_Comparisons._--_Neotoma albigula subsolana_, differs from topotypes of _N. a. leucodon_, the subspecies geographically adjacent to the southwest, as follows: size smaller, especially length of palatal bridge (6.9-8.1 instead of 8.2-9.6), alveolar length of maxillary tooth-row (8.3-8.9 instead of 8.8-9.7), and greatest length of auditory bulla (7.3-7.9 instead of 8.2-8.9); mastoid breadth relatively greater, 51.1 (47.8-52.7) instead of 47.0 (45.5-49.1) per cent of basilar length; posterior process of premaxilla extending only slightly beyond posterior border of nasals; auditory bulla conspicuously smaller; upper parts darker, especially middorsally; over-all color grayish instead of ochraceous or yellowish; lips gray instead of nearly white. _Neotoma albigula subsolana_ differs from _N. a. albigula_, geographically adjacent to the northwest (specimens from Pima County, Arizona) as follows: size averaging slightly larger, except length of nasals; mastoid breadth averaging 18.8 (17.9-20.2) instead of 17.9 (17.7-18.2), its ratio to basilar length therefore greater, 51.1 (47.8-52.7) instead of 49.4 (47.9-50.0); zygomatic arches expanded posteriorly instead of nearly parallel as in _albigula_; interparietal longer and narrower; mesopterygoid fossa broader; auditory bulla slightly smaller; upper parts distinctly darker. _Remarks._--_N. a. subsolana_ is characterized by the combination of small size, dark color, small auditory bulla and relatively broad braincase. Typical specimens have been collected only at higher elevations in the Sierra Madre Oriental where no other species of _Neotoma_ is known to occur. Intergradation between _N. a. subsolana_ and _N. a. leucodon_ occurs at lower elevations on the west side of the Sierra Madre Oriental as shown by specimens from nine miles southwest of Tula, Tamaulipas, and Sierra Guadalupe, Coahuila, from which places some specimens are paler than others, approaching _leucodon_ in color, and are slightly larger than typical _subsolana_. Specimens assigned to _leucodon_ from vicinity of Presa Guadalupe and from 1 to 6 kilometers south of Matehuala, San Luis Potosí, are typical of that subspecies in measurements but are darker than topotypes. _N. a. subsolana_ intergrades with _N. a. albigula_ in southeastern Coahuila (specimens from 6 to 9 miles east of Hermanas and from Panuco) where some individuals average paler and smaller than topotypes of _subsolana_ and some have skulls that combine characters of _subsolana_ and _albigula_. These specimens, which were referred to _N. a. leucodon_ by Baker (Univ. Kansas Publ., Mus. Nat. Hist., 9:281-282, June 15, 1956), are assigned to _subsolana_ on the basis of relatively dark upperparts and broad mesopterygoid fossa (narrow in only one specimen).