No. 3 •] OSTEOL OGY OF MESOHIPPUS AND LEPTOMER YX. 3 29 



external malleolus, which exhibits two facets, — an internal one 

 for the astragalus, and on the posterior side of the distal surface 

 a small one for the calcaneum. 



In AnchitJierium, the tibia and fibula are very much as in the 

 American form, but the former is straighter and decidedly more 

 massive. 



In the horse, the tibia has become nearly straight and very 

 heavy ; the femoral facets are nearly at right angles to the long 

 axis of the shaft ; the cnemial crest is very massive, but does 

 not extend so far down the shaft as in Mesohippus ; the distal 

 end of the fibula is indistinguishably fused with the tibia, but 

 preserves its facet for the calcaneum, at least in some cases. 



The tarsus of Mesohippus (PI. XXIII., Figs. 34, 36-38) is very 

 equine in appearance and construction. The calcaneum is rather 

 long and quite slender and compressed, the dorso-plantar diam- 

 eter being conspicuously small ; the two borders axe straight 

 and nearly parallel. This is true also of Equus, but in this 

 genus the short, massive, and deep tuber has a very different 

 appearance. The distance from the superior or "ectal" astra- 

 galar facet to the distal end is very considerable ; this facet 

 forms a high crest, descending obliquely toward the tibial side, 

 and on its posterior aspect there is, as Osborn has pointed out 

 (No. 41, p. 546), a very distinct fibular facet. This ectal astra- 

 galar facet is slightly prolonged at its infero-external angle. The 

 sustentaculum is very prominent and massive, but has only a 

 proportionately narrow facet for the astragalus. The inferior as- 

 tragalar facet is narrow and of limited antero-posterior extent. 

 The cuboidal facet is narrow, slightly concave, and obliquely 

 placed with reference to the long axis of the bone, descending 

 downwards and forwards. Posteriorly it is very little recurved 

 or inflected toward the tibial side. 



The calcaneum of AncJiitheriitm is more equine in shape, as 

 is especially marked in the increased dorso-plantar diameter of 

 the tuber calcis and the more pronounced beginning of the 

 "facette surnumeraire " by a prolongation of the infero-external 

 angle of the ectal astragalar facet. The inferior astragalar facet, 

 on the other hand, is less equine, being much more extended 

 antero-posteriorly than in Mesohippus. The sustentaculum is 

 more prominent and massive than in the latter genus, and the 

 cuboidal facet but little recurved posteriorly. 



