In three cases we found the deciduous fourth premolar (see fig. 119A), a complicated tooth, consisting primarily of three laminae in which furrows have developed until there are four folds or furrows on the internal side, separating five crests; while on the external side there are three furrows and four crests. Ameghino’s figure of this tooth in _C. prosus_ has four laminae running clear across the tooth. I think the difference is due to his having an unworn deciduous premolar whereas mine are all worn considerably. At first glance, the upper teeth appear strikingly different, resembling those of _Perimys_ to which genus they are probably ancestral. Each molar consists of two laminae, separated by a deep internal fold which extends almost to the external margin. On little worn teeth each lamina shows, on the external side, a shallow furrow extending to about the middle of the tooth, but these furrows early become pits and then disappear with further wear, being preserved on not over a fourth of our specimens. The fourth upper premolar consists of two laminae, but in this case, the separating fold is on the external side and extends nearly to the internal margin, so that this tooth appears to be reversed in its position in the jaw. As in the molars, there is, on the external side of either lamina, a furrow, the one in the anterior lamina shallow and seldom seen, that in the posterior lamina deep and present in all but the most worn teeth. While the upper and lower molars appear so different they may be readily derived from such a tooth as the lower molar, as both have the two laminae and separating furrows in common. In the upper molars, however, the internal fold is prolonged until the external fold is merely indicated or lacking. On upper premolar 4, on the contrary, it is the external fold which is prolonged. The furrows in the external portions of the laminae of the upper molars correspond to those on the internal portions of the same laminae of the lower teeth, reversed, as is typical of all teeth. Ameghino distinguished three species of _Cephalomys_, which are based primarily on size, the other characters which he gave being inconstant. We found these three and no others. UPPER PM. 4 LOWER PM. 4 TO M. 3 TO M. 3 C. arcidens 13-14 mm. 14-15 mm. C. plexus 9.5 mm. 10.5 mm. C. prosus 8.5 mm. 9.5 mm. =Cephalomys arcidens= Ameghino C. arcidens Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 494.