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. _Description of Paratypes._--The fossils are referable to the genus _Heterogeomys_ on the basis of the short lateral angular processes of the lower jaw and on the basis of the associated upper incisors, which have a single distinct sulcus that lies toward the inner margin of each tooth. The isolated lower premolar that is referred to the new species is as large as that of the holotype and has the enamel pattern of _Heterogeomys_. [Illustration: Figs. 1a-1c. x 1-1/2 FIG. 1a. _Heterogeomys onerosus_, lateral view of left lower jaw of holotype. FIG. 1b. _Heterogeomys onerosus_, front view of right, upper incisor. FIG. 1c. _Heterogeomys hispidus_, lateral view of left lower jaw, No. 23979, [Male] adult, from 3 km. E San Andres Tuxtla, Veracruz.] One jaw fragment, L.A.C.M. (C.I.T.) No. 2368, is smaller than the others and probably is from a young individual. Two others L.A.C.M. (C.I.T.) No. 2384 and one unnumbered, are smaller than the holotype, and possibly are the remains of females; however, they have the same characteristic shape as the holotype. Nevertheless, the two rami mentioned above are significantly larger than in adult males of modern _Heterogeomys_ and are especially larger than in females. Another jaw fragment, L.A.C.M. (C.I.T.) No. 2385, is seemingly as large as, or perhaps larger than, the holotype, although the posterior part of the ramus behind the alveolus of m2 is missing. An additional unnumbered ramus is of somewhat lighter construction than the holotype, but is important since it bears not only the incisor and p4 but also the first two lower molars. The only other material referable to _Heterogeomys onerosus_ is a fragmentary and isolated lower molar tooth that has a single posterior enamel blade, a feature characteristic of a number of Recent genera of pocket gophers, and some limb bones which are slightly larger than corresponding elements in Recent species of _Heterogeomys_.