Prosotherium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 426. In founding this genus, Ameghino says that lower pm. 1 is lacking, but our specimens show it present as a vestige, and also show no trace of lower inc. 3 against which Ameghino puts a question mark, making the formula 1 0 4 3 ------- 2 0 4 3 as given above. The upper molars are similar to those of _Pachyrukhos_ except that they have an inner fold which has been lost in _Pachyrukhos_. The premolars are unlike the molars. Lower molar 3 is three-lobed. The description of the skeleton is given under the specific description of _P. garzoni_, and this shows a remarkable resemblance to the skeleton of _Pachyrukhos_, throughout, so that I have no doubt but that _Prosotherium_ is the ancestor of _Pachyrukhos_, the changes in the teeth proceeding in the line of simplification which seems to be general in this order, and is in general characteristic of forms in which the teeth become rootless. Ameghino described four species, _P. garzoni_, _P. triangulidens_, _P. robustum_, and _P. quartum_, the last two of which differ so little from _P. triangulidens_, that I can not consider them as independent species. =Prosotherium garzoni= Ameghino P. garzoni, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 426. This, the most abundant species of typotheres, occurs in our collection from the Chico del Chubut, west of Puerto Visser, fifteen times; and in one case the major part of a skeleton was found, consisting of the skull and jaws, vertebrae of each type, ribs, most of the fore limb, the pelvis and a hind limb. The animal as a whole is smaller than _P. triangulidens_ by about 12%, and is of lighter build. The skull is relatively light and narrow, especially in the rear, where the swollen hollow capsules of the squamosum bones come within ten millimeters of meeting medianly, whereas, in other species, they are twice as far apart. These hollow capsules are in this species the most marked, and in this genus even more developed than in _Pachyrukhos_. The lachrymal bone is larger externally than usual, the lachrymal duct opening about four millimeters in front of the margin of the orbit, and continuing to the margin by an open groove. In _P. triangulidens_, the duct is inside the orbit. The heavy maxilla makes a strong process for the zygomatic arch, extending fully half way back along this arch. The short, but fairly stout jugale has but a short contact with the maxilla. [Illustration: Fig. 30. Left upper dentition; left lower dentition—natural size.] [Illustration: Fig. 31. Left upper dentition No. 3083, showing deciduous premolars.] In the dentition, the premolar and molar teeth are covered with a thin film of cement, which is thicker on the outside of the upper teeth and on the inner side of the lower teeth. On the opposite sides of these teeth this film is so thin that it is often in part worn off.