This species is similar to _P. garzoni_ except for size, the forms running about 12% larger, and being heavier built. In this same line the upper and lower molars are relatively wider and heavier. The top of the skull also is wider. I have drawn carefully the skull and dentition so that the detail can be seen from the figures. Beside triangulidens, Ameghino described _P. robustum_, which, as far as I can see, differs only in being about 5% larger, which is well within individual variation, so I have considered it as a synonym. The same is the case with _P. quartum_, which Ameghino distinguishes as being about the size of _P. robustum_, and having lower pm. 1 present. The latter character we found also characteristic of _P. garzoni_, so only size remains and I do not consider less than 10% enough by itself to make a species. MEASUREMENTS Skull, length 110 mm. Upper dentition, length inc. 1 to m. 3 57 mm. Upper dentition, length pm. 1 to m. 3 35 mm. Upper dentition, incisor 1, width 8 mm. Upper dentition, molar 1, width 4½ mm. Six specimens from Chico del Chubut =Propachyrucos= Ameghino Propachyrucos Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 425. The genus is based on lower jaws, in which the characters of the premolars, the molars and the first two incisors, resemble those of _Pachyrukhos_; but in this genus the third incisor, the canine, and the first premolar are retained and but little reduced. Ameghino has described two species, _P. smithwoodwardi_, and _P. aequilatus_. [Illustration: Fig. 41. Top view of the skull, palatal view—natural size.] =Propachyrucos smithwoodwardi= Ameghino P. smithwoodwardi, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 425. We did not find this species, but I reproduce Ameghino’s figure of it, natural size. The length of the dentition from inc. 1 to m. 3 is 41 mm., height of mandible under m. 1 is 12 mm. [Illustration: Fig. 42. P. smithwoodwardi after Ameghino, right mandible—natural size.] =Propachyrucos aequilatus= Ameghino P. aequilatus, Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 371. This species is based on the anterior lobe of each lower molar, being longer than the posterior. In size, molars 1 to 3 measure 24 mm.[13] [Footnote 13: P. crassus has been described, (loc. cit., p. 425,) based on pm. 2 and 3, of larger size than either of the foregoing but I do not think that the genus can be determined on so small a fragment.] =Phanophilus= Ameghino Phanophilus, Amegh., 1903, Anal. Soc. Cienc., Argen., t. 56, p. 202. This genus is based on isolated upper molars, characterized as similar to _Protypotherium_, but having a pronounced median vertical column, instead of a groove on the external face of the upper molars, a character unique among typotheres. The position of the genus with this scant information is uncertain. One species is described, _P. dorsatus_.