330 SCOTT. [Vol. V. 



In Equus the calcaneum has become relatively shorter and 

 stouter, the dorso-plantar diameter being especially increased ; 

 the sustentaculum is very prominent and enlarged both trans- 

 versely and from above downwards. There are two ectal astra- 

 galar surfaces, the supernumerary facet being completely sepa- 

 rated from the principal one. The cuboidal facet is divided into 

 two portions by a ridge, the smaller anterior one running from 

 before backwards, and the larger posterior one directed nearly 

 transversely. This curvature and transverse extension of the 

 distal end, together with the enlarged sustentaculum, give to 

 the posterior aspect of the horse's calcaneum a very different 

 appearance from that seen in Mcsohippus, though an indication 

 of the modern arrangement is clearly apparent in the older 

 genus. 



The astragalus of Mesohippus has a narrow, deeply incised, and 

 obliquely directed trochlea, the condyles running downward and 

 outward. The internal condyle is longer than the external, 

 extending beyond it both above and below, and inferiorly 

 reaching to the navicular facet, while the external condyle is 

 separated from this facet by a considerable interval. The surface 

 for the external malleolus is narrow and in some specimens quite 

 strongly everted as a distinct process at its lower end ; the facet 

 for the internal malleolus is very much broader, and terminates in- 

 feriorly in a pit, into which the tip of the malleolus fits in extreme 

 flexion. Internal to this is a prominent tubercle for the attach- 

 ment of the astragalo-metatarsal ligament. On the plantar side 

 of the astragalus three calcaneal facets are visible ; the ectal is 

 very deeply concave, and at its infero-external angle there is a 

 small, plane facet, the beginning of the supernumerary facet. 

 The ectal facet is separated by only a narrow interval from the 

 sustentacular, and in some specimens, notably those of the larger 

 animals, the two are in contact. The sustentacular facet is long, 

 narrow, and nearly flat ; distally it forms a projection of the pos- 

 terior margin of the navicular surface. The inferior calcaneal 

 facet is very small and elongate oval in shape. The distal sur- 

 face for articulation with the navicular is relatively narrow, and 

 of irregular shape. It is slightly concave from side to side, and 

 the external margin projects strongly downward ; a similar pro- 

 jection is formed on the posterior margin. As Osborn has 

 pointed out (No. 41, p. 546), the fossa in the navicular surface 



