This genus differs from _Leontinia_ in its formula, 3 0 4 3 ------- 3 0 3 3 but, except for the loss of these canines and the lower premolars, is very similar. In this it seems to approach the line which gave rise to _Colopdon_. The premolars and molars are also narrower in proportion than in _Leontinia_. I have placed _Rodiotherium_ also under this genus as I can not see a generic difference in the descriptions. However we were not fortunate enough to find these forms. Ameghino has described four species as follows: =A. frequens=, 1895, Bol. Inst. Geog. Argen., t. 15, p. 475. Upper dentition, pm. 1 to m. 3 150 mm. Lower dentition, pm. 2 to m. 3 150 mm. Upper molar 3, length 39 mm. =A. lentus=, 1901, Bol. Acad. Nac. Cordoba, t. 16, p. 407. Upper molar 3, length 48 mm. =A. minor=, 1901, loc. cit. Upper molar 3 34 mm. =A. (Rodiotherium) armatum=, see cit. above. Based on an imperfect mandibular symphysis, in which incisor 3 is very large. CHAPTER VIII NESODONTIDAE This family is characterized by the teeth being hypsodont, the second upper incisor and the third lower incisor being enlarged into caniniform teeth, the upper molars complicated by the development of cristae, limbs short, feet tridactyl and semidigitigrade. [Illustration] [Illustration: Fig. 79. _A_, upper and a lower molars 2 of Proadinotherium; _B_, upper and _b_ lower molars 2 of Coresodon; _C_ upper and _c_ lower molars 2 of Neudon—½ natural size.] In the Santa Cruz, the family is represented by the two genera _Nesodon_ and _Adinotherium_. In the Deseado we find _Proadinotherium_ evidently ancestral to _Adinotherium_ and very little differentiated from it. Ameghino has described a genus, _Pronesodon_, which is evidently ancestral to _Nesodon_. I have referred _Coresodon_ to this family because the molars of the upper and lower jaws are very close to those of _Adinotherium_. Ameghino has also described two genera, _Nesohippus_ and _Interhippus_, based on upper molars which are very similar in pattern to _Adinotherium_ and which I believe belong to this family, if they prove to be valid genera, of which I have some doubt, feeling that they will prove to be the deciduous upper premolars of _Proadinotherium_ or some similar form. The genus _Senodon_, which Ameghino also places in this family, I feel will prove to be worn teeth of _Leontinia_. =Proadinotherium= Ameghino Proadinotherium Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 625. [Illustration: Fig. 80. P. leptognathus, rear portion of skull—½ natural size; shaded areas are matrix.] The dental formula is 3 1 4 3 ------- 3 1 4 3