I hestitate to refer this fragment to any of the living species, although I would judge it to represent a form closer to the species castanops than to the merriami group (C. perotensis). The rostrum may represent, and probably does, an undescribed and extinct species of Cratogeomys, but in my opinion it should not be given formal taxonomic status until more adequate material is available.
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 León, México; 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
| Least depth in front of premolar (See A to A′ on Fig. 1c) | Depth of ramus opposite re-entrant angle of p4 (B to B′ on Fig. 1c) | Depth from a point in front of capsule for incisor (See C to C′ on Fig. 1c) | |
| H. onerosus holotype | 11.0 | 17.4 | 11.7 |
| H. h. hispidus ♂ ad., 23979 KU | 9.1 | 15.2 | 10.5 |
The angle between the anterior border of the coronoid process and the dorsal border of the ramus of the mandible is more acute, and the posteroventral margin of the ramus is more nearly straight, in onerosus than in hispidus. The molariform tooth-row in onerosus is only slightly longer (13.9 in contrast to 13.5) than in hispidus and torridus. The ventral border of the massenteric ridge is weakly developed in onerosus and hardly discernable whereas in the living species of Heterogeomys the massenteric ridge is strongly developed posteriorly forming a noticeable prominence.