This species is known only by a single canine 100 mm. long, of which but 15 mm. belongs to the crown. Its antero-posterior diameter is 14 mm., the transverse 12 mm. It seems to me very doubtful whether this is a valid species. Caenolestidae This family, based on the living genus _Caenolestes_, is represented in Tertiary times in Patagonia by three subdivisions, _Palaeothentinae_, _Garzoninae_, and _Abderitinae_. While diprotodonts, as far as known, the family is in strong contrast to the Australian diprotodonts in that there is no sign of syndactylism in the pes. The American forms are characterized by four subequal upper incisors, a normal canine, the first three premolars vestigal, while the fourth is either normal or enlarged into a sectorial tooth. The three molars are progressively smaller from the front back. The first lower incisor is greatly enlarged and procumbrent, the remaining incisors, the canine, and the anterior premolars being vestigal though usually present. Premolar 4 is enlarged and sectorial in most genera, and the molars as in the upper jaw progressively smaller. For the practical purposes of this paper the subfamilies are distinguished as follows: Caenolestinae, lower pm. 4 not developed into a sectorial tooth. Palaeothentinae, lower pm. 4 is developed into a sectorial tooth. Abderitinae, lower pm. 4 is developed into a sectorial tooth and striated. =Palaeothentinae= Sinclair. (= Epanorthidae Ameghino) This group or subfamily was established to hold several genera of tiny marsupials with the dental formula ? 1 4 3 -------; 4 1 2 3 the lower fourth premolar enlarged into a sectorial tooth; and the molars small and buno-lophodont. From the Deseado beds but one genus of this subdivision has been found, _Palaeothentes_, designated by Ameghino first _Epanorthus_ then later _Palaepanorthus_, but as I can see no reason for distinguishing the Deseado species of the genus from those of the Santa Cruz, I have retained the name _Palaeothentes_. The genera of this subfamily are distinguished as follows: LOWER THIRD PREMOLAR Palaeothentes 2-rooted, fairly large, equals pm. 4 in height. Pilchenia 2-rooted, moderate size nearly equals pm. 4 in height. Callomenus 2-rooted, small size much lower than pm. 4. Decastris, 1-rooted, vestigal. =Palaeothentes= (Moreno) Ameghino Palaeothentes Moreno, 1882, Patagonia, Resto de un Continente hoy submergido, p. 22, (nomen nudum). Palaeothentes (Moreno) Ameghino, 1887, Enum. Sist. Espesies Mamif. Fos, Patagonia, p. 5. Epanorthus Ameghino, 1889, Act. Acad. Nac. Cienc. Cordoba, t. 6, p. 271. Epanorthus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 500, (nudum). Palaepanorthus Amegh., 1901, Anal. Soc. Cienc. Argen., t. 51, p. 77, (nomen). Palaepanorthus Amegh., 1902, Bol. Acad. Nac. Cienc. Cordoba, t. 18, p. 123. Palaepanorthus Amegh., 1903, Anal. Mus. Nac. Buenos Aires, t. 9, (ser. 3, t. 2) p. 239.