bspecies of _R. megalotis_ on the central Great
Plains seems justified on the basis of the geographic variation
discussed above. One subspecies, for which the name _R. m. aztecus_ is
applicable, occurs in the southwest and is characterized by the
culmination of trends in the region studied to paler upper parts, longer
tail, longer ear, and longer, relatively narrower rostrum--characters
that appear at least partly independent of each other as concerns
gradation toward the smaller, darker-colored populations to the
northward. The latter, while exhibiting some differences in color
(slightly paler westwardly) and length of tail (shorter northwardly),
stand more or less as a unit in contrast to the mice from the southwest,
and represent, in our judgment, a single subspecies, _R. m. dychei_. The
area of intergradation between the two subspecies is relatively broad,
considering all the characters mentioned, and assignment of some
intergrades is admittedly difficult.

_Reithrodontomys megalotis aztecus_ J. A. Allen

_Reithrodontomys aztecus_ J. A. Allen, Bull. Amer. Mus. Nat. Hist.,
5:79, April 28, 1893 (type locality, La Plata, San Juan Co., New
Mexico).

_Reithrodontomys megalotis aztecus_, A. H. Howell, N. Amer. Fauna,
36:30, June 5, 1914.

_Reithrodontomys megalotis caryi_ A. H. Howell, Jour. Mamm., 16:143,
May 15, 1935 (type locality, Medano Ranch, 15 mi. NE Mosca, Alamosa
Co., Colorado).

_Distribution._--Western and southern Colorado, southeastern Utah,
northeastern Arizona and northern New Mexico, east to the panhandles
of Texas and Oklahoma and to southwestern Kansas.

_External measurements._--Average and extremes of 10 adults (5
males, 5 females) from San Juan County, New Mexico, and adjacent
Montezuma County, Colorado, are: total length, 140.1 (126-150);
length of tail-vertebrae, 67.4 (56-71); length of hind foot, 17.3
(16-18); length of ear from notch, 15.1 (13-17); tail averaging 92.7
per cent of length of body. Corresponding measurements of 13 adults
(7 males, 6 females) from Bernalillo and Guadalupe counties, New
Mexico, are: 142.1 (129-156); 69.4 (60-75); 17.9 (17-19); 16.3
(15-18); tail averaging 95.4 per cent of length of body.
Corresponding measurements of 22 adults (17 males, 5 females) from
Meade County, southwestern Kansas, are: 147.1 (139-162); 71.3
(65-77); 17.6 (17-19); 13.8 (13-15); tail averaging 9