_Diagnosis._--Size larger than any known species; P4 with posteroexternal cusp (metacone) anterior to central (hypocone) and lingual (entostyle) cusps, which are connected by a cingulum; internal cingula of molars undivided, and as high as paracone and metacone; style of each cingulum opposite the straight median valley; rostrum deep and laterally compressed. _Description._--The type consists of the preorbital and interorbital parts of a skull. Its size is comparable to that of the Recent heteromyid, _Liomys pictus_ Merriam. _L. pictus_ is the species referred to in the comparisons below when only the generic name _Liomys_ is mentioned. Both incisors have been broken off. The right tooth-row is missing, but the left row is complete, and its orientation indicates that the tooth rows were parallel. The zygomata are broken off close to the rostrum, which is relatively narrow in comparison with its length and depth. In this narrowness, the specimen resembles _Florentiamys_ Wood more than it does such Recent heteromyids as _Liomys_ or _Heteromys_, where the rostrum is much wider at the dorsal surface than at the ventral surface (correlating with the wide interorbital dimension). In No. 7702 the rostrum is not appreciably expanded on the dorsal surface. The wide interorbital dimension also gives a tapering appearance to the rostrum of the Recent heteromyids, when viewed dorsally, which is not seen in the fossil specimen. Like those of most heteromyids, the nasals and premaxillaries project forward beyond the incisors. _H. tenuiceps_ has a distinctly heteromyidlike appearance, and it is obvious that the features of the anterior part of the skull, which characterize the heteromyids, had been established by middle Oligocene time. The nasal bone extends caudad as far as does the premaxillary; they terminate at the anterior border of the orbit. The nasal is widest anteriorly where it curves downward on the side to meet the anterior projection of the premaxillary bone beyond the incisor. Posteriorly, the two nasals have practically parallel lateral borders much as in _Liomys_. The frontal bone dorsally is relatively narrower than in any Recent heteromyid, and closely resembles that of the geomyids. There is a slight depression in the midline of the skull where the two frontals unite, but no evidence of a ridge for the attachment of the temporal muscle. In lateral view, the ledge seen in _Liomys_ at the dorsal surface is absent, nor is this surface rounded as in _Geomys_. Preservation around the nasolacrimal canal is poor, but traces of sutures indicate that the frontal bone is not involved in the posteromedial wall of that canal. The orbital plate is broad, comparatively flat, and extends farther ventrad than in _Liomys_, and enters into the composition of the sphenopalatine foramina. Ventrally the frontal bone meets the orbital processes of the palatine and maxillary bones, and posterolaterally meets the orbitosphenoid.