NO. 2 A STUDY OF MENISCOTHERIUM — GAZIN Jl 



the posterior extremity of the deep groove for the interosseous liga- 

 ment, separating the ectal and sustentacular facets for most of their 

 length, is the ventral opening of the astragalar foramen. Dorsally 

 this foramen opens near the posterior margin of the tibial surface. 

 Between these apertures the posterior margin of the astragalus shows 

 a smooth, very broad, shallow, and somewhat oblique groove inter- 

 preted as conducting the flexor longus hallucis. The medial surface 

 of the astragalus, as noted by Cope (1884b, p. 505) "is oblique, 

 and has a central fossa and a prominent shelf-like angle below it." 

 There is apparently no articulation with the cuboid. 



The Phenacodus astragalus is noticeably different in exhibiting 

 a much better developed inner crest on the astragalus with a deeper 

 trochlear groove which extends to the posterior margin. The medial 

 or inner surface is essentially vertical, not nearly so oblique as in 

 Meniscotherium. The neck of the astragalus is shorter, and with 

 the head is relatively wider transversely than in Meniscotherium. 



The Hyopsodus astragalus appears anteroposteriorly short, but the 

 most noticeable feature in comparison with Meniscotherium is the 

 much less raised outer crest of the tibial surface. The low, broadly 

 rounded inner crest and the ventral shelflike inner projection are 

 much alike in the two forms, but between the subdued inner and outer 

 crests there is left only a very weakly developed trochlear groove. 

 This also leaves the tibial surface transversely much less oblique than 

 in Meniscotherium. The neck is somewhat oblique, nearly as in 

 Meniscotherium, but appears relatively shorter. Matthew (1915) 

 has described an astragalar facet on the cuboid ; such an articulation, 

 however, is not evident on the head of the astragalus at hand. The 

 facet for the fibula is at a 90-degree angle from the tibial facet, less 

 acute than in Meniscotherium. The lateral prominence of the antero- 

 ventral portion of the fibular surface, which supports anterior exten- 

 sion of the ectal facet beneath, is less projecting than in Menisco- 

 therium. The oblique ectal and sustentacular facets on the ventral 

 surface of the Hyopsodus astragalus are much like those in Menisco- 

 therium but a little less elongate. The groove between them for the 

 interosseous ligament is deeply impressed as in Meniscotherium, but 

 the astragalar foramen in ventral view is more posteriorly placed. 

 The groove on the posterior margin of the astragalus for the flexor 

 longus hallucis is similarly broad and well defined, but dorsoventrally 

 perhaps a little more elongate. 



In the hindfoot of Tetraclaenodon the astragalus has an elongate 

 neck and a relatively large head that is highly convex transversely. 



