MEASUREMENTS Upper dentition, molar 1, length 19 mm. @ Ameghino. Upper dentition, molar 1, width 12 mm. @ Ameghino. Upper dentition, m. 1 to m. 3, length 52 mm. @ Ameghino. Lower dentition, premolar 2, length 13 mm. @ Ameghino. Lower dentition, premolar 3, length 13 mm. @ Ameghino. Lower dentition, premolar 4, length 17 mm. @ Ameghino. Lower dentition, molar 1, length 18 mm., width 7 mm. Lower dentition, molar 2, length 19 mm., width 7 mm. Lower dentition, molar 3, length 20 mm., width 7 mm. =Interhippus= Ameghino Interhippus Amegh., 1904, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 3, p. 183. Interhippus Amegh., 1904, Anal. Soc. Cienc. Argen., t. 56, p. 34 of reprint. This genus was established on isolated teeth which closely resemble those of this family, though the genus was placed among the _Rhynchippidae_ by Ameghino. The teeth described as molars are much elongated and have the cristae greatly developed, and in one species there is a style rising about the middle of the inner side of the molar. Another feature emphasized as characteristic of this and the next genus is, that the crowns are expanded much wider than the roots. While there is not yet enough direct evidence to prove it, I feel that this and the next genus will prove to be deciduous teeth, of either _Proadinotherium_ or some related genus. Two species of this genus have been described, both from the upper Deseado. =I. phorcus= Amegh., loc. cit. above. This species is characterized by its size, the last upper molar (so called) measuring 16 mm. long by 14 mm. wide. =I. deflexus= Amegh., 1904, Anal. Mus. Nac. B. A., ser. 3, t. 3, p. 183. This species is based on a worn tooth designated as molar 1 (probably d. pm. 3) 14 mm. long by 19 mm. wide. [Illustration: Fig. 84. I. phorcus. “Upper molar 3”—natural size, after Ameghino.] [Illustration: Fig. 85. I. deflexus, “Upper molar 1”—natural size, after Ameghino.] [Illustration: Fig. 86. N. insulatus, “Upper molar 1”—natural size, after Ameghino.] =Nesohippus= Ameghino Nesohippus Amegh., 1904, Anal. Soc. Cienc. Argen., t. 56, p. 34 of reprint. Nesohippus Amegh., 1904, Anal. Mus. Nac. B. A., ser. 3, t. 3, p. 218. This genus is described as very like the foregoing, but differs in having a strong perpendicular style on the anterior external face of the upper molars. As in the preceding genus, the crown is considerably expanded above the roots. I feel that this genus will also prove to be the milk teeth of some one of the genera of this family. One species is described, based on a single tooth. =N. insulatus= Amegh., 1904, loc. cit. under the genus. The species is just as described under the genus, the last upper molar measuring 24 mm. long by 16 mm. wide; given as from the upper Deseado. CHAPTER IX ISOTEMNIDAE