maxillary process of the left palatine bone is united to the maxillary by a highly sinuous suture. The union of the palatines to the maxillaries make a suture in the shape of a "V" with the base forward and somewhat blunt. The canal for the palatine artery and nerve has a multiple opening on the palate. One major foramen opens on each side of the palatomaxillary suture, and two or possibly three smaller foramina open posteriorly on the palatine bone. Prominent on the palatine bone, posteromedial to the third molar, is the foramen (palatine pit) for the palatine vein. Collectively, this complex of foramina is often known as the posterior palatine foramina. Wood (1933) states that H. gregoryi has two posterior palatine foramina as in Recent genera, the anterior one opening opposite the posterior end of M1, and the posterior one opposite the median part of M3. The orbital process of the left palatine bone lies inside (medial to) the palatine process of the maxillary. Anteriorly this orbital process meets the orbital process of the maxillary bone, and the sphenopalatine foramen is found in the suture between these two bones and the frontal.

As previously mentioned, the preserved dentition of this specimen consists of the complete left row of cheek teeth and roots of the incisors.

The incisor is compressed laterally, more so than in any Recent heteromyid. The anterior face is rounded, asulcate, and covered with a heavy band of enamel, whereas the posterior side, due to lateral compression, is drawn out into a thin blade. The root of the incisor is at the lateral border of the premaxillary, so it is obvious that the two incisors converged on each other at the midline to form a cutting surface. The writer has not examined the asulcate, laterally compressed incisors of H. hatcheri, and cannot say how they compare with this specimen.

The most significant features of the cheek teeth are their size, and the undivided internal cingulum. The molars are well worn, but the pattern, as a whole, is easily discernable.

P4 has an anterior cusp and three posterior cusps as in other members of the genus. However, the buccal cusp (metacone) of the metaloph is considerably anterior to the central (hypocone) and lingual (entostyle) cusps, and the three cusps do not form a curve as in other species. In size the central cusp is largest, the buccal cusp is practically as large, and the lingual cusp is small. A cingulum connects the lingual and central cusps at the posterior margin of the tooth. In the Pipestone Springs specimen of Heliscomys reported by McGrew (1941) the central and buccal cusps were connected by a cingulum, and some H. hatcheri specimens have all three cusps connected in a similar manner. A low arm or ridge extends from the lingual cusp forward to the lingual side of the base of the anterior cusp. The valleys between the posterior cusps are shallow. There is no sign of the small cuspule on the anteroexternal base of the anterior cusp seen in H. gregoryi, H. hatcheri, and the Pipestone Springs specimen. However, when one sees the variability of the cuspules on P4 of H. hatcheri, the presence of a minor cuspule does not seem to be of taxonomic importance.

M1 deviates from the pattern typical of Heliscomys more than do any of the other molar teeth. However, it must be kept in mind that some of the differences may be due to wear. For example, the protocone and paracone, and the hypocone and metacone are united to form protoloph and metaloph respectively. If the height of the external border of the paracone and metacone is taken into account and compared with the worn inner parts of these two cusps and the equally well-worn protocone and hypocone, it appears that these cusps formed no more of a true bilophodont tooth than do the cusps in other species of Heliscomys; in each of the species the cusps generally are separate entities. H. gregoryi is reported to have an