The genus is based primarily on the three-lobed character of the molars, which is a secondary effect of an infolding on the inner side of the posterior lobe. They are rooted, but strongly hypsodont. The margins of the crescents are well developed and the “pillar” is a prominent feature in the posterior crescent. =Tricoelodus bicuspidatus= Ameghino T. tricuspidatus Amegh., loc. cit. above. [Illustration: Fig. 22. Lower right pm. 3-m. 1—natural size, after Ameghino.] The species is the only one known of the genus, and its features are those of the genus. The following measurements indicate the size, lower pm. 3 to m. 1, 25 mm.; height of the mandible under molar 1 is 12 mm. =Proadianthus= Ameghino Proadianthus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 455. This genus is known only by the last two premolars of the lower jaw, which are compressed and moderately high. The two crescents are markedly separated by a cleft from the external side of the tooth, opposite to which is a high denticle, made by the fusion of the two ends of the crescents where they come together. =Proadianthus excavatus= Ameghino P. excavatus Amegh., loc. cit. above. The species is based on the two lower premolars described above. I reproduce the figure given by Ameghino. The measurements are: length of pm. 3 and 4, 10 mm.; height of mandible under pm. 4, 8 mm. [Illustration: Fig. 23. Lower right pm. 3 and 4—natural size, after Ameghino.] CHAPTER V TYPOTHERIA In the Deseado beds this group of running and hopping animals is well represented, making about 14% of the Amherst collection, and varying in size from a little larger than a rat to larger than a sheep.