The species is simply said to be much smaller than the preceding, which, by the measurements, would figure out about 27%. The measurements given are pm. 1 to m. 3, 33 mm. =Argyrohyrax acuticostatus= Ameghino A. acuticostatus Amegh. 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 361. This species is described as differing from _A. proavus_ in being somewhat smaller, but I find it about the same size. The specific character is in the teeth being more compressed, and in the anterior vertical margin of the upper molars being developed in the form of a very salient crest. =Isoproedrium= Ameghino Proedrium Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 623. Proedrium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 431. Isoproedrium, Amegh., 1903, Anal. Soc. Cient. Argen., t. 56, p. 18. This genus is based on a toothless mandibular symphysis of large size, on which the second incisor is indicated as larger than the first. We found one lower jaw, which, though imperfect, confirms the above as a genus and adds the following for generic characters; premolars with a shallow inflexion on the inner and outer sides, the molars with a shallow inner, and a narrow deep outer inflexion, heavy layer of cement on the premolars and molars, enamel reduced on the anterior internal, and the posterior internal corners of at least the premolars. =Isoproedrium solitarium= Ameghino loc. cit. above. [Illustration: M1 4 3 2 pm1 2 i1 Fig. 51. Right mandible, dotted line indicated alveolus—natural size.] I have assigned the specimen shown in fig. 51 to this species. Alveoli of the first and second incisors show inc. 2 considerably larger than inc. 1. About the third incisor and the canine my specimen shows nothing, being broken down in that region. Pm. 1 is represented by a moderately large alveolus, 10 mm. long. Pm. 2 is 13 mm. long by 10 mm. wide, and slightly constricted medianly as far as the enamel is concerned, the furrow in either side being filled with cement. Pm. 3 is 18 mm. long by 12 wide and similar. There is only an alveolus for pm. 4, which is 23 mm. long. M. 1 is incomplete, but was about 23 mm. long by 10½ mm. wide and is distinguished by the deep outer inflexion. Each tooth present is covered with a heavy layer of enamel nearly a millimeter thick; and each of the teeth is unique in that the enamel is wanting on the anterior internal and the posterior internal corners of the teeth. CHAPTER VI TOXODONTIA The toxodonts of the Deseado are much more varied than those of the Santa Cruz, and less so than those of the Casamayor; the teeth less hypsodont than in the Santa Cruz, and more hypsodont than in the Casamayor; are smaller than those of the Santa Cruz, and larger than those from the Casamayor. It is a group of heavy, short-limbed, nonadaptive animals, which, as time and competition progressed, gradually diminished in numbers and variety.