69 



into the deeper excavation on the inner part of the tibial articular surface, and forms a 

 kind of pivot on which the foot worked. 



The under or calcaneal side of the astragalus slopes from behind downward and 

 forward, and is divided by an oblique groove, about an inch broad, into two facets ; the 

 outer and posterior one is for the calcaneum exclusively ; the inner and anterior one is 

 for the calcaneum, but is continuous with the surface for the naviculare. The outer 

 calcaneal surface is ovate, concave lengthwise, convex across its posterior broader end, 

 nearly flat at its anterior end. The inner calcaneal surface is of much less extent and 

 is nearly flat ; its anterior end suddenly bends forward and upward to be continued into 

 the outer convex part of the navicular surface, which surface is divided into this convex 

 portion and a contracted subcircular concavity. The lower part of the convex facet rests 

 upon an articular concave surface on the cuboides. 



Only a small proportion of the outer surface of the astragalus is non-articular, and this 

 is chiefly on the outer, or, in the megatherian position of the foot, the upper surface 

 (Plate XXVI. fig. 1). The groove dividing the calcaneal surfaces begins at their lower 

 part and passes backward and inward, with slight terminal bends, which give it the form 

 of the italic f. It is divided by a ridge from a wider channel, excavating the under part 

 of the inner process ; from which a third channel extends backward, dividing part of the 

 inner calcaneal facet from the naviculo-cuboid surface. Both this channel and the 

 calcaneal one are perforated by conspicuous vascular canals. 



The calcaneum (Plates XXV. & XXVI. fig. 1, b) is a long irregular pyramidal bone, 

 with an obtuse apex : the base is obliquely truncated, forms the fore part of the bone, and 

 supports two articular surfaces (Plate XXVI. fig. 2) : the upper and outer surface (as) 

 is ovate, concave where that on the astragalus is convex, and vice versa: the oblique 

 channel which divides this astragalar surface from the astragalo-cuboidal surface (as 1 ) is 

 as deep as in the astragalus, and, like it, is perforated by vascular canals. The slightly 

 concave triangular inner astragalar facet (as!) is continued below, at an obtuse angle, 

 into the semielliptic slightly concave surface (cb) for the os cuboides. External to the 

 astragalar facet a strong vertically extended tuberosity (Plate XXVI. fig. 1, t) forms 

 the fore part of a wide and deep tendinal canal (u) ; a narrow and feeble ridge bounds it 

 behind ; a second and more shallow groove (v) succeeds ; and, behind this, is a wide and 

 deep vascular perforation (w). The under part of the calcaneum is very rugged and flat. 

 The obtuse hinder extremity of the heel-bone shows by its sculpturing and the out- 

 standing osseous spiculae, the force with which the attached cable-like ' tendo Achillis' 

 must have acted on so unusually produced a calcaneal lever. 



The os naviculare (Plates XXV. & XXVI. tig. 1, c) is a transversely oblong bone, corn- 

 ed from before backward. Its posterior surface is occupied by the articulation for 

 the astragalus, which is equally divided into an inner concavity and an outer convexity, 

 the latter approaching the conical form. From the lower part of this is continued at a 

 right angle a small flat triangular surface for the cuboides (Plate XXV. fig. 3, cb"). 

 The upper non-articular surface is narrowest at its middle, and is developed into a low 



