We found no specimens of this species, but Ameghino has described a complete skull, a figure of which is reproduced here. It indicates a smaller lighter built animal, differing from _R. equinus_ not only in small size, but also in having a relatively longer and narrower head. The individual is a rather old one, so that the pits in inc. 1 and 2 have disappeared, as is also the case with the cingulum on the ant. int. corners of the premolars. Ameghino gives the following measurements in his description. Skull, length over all 155 mm. Upper dentition, from inc. 1 to m. 3 80 mm. =Rhynchippus medianus= Ameghino R. medianus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 375. This third species is intermediate in size between the two foregoing. No figure is given, but the following measurements give the size: Upper molar 2, length, 17 mm., width 11 mm. Length of lower molars 1 to 3 40 mm. Height of mandible under m. 2 24 mm. These figures would indicate a form about 15% larger than _R. pumulis_, and 35% smaller than _R. equinus_. =Morphippus= Ameghino Morphippus, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 459. This genus is very similar to _Rhynchippus_, with the same dental formula, the same grooves in the upper incisors, and the same pattern of the premolars and molars. It differs, however, in the lower incisors having no cingulum at their base, in the upper molars having a shallower basin, and in bays 3 and 4 being absent from the lower molars. These features simply indicate a slightly less advanced specialization, less hypsodont teeth. I do not think that the bays are any of them lacking in unworn teeth, but in a less hypsodont tooth, with the pits extending a less distance into the crown, all indication of the bays disappears early. _M. imbricatus_ is described as the type species, and four others have been described, all equal in size to _M. imbricatus_, and distinguished by the teeth being slightly more compressed, by the external cleft of the lower molars being deeper, or by variations in the pits. All these features I consider to be either age characters or individual variations, so that all five species of this genus are lumped under _M. imbricatus_. =Morphippus imbricatus= Ameghino M. imbricatus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 459. M. hypsolodus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 461. M. complicatus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 461. M. fraternus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 374. M. quadrilobus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 374. The general characters of this species are given under the generic description and I will here give only Ameghino’s measurements which go with the figure: