_Remarks._--The features which distinguish _Pappogeomys alcorni_ seem to be beyond the range of variation in _Pappogeomys bulleri_. In view of the absolute quality of the differences between _P. alcorni_ and _P. bulleri_, it seems best to regard the former as a species, rather than as a subspecies of _P. bulleri_. Moreover, it seems unlikely that actual intergradation of the two species can occur, since the broad, low valleys between the higher terrain, where pocket gophers of this genus are found, do not offer suitable habitat for _Pappogeomys_. In every example of _P. bulleri_ that I have seen (more than 100 specimens, representing all of the named subspecies) the anterior palatine foramina are long and slitlike and the nasals are always narrow and emarginate posteriorly, whereas in _P. alcorni_ the anterior palatine foramina are short and round and the nasals broad and squarely truncate posteriorly. The conspicuous nasal patch of _P. alcorni_ is large and bright cinnamon or buffy, and, although the nasal patch may be large in some subspecies of _P. bulleri_, in each specimen possessing the patch the hairs are whitish with little or no trace of pigmentation. One of the most interesting features of _P. alcorni_ is the reduction of enamel on the posterior wall of the first upper molar. In _P. alcorni_ the enamel present is thick, but it occurs only on the inner one-fourth of the posterior wall of the tooth. The enamel is always complete in _P. bulleri_; but in some old individuals it becomes thin with wear, and at a casual glance may appear to be partly or entirely absent. Close examination under magnification reveals, however, in every specimen of _P. bulleri_, a fine line of enamel completely across the posterior wall. It seems that the posterior enamel plate of M1 is disappearing in both _P. bulleri_ and _P. alcorni_. In both species the enamel on the posterior wall of M1 does not extend down the crown so far as the level of the alveolus, whereas the anterior plate of enamel on M1, for example, extends well below the alveolus of the tooth. Even though disappearance of the posterior enamel seems to be a trend in both species, it has proceeded farther in _P. alcorni_ than in _P. bulleri_. Examination of the posterior wall of M1 in _P. alcorni_ disclosed only the vestige of enamel on the inner side of the tooth, and no enamel, not even a thin plate, was present on the remainder of the posterior wall of the tooth. The name _alcorni_ is proposed as a token of appreciation to Joseph Raymond Alcorn, whose collecting has greatly enriched our knowledge of the mammals of México. _Transmitted August 30, 1956._ 26-5801 * * * * * Transcriber's Notes: Bold text is shown within =equal signs=. Italic text is shown within _underscores_.