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 "incipient tendency to form lophs," and _H. hatcheri_ does the same when worn, but by union with the anterior cingulum. If cusps in _H. tenuiceps_ do form lophs, the process is definitely not by union of the cusps with the anterior cingulum. The transverse median valley is deep and divides the tooth on the buccal side. The anteroposterior valleys are shallow and hanging, and can be said to exist only as indentations between the two sets of cusps. The paracone and metacone are much higher than the other two cusps, but much of this disparity in height may be the result of greater wear on the protocone and hypocone; _H. gregoryi_ agrees with _H. tenuiceps_ in these respects. Possibly the protocone and hypocone were much larger than the paracone and metacone. The internal cingulum of M1 exhibits only one large cusp opposite the medial end of the transverse valley, and shows no evidence of having been divided into two cusps. It is barely possible that there may have been two cusps and that wear makes it appear that there was only one. I doubt that there