the Brule formation in "Chimney Canyon," Sec. 3, T. 11 N, R. 54 W, Logan County, Colorado.

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.

In the anterodorsal angle of the rim of the orbit the lacrimal bone rests against the frontal and maxillary bones, where the body of the lacrimal contributes to the formation of the posteromedial wall of the nasolacrimal canal. Only a slight part of the maxillary process of the lacrimal remains on each side.

The premaxillary bone, which constitutes most of the anterior part of the rostrum, is typically heteromyid in shape. The frontal process is long and slender. On the side of the rostrum the premaxillary forms the anterointernal border of the infraorbital foramen. The ventrolateral border of the bone is expanded slightly and aids in the formation of the tuberosity made by the maxillary bone at the ventroposterior border of the foramen. Ventrally the premaxillary makes up the anterior two-thirds of the lateral wall of the incisive (anterior palatine)