118 



malleolus and descends upon the inner side of the astragalus, here on the con- 

 trary ascends at nearly a right angle to the plane of the middle reniform surface, 

 as it were, from its pelvis, in the form of a full convex semi-elhptical tuber, 

 which ascends to fill the corresponding concavity or excavation on the fore and 

 inner side of the distal articulation of the tibia. Below the extremity of this sur- 

 face 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 channel and the two depressions separating 

 this part of the astragalus from the calcaneo-navicular articulation : in these 

 depressions open many large vascular canals. A third rough and perforated 

 depression separates the fibular articular surface from the cuboidal one : a more 

 shallow depression indents the outer side of the astragalus behind the fibular 

 surface. The inferior and anterior part of the astragalus is occupied by one 

 extensive elongated articular surface adapted to the calcaneum, cuboides and 

 naviculare. The navicular surface* is flat at its upper half, convex at its lower 

 half; the latter part being continued uninterruptedly into the convex cuboidal 

 surfacef . The calcaneal surface:}:, which is continued backwards from the cu- 

 boidal one, is elongated, being continued to the posterior apex of the bone ; it 

 is rather narrow, and is constricted near its middle part. It is slightly convex 

 at its posterior extremity ; in a still less degree convex through the rest of its 

 extent, and that only from side to side ; it is gently concave from before back- 

 wards : it is perfectly smooth through the whole of its extent. 



The OS calcis^ of the Mylodon, which equals in size that of the Elephant, is 

 chiefly remarkable for the great breadth and length of its rugged posterior por- 

 tion, for its broad, concave, triangular basis, perforated by many large vascular 

 foramina, and for the large and deep tendinous groove, sometimes converted 

 into a canal, at the outer side of the bone, which canal or groove is nearly an 

 inch in diameter. Above it, at the anterior part of the outside of the bone, there 

 is a wider and shallower canal, mth a less smooth surface, bounded above by a 

 small tuberosity, and in front by a depression and a second tuberosity : the 

 broad and deep outer surface of the calcaneum behind the foregoing canals is of 

 a rhomboidal figure and is slightly concave, separated from the inferior surface 



* PI. XXIII. fig. 1. c. t PI. XXIII. fig. 2. d. 



1 PI. XXIII. fig. 2. e,/. § Plates XXI. and XXII. b. 



