thern and eastern
populations.

A "pectoral spot," fairly common in some populations of _R. megalotis_
east of the Missouri River (see Hoffmeister and Warnock, 1955:162-163),
is present in only a small percentage of the specimens we have studied,
and when present is usually only faintly developed.

_External and Cranial Size_

[Illustration: FIG. 1. Geographic variation in five measurements of
_Reithrodontomys megalotis_ on the central Great Plains. The size of
each sample is given, along with total length, length of tail expressed
as a percentage of the head and body, length of ear, greatest length of
skull, and length of rostrum. The approximate distribution of the
species in the region shown and the approximate boundary between the
subspecies _R. m. aztecus_ and _R. m. dychei_ also are indicated.]

As seen in Figure 1, the tail and especially the ear are longer in mice
from New Mexico and adjacent areas than in specimens from northern
localities. The ear, only slightly variable in size in the northern part
of the region, is markedly longer in the southwest, averaging more than
2 mm. longer in specimens from New Mexico and adjacent southwestern
Colorado than in specimens from Nebraska and eastern Kansas; specimens
in a zone from central Colorado through southwestern Kansas and adjacent
Oklahoma generally have ears of a size between the two extremes. As
concerns the tail we note a slight trend toward increasing length (best
expressed as percentage of length of body) from north to south
throughout the central plains, but in general the trend is more
pronounced southwestwardly. Variation in length of tail and length of
ear, therefore, appear to be in accord with Allen's Rule. Length of body
and length of hind foot seem not to vary significantly in specimens we
have studied.

The skulls of specimens examined differed only slightly, except that the
rostrum is significantly longer and relatively, if not actually,
narrower in specimens from the south and southwest than in mice from the
rest of the region under study. The rostrum is longest (average 7.7 mm.)
in specimens from the vicinity of the type locality of _R. m. aztecus_,
but is relatively long (7.5-7.6 mm.) in populations from as far north as
northeastern Colorado and southwestern Nebraska. An average greater
occipitonasal length (greatest length of skull) in specimens from the
south and southwest results mostly from the longer rostrum.

Recognition of two su