If the fossil is actually _Cratogeomys castanops_, and if the fragment is from an earlier deposit in the cave than is the material here assigned to _Cratogeomys castanops_, the fossil stock could be ancestral to the group of small subspecies provided there had been a trend in evolution toward smaller size. Another possibility is that a shift in geographic range of the kinds of _Cratogeomys_ that lived in the vicinity of the cave has occurred, and that the fossil represents an evolutionary line with no close relationship to Recent species and now is extinct. Additional material is needed before the history of these species can be reconstructed with validity. #Heterogeomys onerosus# new species _Holotype._--Los Angeles County Museum (C.I.T.) No. 2384, an incomplete left ramus, bearing incisor and p4; the alveolus of m1-m3 is present (Fig. 1a). Paratypes: Two isolated and unnumbered right upper incisors, one isolated premolar, and five additional rami, Nos. 2385, 2386, 2388, and two with no number. _Horizon and type locality._--Upper Pleistocene, Cueva de San Josecito, province of Aramberri, near the town of Aramberri, Nuevo Leon, Mexico; California Institute of Technology, Vertebrate Paleontology Locality 192. _Description of Holotype._--Differs from any known living species of _Heterogeomys_, by the significantly heavier and deeper ramus (see Table 1 and Fig. 1). The holotype is compared with the largest adult male of _Heterogeomys hispidus_ (_H. torridus_ is smaller than _hispidus_) available to me in Table 1. Relative to the length of the ramus (measured from the anterior mental foramen to the posterior margin of the capsule that surrounds the root of the lower incisor), the depth of the ramus anterior to the molariform tooth-row is 33.0 per cent in _H. onerosus_ compared with 27.3 per cent in _H. hispidus_. If the fossil ramus is that of a female (females are significantly smaller than males in _Heterogeomys_) then the differences would be greater than recorded. TABLE 1. DEPTH OF MANDIBULAR RAMUS A: Least depth in front of premolar (See A to A' on Fig. 1c) B: Depth of ramus opposite re-entrant angle of p4 (B to B' on Fig. 1c) C: Depth from a point in front of capsule for incisor (See C to C' on Fig. 1c) =====================================+======+======+====== | A | B | C -------------------------------------+------+------+------ _H. onerosus_ holotype | 11.0 | 17.4 | 11.7 -------------------------------------+------+------+------ _H. h. hispidus_ [Male] ad., 23979 KU| 9.1 | 15.2 | 10.5 -------------------------------------+------+------+------