From the table of measurements it will readily be seen that Hoplophoneus mentalis is considerably smaller than the large H. occidentalis, approximating in size individuals of the insolens and robustus groups. A comparison with the specimens in the Princeton collection used by Adams in defining these two forms shows that, although the jaw depth back of the flange is the same in H. robustus and H. insolens as in the new form, the flange is absolutely larger in the latter and tapers less rapidly in width distally; both premolars are smaller, and in p4 the paraconid is disproportionately smaller, than in H. insolens (Princeton Univ. Geol. Mus. No. 11372) and is out of line with the other two cusps, toward the inner side of the jaw as in Eusmilus and hoplophonids in general, and has sharper cutting edges than in H. insolens; also the alveolus for mī is very much wider transversely, in front, than in the latter species. Between H. robustus and H. mentalis there is the same striking difference in the shape of the jaw flange, which is decidedly V-shaped in robustus, while in the new species it is U-shaped and of the Eusmilus type. The third premolar is as small as in robustus, but rakes backward a bit stronger and has a larger posterior cuspule. To the paraconid of p4 the same remarks are applicable as in the comparison with H. insolens.