No . 3 . ] OSTEOL OGY OF MESOHIPPUS AND LEPTOMER YX. 3 3 1 



varies much in depth and distinctness. The cuboidal surface is 

 very small, and abruptly truncates the projection of the external 

 margin already mentioned. 



The astragalus of Anchitlierium is very much like that of 

 Mesohippus, but is relatively broader, especially the distal end, 

 and the neck is shorter, so that the external condyle projects 

 downwards as far as the navicular surface. The projection of 

 the external margin of the latter surface is less pronounced 

 than in Mesohippus ; that from the plantar margin rather more 

 so. According to Kowalevsky, there is no fossa in the navicu- 

 lar facet : " J'ai examine plus de quarante astragales de YAnchi- 

 therium sans pouvoir trouver un seul qui presenterait cette 

 echancrure caracteristique, toute la surface tarsienne est invari- 

 ablement d'un poli uniforme. Sur deux astragales seulement 

 j'ai pu remarquer a l'endroit ou se trouve l'echancrure chez les 

 chevaux une legere rugosite " (No. 25, p. 39). 



The astragalus of the horse has become decidedly broader, 

 the trochlea more widely open, and the condyles thicker ; the 

 neck is shorter, and the condyles overhang so far as to project 

 below the navicular surface. The supernumerary calcaneal 

 facet is quite widely separated from the ectal. The surface 

 for the external malleolus is broader. The anterior margin of 

 the navicular facet forms a nearly straight line, there being no 

 projection of the outer edge, but the posterior projection is very 

 strongly developed. The fossa of the navicular facet is very 

 distinct. The cuboidal facet is more completely distal than 

 in the earlier forms. 



The cuboid of Mesohippus is high and narrow, but deep antero- 

 posteriorly, and with a short but broad and rugose posterior 

 hook. The proximal surface is somewhat oblique to the verti- 

 cal axis of the bone, descending anteriorly ; it is obscurely 

 divided into two portions, of which the anterior is the broader, 

 but there is no such difference as in Anchitlierium ; the two 

 are in the same antero-posterior line. The astragalar facet 

 is very small and placed on the tibial side of the cuboid. 

 There is no antero-superior facet for the navicular, the two 

 bones being separated at this point by the downward projec- 

 tion of the astragalus, but there is a well-marked anteroin- 

 ferior facet. Posteriorly there is a small projecting process, 

 which carries two facets, a superior one for the navicular and 



