370 PATAGONIAN EXPEDITIONS I PAL/EONTOLOGY. 



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 Thylacymts. 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 Amphiprom- 

 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. 20), 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 Prothylacynus are almost the same 

 as in Sarcophilus. The chief difference is in the facet for the outer cunei- 



