_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 94.1 per cent of length of body. For cranial measurements see Table 2. _Remarks._--For comparisons with _Reithrodontomys megalotis dychei_, geographically adjacent to the northeast, see account of that subspecies. When Howell (1935:143) named _Reithrodontomys megalotis caryi_ from the San Luis Valley of Colorado he compared it directly only with _R. m. megalotis_ from southern New Mexico and northern Chihuahua. Few adults were available to Howell from the San Luis Valley, accounting for the fact, we think, that the published measurements of _caryi_ average less than those given for _R. m. aztecus_ by Howell (_op. cit._:144) and herein. We have examined 16 of the 23 specimens from Medano Ranch and the single specimen from Del Norte that Howell listed. Unfortunately, none is fully adult. The specimens from Medano Ranch, collected in late October and early November, are mostly in fresh winter pelage or molting from subadult pelage, and closely resemble topotypes of _aztecus_ in comparable pelages. Comparison of skulls of the specimens from Medano Ranch with skulls of topotypes and other individuals of _aztecus_ of approximately equal age indicates that the Coloradan specimens may average slightly smaller and have somewhat shorter rostra. Externally, topotypes of _caryi_ have the relatively long tail of _aztecus_ and approach it in length of ear (measured on dry specimens). To us, they appear to be intergrades between _aztecus_ and _dychei_, but to bear closer resemblance to the former, and we tentatively regard _caryi_ as a synonym of _aztecus_. Benson (1935:140) noted that two adult topotypes of _caryi_ were "similar to adult topotypes of _aztecus_." Specimens from southern Colorado east of the San Luis Valley, assigned to