119 



by a broad, rugged, elevated border, and from the superior articular surface by 

 a well-defined margin. The posterior surface of the calcaneum is high but 

 narrow, rising from the tuberous extremity of the heel obliquely upwards with a 

 gentle concave curve, to the superior surface, from which it is separated by a 

 small rugged tubercle : this tubercle, and a corresponding one on the extremity 

 of the astragalus above, indicate the points of attachment of a strong posterior 

 hgament. The inner surface of the calcaneum is separated from its posterior 

 surface by a broad and rugged obhque ridge: it gradually deepens to a wide 

 concavity bounded by three large tuberosities ; one above, which supports the 

 inner extremity of the astragalar articulation ; another below, forming the in- 

 ternal and anterior angle of the inferior subconcave surface ; the third in front, 

 constituting the anterior prolongation of that surface. There is but one ar- 

 ticular surface in the os calcis of the Mylodon, which occupies the whole of 

 the narrow superior facet, and bends down over part of the anterior surface : 

 the upper division of the articular surface supports the astragalus : it is slightly 

 concave transversely, convex from behind forwards at its middle part : its 

 breadth is not quite equal to the surface of the astragalus which plays upon it, 

 and it is evident that a certain freedom of lateral motion was allowed between 

 these two bones. The anterior deflected division of this long and narrow ar- 

 ticular surface is equal to one-fourth of its entire extent ; it is shghtly concave, 

 and is adapted to the os cuboides. 



The naviculare* is an irregular, thick, oval plate of bone, concave towards 

 the astragalus, convex next the metatarsus. Its thickest part is towards the 

 upper and outer end, where the articular surface for the astragalus is slightly 

 convex ; the rest of that surface which extends to the tibial and lower border of 

 the naviculare is concave. The upper and posterior border of the bone presents 

 accordingly a sigmoid curve. The thickest part of the margin of the bone, 

 which is above the convex articulation with the astragalus, is rough and flat- 

 tened : the angle between this and the outer margin is formed by a thick tu- 

 berosity. Only a small part of the anterior convex surface is modified for 

 articular union, and this part presents two distinct surfaces for two cuneiform 

 bones : of these articulations, the outer one is more than double the extent of 

 the iimer one, and forms an oval of one inch and a half by one inch in diameter ; 



• Plates XXI. and XXII. c. 



