_P. holmbergi_ is the type species, and of considerable size, and to it I have assigned my material. In such a large animal, variations in size are to be expected. _P. troussarti_, as described, is a tenth smaller than _P. holmbergi_, the only structural character differentiating it being the isolation of the pillar in the lower molars, which is a character due to youth; so I have considered it a synonym of _P. holmbergi_. _P. ephebicum_ is a much smaller and distinct species, with which I should associate the single upper molar to which the name _P. lemoinei_ has been given. _P. martiale_ is a large species, distinguished by the strong development of the cingulum on the internal side of the upper molars, and on the inner side of the lower molars; and by lower premolar 3 being well developed with two roots. _P. superabile_ is of the same size as the foregoing, but has the cingulum on upper premolar 4 (the only tooth known) less developed. I should therefore consider it a synonym of _P. martiale_. _P. insuperabile_ is the largest species, and is distinguished by the excessive development of the cingulum. _Liarthrus_ is founded on an upper pm. 4 with a part of pm. 3, but, as far as I can see, does not differ in character or size from _P. holmbergi_. _Traspoatherium_ is based on upper premolars which are distinguished by the roots being fused from side to side. I think it is an age character and for the present would consider it the same as _P. holmbergi_, probably the tooth being pm. 3. =Parastrapotherium holmbergi= Ameghino P. holmbergi Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 636. P. holmbergi Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 449. P. troussarti Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 638. P. troussarti Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 449. Liarthrus copei Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 641. Liarthrus copei Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 451. Traspoatherium convexidens Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 641. Traspoatherium convexidens Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 450. Parastrapotherium holmbergi Tournier, 1905, Bul. Soc. Geol. France, ser. 4, t. 5, p. 305. Astrapotherium holmbergi Gaudry, 1906, Anal. Palaeontologie, t. 1, p. 5. To this, the type species, I have assigned all the material we found on the Chico del Chubut, west of Puerto Visser, as enumerated under the generic description. The lower jaws belonged to an old individual. The humerus and scapulae were found associated, the femur isolated. Of the upper dentition, the only available measurements are those of Ameghino for the first molar, and the canine. Upper dentition, canine, length 256 mm. Upper dentition, molar 1, length 57 mm., width 57 mm. [Illustration: Fig. 103. Upper molar 1 of the left side—natural size, after Ameghino.] The measurements of the complete pair of lower jaws which we found are