79 



The third compartment is formed as it were by an excavation of the an- 

 terior and inner side of the distal articular surface, causing the concavity 

 of the preceding crescentic surface, and the wide and deep semicircular 

 notch which characterizes the fore-part of the distal end of the tibia. 

 The third and the crescentic compartments of the distal articulation are 

 exclusively articulated with the astragalus, the singular form of which 

 they sufficiently indicate. The inner margin of the anterior excavation 

 is pierced by a row of deep vertical canals. Purchased. 



The following bones, from 443 to 452 inclusive, belong to the same left 

 hind-foot of the Mylodon robuslus, are from the same stratum and locality as the 

 skeleton, and were purchased by the College. 



443. The astragalus. 



This characteristic bone, when in its natural relations with the rest of 

 the tarsus, has its fibular or outer side uppermost, and the articular surface 

 of the tibia looks inwards ; when articulated, therefore, to the leg, placed 

 vertically above it, the foot rests upon the ground by its outer edge, not 

 by its sole, and the peculiarities of the metatarsal structure relate to this 

 inversion of the foot. The articular surface which the astragalus pre- 

 sents to the bones of the leg is divided into three parts, the general 

 planes of which are at right angles to each other ; the surface, which in 

 the naturally inverted position of the foot is horizontal, presents a uni- 

 form figure : it is slightly convex anteriorly, concave in a less degree 

 posteriorly : at the antero lateral part of its outer convex border the ar- 

 ticulation adapted to the malleolar process of the fibula is continued 

 upon the external surface : that which answers to the inner malleolus 

 presents the form of a full convex semi elliptical tuber, which ascends to 

 fill the corresponding concavity or excavation on the inner side of the 

 distal articulation of the tibia ; below the extremity of this surface the 

 astragalus swells out into an oblong tubercle, and below this there is a 

 wide channel sinking into a deep depression at the fore and at the back 

 part of the base of the above process ; the canal and the two depressions 

 separating this part of the astragalus from the calcaneonavicular articu- 



