This, the type species of the above genus, is based on specimen 3063, a lower jaw with the incisor, premolar 4 and the first two molars. The incisor is fairly large and heavy, the anterior face slightly convex, and the anterio-posterior diameter greater than the transverse diameter. Premolar 4 has a deep external fold, dividing the crown into an anterior and posterior lamina, the former being then subdivided by another external fold, making the tooth three-lobed. Just internal to the median fold is a tiny pit, apparently the last vestige of an internal fold. Each molar consists of two laminae separated by a deep external fold, around the inner end of which the laminae are connected by a narrow bar. In the present condition of wear there is no indication of secondary furrows. The premolar is smaller than the molars. MEASUREMENTS SPECIMEN 3063 Lower dentition, in. 1 to pm. 4 8 mm. Lower dentition, premolar 4 length 3 mm., width 2.5 mm. Lower dentition, molar 1 length 2.5 mm., width 2.5 mm. Lower dentition, molar 2, length 2.75 mm., width 3 mm. Height of mandible under pm. 4 5.5 mm. [Illustration: Fig. 126. Left mandible external side, × 4/1.] =Litodontomys= gen. nov. One set of lower teeth found by the Amherst party shows a simplicity of pattern found in no other genus of South America; and this is, therefore, named _Litodontomys_. The teeth are brachydont, the premolar and the molars each being divided into two laminae by an external and an internal fold, the distinctive generic feature being in that this fold is narrowest at the margin of the tooth and expands internally. In connection with the expanded folds, the ends of the laminae are curved toward each other, so that in a worn specimen they would meet on the margins of the tooth, and leave the folds to appear as pits. No indication of a furrow is evident on either lamina. =Litodontomys chubutensis= sp. nov. The type is number 3086 of the Amherst collection, from the Deseado beds on the Chico del Chubut River, west of Puerto Visser, and consists of the lower premolar-molar series. [Illustration: Fig. 127. Right lower premolar 4—molar 3, × 4/1.] Premolar 4 is elongated, the anterior lamina being considerably longer than it is wide, whereas the laminae of the other teeth are wider than they are long. The following measurements with the figure give the specific details. Lower dentition, premolar 4, to molar 3 10.5 mm. Lower dentition, premolar 4, length 3.5 mm. Lower dentition, molar 1, length 2. mm. Lower dentition, molar 2, length 2.5 mm. Lower dentition, molar 3, length 2.5 mm. Lower dentition, width of molars 2. mm. ERETHIZONTIDAE =Asteromys= Ameghino Asteromys Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 495.