rts of Nebraska and Kansas, and many individuals from the two areas can be matched almost exactly.
To the southwest, on the other hand, a trend toward paler (pale brownish, less blackish) upper parts is apparent. Specimens from southwestern Kansas and adjacent parts of Colorado and Oklahoma average slightly paler in comparable pelages than specimens from northeastern Kansas and eastern Nebraska, but most specimens from farther southwest, in northern New Mexico and southwestern Colorado, are discernibly, although not markedly, paler than mice from northern 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