This species is founded on upper teeth, which are said to be relatively smaller anteriorly, and actually smaller throughout, by about 15% than are those of the preceding species. Molar 1 measures 17 mm. long by 9 mm. wide. =Eutrachytherus grandis= sp. nov. This species is based on upper molars 1 and 2 of the left side, from the Deseado beds, of the Chico del Chubut, west of Puerto Visser. The teeth are typically those of the genus and differ only from other species in their large size, being some 50% larger than the corresponding teeth of _E. spegazzinianus_. Each is covered with a layer of fully half a millimeter of cement. [Illustration: M¹ M² Fig. 49. E. grandis, molars of the left side—natural size.] MEASUREMENTS Upper molar 1, length 29 mm., width 21 mm. Upper molar 2, length 30½ mm., width 18 mm. =Argyrohyrax= Ameghino Argyrohyrax Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 435. The genus is distinguished by the full dentition in closed series, and by the upper molars having a deep internal inflexion which is bifurcated, making the teeth at least incipiently three-lobed. Incisor 1 is relatively somewhat wider than in the preceding genus. It seems to me that it is from such a genus that _Typotherium_ arose, by the reduction of the lateral incisors, the canine, and the first two premolars, and by the increase of the bifurcated internal fold. Three species have been described, _A. proavus_, the type species, _A. acuticostatus_, of a little smaller size, and _A. proavunculus_ of still smaller size. =Argyrohyrax proavus= Ameghino A. proavus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436. This species occurs three times in the Amherst collection. The best specimen has pm. 1, 3, 4, and m. 1 and 2 of the upper jaw. The species is characterized by a narrow furrow near the anterior external margin of the premolars and molars; and by pm. 3 and 4 and the molars having a deep internal bifurcated inflexion, which tends to make these teeth three-lobed. [Illustration: Fig. 50. Right upper dentition, the outline teeth after Ameghino—natural size.] The genus is known only by the upper dentition, and while I did not find any associated lower teeth, I believe that some one of the genera known only by the lower dentition, like _Plagiarthrus_, is that lower dentition. MEASUREMENTS Upper dentition, length inc. 1 to m. 3 (@ Ameghino) 71 mm. premolar 1, length 7 mm., width 4 mm. premolar 3, length 7 mm., width 6 mm. premolar 4, length 8 mm., width 6 mm. molar 1, length 8 mm., width 6½ mm. =Argyrohyrax proavunculus= Ameghino A. proavunculus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436.