37 ° 
PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 
much lower than in Thylacynus. Unlike the latter, the distal end of the 
shaft is curved backward. The articular surface for the astragalar trochlea 
is broadly concave, that for contact with the inner side of the astragalus 
convex. The internal malleolus is much heavier than in Thylacynus and 
the fibular facet larger. The shaft is triangular in cross section. 
The fibula (PI. LIII, fig. i) is much heavier than in Thylacynus. The 
proximal end is greatly thickened, supporting two approximately plane 
facets for articulation with the tibia and fabella respectively. The distal 
end carries three articular surfaces, a round tubercle for lateral contact 
with the tibia, a flat irregularly triangular facet for the outer margin of 
the astragalar trochlea, and a concave elliptical facet for the calcaneum. 
The peroneal groove is broader than in Thylacynus and less perfectly 
defined. The shaft is straight, with strongly marked interosseous ridge. 
In contrast with the robust femur and tibia, the pes is rather feeble. 
The astragalar trochlea is remarkably flat, with the articular surfaces for 
the tibia and fibula separated by a faint groove. The tibial surface is 
produced distally down the dorsal aspect of the neck, as in Amphiprovi- 
verra, but unlike Sarcophilus and Thylacynus. The head is less obliquely 
placed than in the latter genus and is supported on a long heavy neck. 
Its distal end bears a single convex facet for the navicular. The inner 
side of the neck is deeply grooved for articulation with the internal tibial 
malleolus (PI. LIV, fig. 2). In plantar aspect (PI. LIV, fig. 2 a), the 
semicircular ectal facet is seen to be deeply concave antero-posteriorly. 
The sustentacular facet is irregularly lobate in outline and strongly con- 
vex in dorso-plantar section. Several small foramina pierce the body of 
the astragalus at the point occupied by the astragalar foramen in certain 
primitive placentals. These are not visible in the dorsal view (PI. LIV, 
fig. 8). The navicular has much the same shape as in Thylacynus , but 
is considerably larger and differs also in supporting all three cuneiforms, 
whereas in the recent genus, the outer cuneiform is supported almost 
entirely by the cuboid (cf. PI. LIV, fig. 8, and text-fig. 4, b ). The cuboid 
has the same shape as that of Sarcophilus , from which it differs slightly in 
the arrangement of some of the facets. 
The cuboid in Thylacynus does not lend itself to comparison, owing to 
the outward shifting of the lateral cuneiform just mentioned. The proximal 
and distal surfaces of this element in Prothylacymis are almost the same 
as in Sarcophilus. The chief difference is in the facet for the outer cunei- 
