Among the Santa Cruz specimens, this genus is distinguished by having in the lower jaw the large first incisor, then five vestigal teeth, followed by a two-rooted, though somewhat reduced, third premolar, next the enlarged fourth premolar, making the sectorial tooth, and lastly three buno-lophodont molars. There is considerable confusion as to the use of the generic name. Moreno designated the first specimen, _Palaeothentes_, without a description; then Ameghino used this term describing the species; later Ameghino thinking that the name _Palaeothentes_ was the same as _Palaeothentes_ proposed the name, _Epanorthus_, using this for the first description of the Deseado species. Later, however, he changed this for _Palaepanorthus_. As I can see no generic differences between the Deseado and Santa Cruz species, I shall follow Sinclair in using the generic term _Palaeothentes_. =Palaeothentes chubutensis= Ameghino Epanorthus chubutensis Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 500. Palaepanorthus chubutensis Amegh., 1901, Anal. Soc. Cienc. Argen., t. 51, p. 77. Palaepanorthus chubutensis Amegh., 1902, Bol. Acad. Nac. Cienc. Cordoba, t. 18, p. 123. Palaepanorthus chubutensis Amegh., 1903, Anal. Mus. Nac. B. A., t. 9 (ser. 3, t. 2) p. 239. [Illustration: Fig. 145. Right mandible—2 times natural size, after Ameghino.] The species is founded on a tiny mandible with premolar 3-molar 3, on which the third premolar, while reduced, has two roots and reaches the height of the fourth premolar, being in about the same stage of development as the Santa Cruz species. As we found no specimens of this species I reproduce Ameghino’s figure and measurements. Lower dentition, premolar 3 to molar 3 19 mm. Lower dentition, height under premolar 4 12 mm. =Pilchenia= Ameghino Pilchenia Ameghino, 1903, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 2, p. 128. Pilchenia Ameghino, 1904, Anal. Soc. Cienc. Rep. Argen., t. 58, p. 259.