_Comparisons._--This subspecies needs close comparison only with the adjacent subspecies _C. g. limitis_. As compared with topotypes of _limitis_, _C. g. arizonensis_ has darker pelage, narrower rostrum, greater width across zygomatic arches, lesser lambdoidal breadth, longer nasals, wider palate, and more inflated auditory bullae. The posterior border of the hard palate is straight in five skulls of the series that are complete (two skulls have the palatal regions broken); all _C. g. limitis_ examined have a median posterior projection on the posterior border of the hard palate. _Measurements._--External and cranial measurements of the type, and the average and extreme measurements of three adult males and two adult females from the type locality (including the type) are: Total length, 160, 145.6 (137-160); tail, 44, 40.8 (37-46); hind foot, 18.5, 19.3 (18-20); condylobasilar length, 23.3, 22.8 (22.1-23.5); zygomatic breadth, 13.8, 13.4 (12.6-13.8); lambdoidal breadth, 11.5, 11.4 (11.0-11.6); alveolar length upper cheek-teeth, 5.5, 5.4 (5.2-5.5); interorbital breadth, 3.8, 3.9 (3.8-4.0); length of nasals, 7.6, 7.1 (6.9-7.6); breadth of rostrum, 3.1, 3.1 (3.0-3.2); length of incisive foramina, 5.5, 5.2 (5.0-5.5). _Remarks._--Hall and Davis (Proc. Biol. Soc. Washington, 47:55, February 9, 1934) reported 12 specimens of red-backed mice from Hannagan Meadow, 9500 to 9600 ft., and ten from Hannagan Creek, 8600 ft., all in Greenlee County, Arizona. Although they pointed out most of the cranial differences here described as diagnostic of _C. g. arizonensis_, they did not name the animals as new since they had no seasonally comparable materials; thus they were unable to evaluate the differences noted in pelage. We have not examined the material referred to by Hall and Davis (_loc. cit._), but, on the basis of their description, here refer it to _C. g. arizonensis_. _Specimens examined._--Total, 7, all from the type locality and all in the Biological Surveys Collection in the United States National Museum. Grateful acknowledgment is made of the opportunity to study the specimens from New Mexico and Arizona in the Biological Surveys Collection of the United States National Museum and the material from South Dakota in the University of Michigan Museum of Zoology, as well as for the financial support afforded one of us (Cockrum) by the University of Kansas from its Research appropriation. Cockrum's work was part of a larger investigation of the geographic distribution of all North American native mammals, aided by a contract between the Office of Naval Research, Department of the Navy, and the University of Kansas (NR. 161-791). Also, assistance with some of field work was given by the Kansas University Endowment Association. _Transmitted June 21, 1952._ [Illustration: printers mark] 24-4369 Transcriber's Notes. This file was derived from scanned images. The original text is presented.