This genus was founded on a single lower molar which, in the light of the specimen we found, I take to be the third or last. Our specimen shows pm. 3 and 4 and the three molars. The third premolar is a small two-rooted tooth with a simple crown and no heel. Premolar 4 is an enlarged sectorial tooth, the anterior part consisting of two cusps, closely set near the median line, with an incipient cusp on the inner face of the large anterior cusp. The posterior part of this tooth is arranged as a typical talonid, with one internal and two external cusps on the margin of a shallow inclosed basin. On the rear of the tooth is a small crescent-like cingulum, which occurs in the same place on molars 1 and 2, but is lacking on molar 3. This is a characteristic feature of the genus. On the anterior part of the molars is developed a sort of trigonid of small size, and the cusps are indistinct. The posterior portion of each molar is a large talonid with a shallow basin surrounded by a low wall on which are three tiny cups (the entoconid, hypoconid, and hypoconulid). =Pilchenia lucina= Ameghino P. lucina Amegh., 1903, Anal. Mus. Nac. B. A., ser. 3, t. 2, p. 128. P. lucina Amegh., 1904, Anal. Soc. Cienc. Rep. Argen., t. 58, p. 259. [Illustration: Fig. 146. Left mandible with premolar 3 to molar 4—4 times natural size.] In the Deseado beds, on the Chico del Chubut River, west of Puerto Visser, we found a single specimen of this species which agrees with the single tooth figured by Ameghino as the type, and which I interpret as molar 3. The description is given under the genus, the measurements are as follows: SPECIMEN NO. 3110 Lower dentition, distance from premolar 3 to molar 3 14 mm. Lower dentition, premolar 3, length 2 mm., width .75 mm. Lower dentition, premolar 4, length 5 mm., width 2.5 mm. Lower dentition, molar 1, length 3 mm., width 2 mm. Lower dentition, molar 2, length 2.5 mm., width 2 mm. Lower dentition, molar 3, length 2 mm., width 1.75 mm. Lower dentition, height under pm. 4 5 mm. =Callomenus= Ameghino Callomenus Amegh., 1891, Neuvos Restos Mamif. Fos. Patagonia Austral, p. 20. Callomenus Sinclair, 1901-6 Princeton Patag. Expeditions, vol. 4, p. 434. This genus has not been previously reported from the Deseado beds, but we found a tiny lower jaw with three teeth to represent it. The genus is distinguished by premolar 3 being two-rooted, but so small as not to attain the height of premolar 4. =Callomenus praecursor= sp. nov.