Ameghino has described two upper teeth. Specimen No. 3001 of the Amherst Collection from the Chico del Chubut River, west of Puerto Visser, adds to this the knowledge of twelve vertebrae (seven dorsal and five lumbar), the lower dentition complete, the left hind limb complete, and the right hind limb complete except for the femur. In general, the animal is about ⅗ the size of _Theosodon garrettorum_, but in parts varies from this as follows. The lower jaw is ⅗, the vertebrae are ⅖ in length, the hind leg is ⅗ in length but ¾ in diameter of bones, while the metatarsus is only ⅓ in length. This makes an almost plantigrade form of heavy, clumsy proportions. [Illustration: Fig. 14. Upper molar. 2 of the right side—natural size, after Ameghino.] Of the upper dentition we know only what Ameghino has given us. The molar is distinguished by the presence of both the protoconule and metaconule, by the development of the posterior cingulum and by the presence of three external styles. MEASUREMENTS Upper premolar 4, length 12 mm., width 15 mm. Upper molar 2, length 14 mm., width 17 mm. [Illustration: Fig. 15. Right lower dentition—natural size.] In the lower dentition, none of the teeth are reduced, and all are in a continuous series, except that there is a small diastema either side of pm. 1. The incisors are simple, compressed teeth, with but a trace of a cingulum. The canine is incisiform and a trifle larger than the incisors. Premolar 1 is also incisiform, and is isolated by a small diastema on either side. The second premolar is longer and wider than the first, and begins to show molariform characters, the anterior portion being composed of a high compressed cusp, the posterior portion by a low crescent on which but one cusp is fully developed. The third premolar is composed of two complete crescents, and has the “pillar” already developed opposite the posterior end of the back crescent. In fact, the tooth is molariform, except as to the tiny extra cusp found on the molars. Premolar 4 is more completely molariform consisting of the same parts as the preceding tooth. The molars may be distinguished by the presence of a tiny median cusp on the rear of the tooth, behind the crescent, which, when the tooth is worn, makes a median spur to the rear. In both the premolars and molars, the teeth are characterized by their plumpness, and the isolation and lowness of the cusps. [Illustration: Fig. 16. Right mandible—½ natural size.] The two halves of the lower jaw are completely fused at the symphysis. The horizontal ramus is thick, but low dorso-ventrally, giving the appearance of a slender jaw. The posterior angle is prolonged backward and bent inward. The fossa for the masseter muscle, while large, is but faintly outlined. The ascending ramus hardly rises above the level of the teeth, except as the slender coronoid projects to a good height above the articular condyle and curves backward over it. MEASUREMENTS