Two species are described. =Pronesodon cristatus= Ameghino P. cristatus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 626. P. cristatus Amegh., 1897, Bol. Inst. Geog. Argen., t. 15, p. 467. This species is very imperfectly known, is characterized by a large external anterior style, molars said to be 15 mm. wide. =Pronesodon robustum= Ameghino P. robustum Amegh., loc. cit. above. This is a larger species, of which the three lower molars are known, and which measure 16, 22, and 30 mm. in length respectively, while they are 9-10 mm. wide. =Coresodon= Ameghino Coresodon Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 630. Coresodon Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 459. Coresodon Gaudry in part, 1908, Anal. Palaeontologie, t. 1, p. 46. In this genus, the pattern of the upper molars is essentially the same as in _Proadinotherium_, and they are of the same hypsodont character, and with roots. I can now find only the fact that in _Coresodon_ the teeth are more compressed and somewhat more hypsodont, as a feature by which to distinguish this genus from _Proadinotherium_. Gaudry figures the front of a lower jaw under the name _Coresodon_ which lacks the caniniform incisors. I have doubted the association, but should it prove correct, then this genus would be markedly different in that respect. Two species have been described, _C. scalpridens_, and _C. cancellatus_, both of which I consider the same. =Coresodon scalpridens= Ameghino C. scalpridens Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 630. C. scalpridens Amegh., 1897, Bol. Inst., Geog. Argen., t. 18, p. 459. C. cancellatus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 374. [Illustration: Fig. 82. Sections of second lower molar; _A_, top; _B_, 4 mm. down; _C_, 10 mm. down; _D_, 18 mm. down—natural size.] Of this species we found two specimens, one containing the three lower molars, the other the second lower molar only. In establishing _C. cancellatus_, Ameghino says it is of the same size as _C. scalpridens_, but distinguished by the basin in the upper molars being narrower, the internal fold not being bifurcated, and by the absence of islets of enamel. All these features seem to me to be the results of wear. [Illustration: Fig. 83. Molars 1 to 3—natural size.] While the pattern of the upper molars is the same as in _Proadinotherium_, these teeth are about as wide as they are long. The lower molars, however, are more compressed, with the anterior crescent occupying about a third of the tooth, and having in the early stages a deep pit, which disappears when the tooth is worn down. In the middle of the basin of the posterior crescent is a large pillar, and between this and the median horn of the crescent is a tiny septum, which early unites with the pillar, leaving a tiny pit (3) which soon disappears entirely. In fact, in an old tooth, the second and fourth bays, having become pits, may even be lost also.