as in _Adinotherium_, the chief difference being that the teeth are less hypsodont than in the Santa Cruz genus. Little is known as yet of the skeleton, but when more is known probably more distinctive characters will appear. Ameghino made two species, _P. leptognathus_ which we also found, and _P. angustidens_ a much smaller form. =Proadinotherium leptognathus= Ameghino P. leptognathus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 625. P. leptognathus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 467. [Illustration: Fig. 81. Left lower jaw with incisor 3 and molar 2—½ natural size.] Of this species we found on the Chico del Chubut River, west of Puerto Visser, three specimens; the back of a skull as far forward as molar 2, and two lower jaws. In general, the species is very similar, even to size, to _Adinotherium ovinum_ of the Santa Cruz. The upper molars are strongly hypsodont, curved teeth. On the upper surface, the basin is subdivided by two strong cristae into three smaller bays. In an early stage of wear, the second crista unites with the posterior lobe, converting bay 3 into a pit. On the posterior margin of the tooth, the cingulum is developed so as to appear like a third crista, which inclosed bay 4, and when the tooth is worn, bay 4 becomes a pit also. In my lower jaw incisor 3 is developed into a strong caniniform tush. Most of the teeth are lacking, but lower molar 2 is a strongly compressed, hypsodont tooth, surrounded by a thick layer of enamel. This tooth rises 22 mm. above the well-developed roots, and is already considerably worn down. The pillar is prominent as a strong fold in the middle of the posterior crescent. In this specimen there is no trace of the usual pit (3) indicative of the septum, but I should expect to find it in a younger specimen. The mandible broadens in front into a scoop-like anterior end, and the alveoli of the first two incisors would indicate that they were proclivous. The alveoli for the other teeth are arranged as in _Adinotherium_. MEASUREMENTS Skull, width across the zygomatic arches 148 mm. Skull, width across opposite m. 3 (outside) 73 mm. Upper dentition, molar 2, length 25 mm., width 13 mm. Upper dentition, molar 3, length 23 mm., width 12 mm. Lower dentition, incisor 3, length 13 mm., width 7 mm. Lower dentition, molar 2, length 20 mm., width 7½ mm. =Proadinotherium angustidens= Ameghino P. angustidens Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 467. This is based on a single lower tooth, which is considered either pm. 4 or m. 1, and measures 13 mm. long by 4½ mm. wide. =Pronesodon= Ameghino Pronesodon Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 626. The genus is said to resemble _Proadinotherium_, but with the caniniform incisors proportionally much shorter. An associated calcaneum is shorter than that of _Adinotherium_ and longer than that of _Nesodon_.