The calcaneum is longer than in _Nesodontidae_. It is, however, heavy and stout, the tuber broadening slightly toward the free end, on the plantar side of which there is a tendinous sulcus as in Nesodon. The fibular facet is wide, rectangular, and convex. Of the astragular facets, the sustentacular extends well back onto the tuber, and the ectal is the usual ovoid surface. Distally there is a broad slightly concave facet for the cuboid, and external to this a narrow surface for contact on the navicular. Except in length, this bone is very like that of Adinotherium. The astragulus and rest of the tarsal bones are wanting. Parts of the metatarsals and a phalanx indicate that the hind foot is of the same tridactyl type as the front, differing only in that the median digit seems to be relatively a little heavier. Ameghino described three species of _Rhynchippus, R. equinus, R. pumulis_, and _R. medianus_. =Rhynchippus equinus= Ameghino R. equinus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 463. This species is the dominant one in the Deseado from the Chico del Chubut, west of Puerto Pisser, no less than fourteen individuals being represented in our collection, three by skulls, and one by the major part of a skeleton which was found associated with the skull of _Leontinia_, but was determined as belonging to this species, by the duplication of the radius with a specimen having the radius and lower jaws associated. The description of the generic characters is largely based on this skeleton. The three species are differentiated largely by their size, _R. equinus_ being the largest, but as compared with _R. pumulis_ it is not only larger but much heavier built. [Illustration: Fig. 53. Dorsal view of skull—½ natural size.] The skull has been described under the generic discussion. In young individuals, the furrow in the incisors and canine is marked as a groove, later as a pit, and still later is lacking altogether, as is the case in the specimen figured, for all three of my more complete specimens are old individuals, as is also Ameghino’s type. All the incisors show the cingulum near the base of the enamel. On account of the development of the cingulum on the inner anterior corner of the premolars, these teeth are broadened out and overhang the palate in a marked degree, and are also much wider than long. Molar 1 is intermediate in character between the premolars and the succeeding molars, being only slightly rhomboidal in outline, while in the last two molars the anterior external corner is markedly prolonged. [Illustration: Fig. 54. Left upper dentition, old individual—½ natural size.] [Illustration: Fig. 55. Right lower dentition of R. equinus; _A_, of a young individual; _B_, of an old individual—½ natural size.]