=Phanophilus dorsatus= Ameghino P. dorsatus, Amegh., loc. cit. p. 202. In our collection, two isolated upper molars of this unusual form occur, corresponding in size and pattern to the one described by Ameghino. The external column, as seen by fig. 42, is narrow and high. A single tooth measures 5½ mm. from front to back, and 3¾ mm. in width. [Illustration: Fig. 43. External view of molar 1—natural size.] [Illustration: Fig. 44. Milk dentition, genus and species?—natural size.] Specimen 3142 gen. and sp.? This specimen is the mandible with the milk dentition. The molars present suggest _Prosotherium triangulidens_, but inc. 3 and the canine are present, and the first two incisors are not enlarged, so it would seem to represent a genus which I have not been able to identify. Molar 1 is bilobed and similar to that of _Prosotherium_. The deciduous premolars are all present, all rooted, and all remarkable for their great antero-posterior elongation. Roots of the incisors and canine are present, that of the canine being the largest, and those of the incisors being about equal in size. This would suggest such a form as _Protypotherium_, were this genus represented in the Deseado beds. =Interatheriidae= The family is characterized by the incisors not being enlarged, by upper and lower premolars and molars being inflexed on both the inner and out sides, and by the inflated mastoid bulla being filled with cancellous tissue. The only genus referable to this family, from the Deseado beds, is _Archaeophylus_. =Archaeophylus= Ameghino Archaeophylus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 423. The genus is based on a skull which preserved most of the upper dentition except the incisors, and which shows the characteristics of the family in their inception. =Archaeophylus patrius= Ameghino [Illustration: Fig. 45. Archaeophylus patrius after Ameghino, right upper dentition—natural size.] A. patrius Amegh., loc. cit. We found no specimen of this interesting type, but I give a diagram of Ameghino’s type, which shows the characteristics of the species and genus. The length of the premolar-molar series is 27 mm. =Eutrachytheridae= Ameghino The family consists of larger forms than the other families, and is characterized by the upper molars having the inflexion bifurcated, so that the teeth are at least incipiently three-lobed. I would place in the family the genera, _Eutrachytherus, Argyrohyrax_, and _Isopoedrium_. =Eutrachytherus= Ameghino Trachytherus Amegh., 1889, Act. Acad. Nac. Cienc. Cordoba, t. VI, p. 918. Trachytherus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 623. Eutrachytherus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 429. This genus was first called _Trachytherus_, and when this was found to be preoccupied, changed to _Eutrachytherus_. Ameghino gives the dental formula as 3 0 4 3 -------, 2 1 4 3