_Diagnosis._--Size medium for genus (see measurements); tail naked, short relative to length of head and body; hind foot short; hairs of upper parts and underparts Plumbeous basally and Orange-Cinnamon apically (capitalized color terms after Ridgway, Color Standards and Color Nomenclature, Washington, D. C., 1912); large nasal patch Cinnamon-Buff in two specimens, but Pale Pinkish-Buff in holotype; white throat spot small and inconspicuous, throat mostly bright Cinnamon-Buff; auricular patch pure Plumbeous, hairs lacking cinnamon-colored tips; tarsi with Cinnamon-Buff hairs; dentition as in _P. bulleri_ except that enamel plate of posterior wall of M1 reduced to a vestige present only on inner fourth, outer three-fourths of posterior wall of M1 without trace of enamel; zygomata slender, bowed outward; jugal long, widely separating maxillary and squamosal arms of zygoma; skull deep (measured from a point on the frontal to a point on the palate directly below and between the maxillary teeth); rostrum narrow and short; nasals broadly truncate posteriorly, and not decurved anteriorly; narrow across mastoid processes of squamosals; anterior palatine foramina small and rounded in outline, not slitlike. _Comparisons._--Compared with _Pappogeomys bulleri_, the only other named species of the genus, _P. alcorni_ differs, as follows: Nasal patch cinnamon or buffy instead of white; enamel plate of posterior wall of M1 reduced to inner fourth rather than developed completely across posterior wall of tooth; nasals broadly truncate posteriorly instead of narrow and emarginate; anterior palatine foramina short and round instead of long and slitlike. _Measurements._--The type and an adult female topotype (in parentheses) measure, as follows: Total length, 210 (210); length of tail, 61 (63); length of hind foot, 29 (28); condylobasal length, 38.0 (36.9); basilar length, 32.8 (31.9); breadth across zygomata, 24.2 (24.8); palato-frontal depth, 15.0 (14.8); palatal length, 24.7 (24.1); length of nasals, 12.7 (12.8); breadth of braincase, 18.1 (17.5); breadth across mastoid processes of squamosals, 21.5 (21.4); breadth of rostrum, 8.4 (8.1); length of rostrum, 16.9 (16.3); alveolar length of maxillary tooth-row, 9.3 (8.8); breadth across angular processes of rami, 26.1 (26.2). _Specimens examined._--Three, all from Jalisco, as follows: 4 mi. W Mazamitla, 2; 3 mi. WSW Mazamitla, 1.