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.
