The calcaneum is of moderate size, has a narrow fibular facet, a broad ectal facet, and a moderately large sustentacular one. The facet for the cuboid is slightly concave, and occupies the whole of the distal end of the calcaneum. [Illustration: Fig. 39. Right foot—natural size.] The metatarsals are moderately long and rather heavy, not quite as long and slender as those of _Pachyrukhos_. The phalanges are also shorter and slightly heavier than those of _Pachyrukhos_. We found four proximal and four of the second series, all associated, which probably indicates the full number of the toes. The ungual phalanges are proximally narrow and high, then expand toward the tip, developing into marginal expansions. There is but a trace of a cleft in the end of these ungual phalanges. MEASUREMENTS Skull, greatest length 99 mm. Upper dentition, length inc. 1 to m. 3 55 mm. Upper dentition, length pm. 1 to m. 3 31 mm. Upper dentition, incisor 1, width 6½ mm. Upper dentition, molar 1, length 6 mm. Upper dentition, molar, width 4½ mm. Mandible, greatest length 82 mm. Lower dentition, length inc. 1 to m. 3 53 mm. Lower dentition, length pm. 1 to m. 3 32 mm. Lower dentition, molar 1, length 6½ mm. Lower dentition, molar, width 3 mm. Third metacarpus, greatest length 28 mm. Pelvis, length front to back 83 mm. Femur, greatest length (computed) 93 mm. Femur, diameter of middle of shaft 9 mm. Tibia, greatest length 90 mm. Astragulus, length 14 mm. Astragulus, width 11 mm. Calcaneum, length 25½ mm. Metatarsus III, length 32 mm. First phalanx of digit III, length 12 mm. Ungual phalanx of digit III, length 9 mm. To make the similarity of Prosotherium with Pachyrukhos clearer, I have restored Prosotherium, figure 40, from which it will be seen that this genus is also a hopping form with a plantigrade hind foot and a semidigitigrade front foot. In general it compares very closely with Pachyrukhos, but the limbs are shorter and the grade of specialization is not quite as high. It is, however, very evidently the ancestor of Pachyrukhos. [Illustration: Fig. 40. Restoration of Prosotherium garzoni—⅓ natural size. (Half tone.)] =Prosotherium triangulidens= Ameghino P. triangulidens Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 427. P. robustum, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 427. P. quartum Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 371.