Incisor 1 is a little larger than the other incisors. Each upper molar has a vertical groove near the anterior external margin. In each upper premolar (after the first) and molar, there is a central pit surrounded by enamel, which is opposite the internal inflexion, and in a young individual, is presumably connected with the fold. In the same way, the last three lower premolars and the lower molars each have an internal pit, adjacent to the external inflexion. With advanced age all the teeth show closed roots, another primitive feature. In spite of the closed roots, the full dentition, and the enamel on the incisor; and on account of the deep inflexions and the isolated pits, I consider this genus a specialized side line, retaining many primitive features, and expect to find the ancestor of the typotheres in some one of the related Casamayor genera. [Illustration: Fig. 27. Archaeohyrax patagonicus, after Ameghino—natural size.] Ameghino described three species, _A. patagonicus_, which we have figured, and which has a length of 84 mm. from inc. 1 to m. 3 in both the upper and lower dentitions; _A. propheticus_, of the same size, but with the dental series closed; and _A. concentricus_ of larger size, the three lower molars having a length of 38 mm. =Plagiarthrus= Ameghino Plagiarthrus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436. This genus is known only by the lower premolars and molars, which are permanently growing teeth, composed of two subcylindrical cylinders almost entirely separated by the external and internal folds which almost meet in the median line. On the outside, each tooth is coated with a layer of cement. When better known it may prove that this genus, so specialized in the character of the teeth, does not belong in this family. =Plagiarthrus clivus= Ameghino [Illustration: Fig 28. Left lower premolars 3 and 4 and molars 1-3—natural size.] P. clivus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436. This species is represented by a single specimen from the Chico del Chubut, west of Puerto Visser, which preserves pm. 3 and 4 and the molars. The characters of this, the type species, are those of the genus. The total length of the five teeth is 36 mm., and fig. 28 shows in natural size the various individual teeth. =Hegetotheriidae= Ameghino