States Biological
Surveys Collection from Colorado discloses that there is a cline of
increasing intensity of color from the geographic range of _T. t.
cheyennensis_ at Pine Bluffs, Wyoming, westward to the eastern side of
the Sierra Madre at a locality three miles east and five miles north of
Savery, Wyoming. A further deterrent to setting apart the animals of the
Medicine Bow Mountains as a separate subspecies is the large size of
males from the North Platte River Valley southeast of Saratoga. The
males from the valley of the North Platte are intermediate in size
between those from the Medicine Bow Mountains and those from the Laramie
River Valley. Females from the same places are available in longer
series and show less variation. If there is a difference in size in the
females, those from the mountains are larger than those from lower
elevations on either side.

The examination that one of us, Hall, has made of the related materials
from Colorado reveals, as we supposed would be the case, that a large
area formerly assigned to the geographic range of _Thomomys talpoides
fossor_ is to be assigned to the geographic range of the newly named
_Thomomys talp