Fig. 37. Calcaneum,
astragulus and cuboid,
from upper side—
natural size.
Fig. 38. Astragulus
from below—
natural size.
The astragulus is also quite characteristic, the trochlear surface being entirely on the dorsal surface, and the condylar ridges being relatively low and flat. This trochlear surface is far from being symmetrical, the inner ridge being much flatter and lower than the outer. The head of the astragulus is rounded, on a long neck, and directed obliquely inward. The fibular facet for the fibula is crescent-shaped and vertical except that the small proximal end of the crescent flares out. The outline of the sustentacular facet is that of an acute ovoid, and is situated mostly on the neck of this bone. The ectal facet is roughly rectangular in outline, strongly concave, and is separated from the sustentacular facet by a deep groove.
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.
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.