yO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



for the astragalus. The lateral margin of the distal surface is turned 

 outward, evidently for some articulation with the calcaneum, but 

 this marginal facet is anteroposteriorly convex rather than concave 

 as in Meniscotherium. The lateral tubercle of the malleolus that 

 bounds the groove for the peroneus longus anteriorly is somewhat 

 more posterior in position, and the peroneal groove though narrower 

 is well defined. The posterior margin of the preserved portion of 

 the fibula is rather more sharply ridged than in Meniscotherium. 

 Medially, the external malleolus is slightly roughened proximal to 

 the astragalar facet, and there is no evidence of a facet for the tibia. 



PES 



The hindfoot of Meniscotherium (see pi. 10) appears to be as 

 much as 25 percent larger than the forefoot, as determined by the 

 length of the metatarsals relative to that of the metacarpals. The 

 tarsus, however, does not appear to be so proximodistally compressed 

 as the carpus, so that in dorsal aspect the tarsus more nearly resem- 

 bles that of Phcnacodus. Most noticeable differences from the Phenac- 

 od us foot lie in the appearance of the tibial surface of the astrag- 

 alus and the shorter, more wedge-shaped cuboid in Meniscotherium. 

 Moreover, the distal extremities of the metatarsals are relatively not 

 so broad as in Phcnacodus primacvus. The Meniscotherium hind- 

 foot bears a resemblance also to the Tetraclacnodon hindfoot, but 

 in the latter the elements are relatively more elongate and slender. 



Astragalus. — The astragalus is a distinctive bone in Menisco- 

 therium, somewhat compressed dorsoventrally on the medial side but 

 not on the lateral side. The inner crest of the trochlea or tibial sur- 

 face is subdued and rounded, whereas the lateral crest is high and 

 acute. The neck is sturdy but elongate and directed medially as well 

 as distally. The transversely broad head has a strongly biconvex 

 navicular surface that tapers somewhat medially. The ventral sur- 

 face of the astragalus, or that facing the calcaneum, exhibits two 

 obliquely elongate and nearly parallel facets. The lateral or ectal 

 facet is uniformly concave in its long direction and extends out on a 

 prominent distolateral process. This process also deflects laterally 

 the distoventral margin of the large, otherwise nearly vertical facet 

 for the fibula. The medial or sustentacular facet is longitudinally 

 convex and broader distally where it extends onto the neck. Pos- 

 teriorly (proximally) this facet narrows and near its extremity is 

 sharply flexed ventrally where it articulates with the acute postero- 

 dorsal margin of the sustentaculum. Lateral to this flexure and at 



