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.

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.

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