68 



dittos, to which this structure is peculiar amongst existing Mammals. But, since it is 

 known only in the Order Bruta, it forms an interesting additional proof of the essential 

 relations of the huge extinct animal under description to that now anomalous group of 

 Mammals. The tibia of the Mylodon is proportionally shorter and thicker than in the 

 Megatherium: the outer articular surface at the upper end is of a subcircular form 

 and slightly concave : in the Megalonyx it is convex, but in a less degree than in the 

 Megatherium. The lower articular surface in both Mylodon, Scelidotherium, and Mega- 

 lonyx, presents the additional facet for the fibula. The hemispheric excavation on the 

 inner side of the distal articulation is relatively larger in the Mylodon than in the Mega- 

 therium. This excavation, with the concomitant protuberance of the astragalus, is pecu- 

 liar to the great extinct Sloth-like quadrupeds ; in which so secure an interlocking of 

 the foot with the leg bespeaks some habits peculiar to them, connected with the require- 

 ment of unusual resistance in the foot to the forces acting upon it from the leg and 

 thigh. 



In the series of existing animals Man presents the plantigrade foot in which the 

 weight of the body presses most nearly upon the crown of the tarsal arch ; but, in the 

 Megatherium, owing to the length of the heel and the shortness of the toes, the leg 

 transmits the superincumbent weight nearly upon the middle of the foot. So singularly 

 shaped and adjusted, however, are the tarsal bones in the Megatherium, that the tibia 

 articulates with the side instead of the summit of the tarsus, so that the whole foot is 

 turned inward and rests upon its outer edge instead of its sole (Plate XXVI. fig. 1, and 

 Plate I. cl, a). 



The number of tarsal bones is reduced to six, through the absence of the entocunei- 

 form*; but the astragalus, and especially the calcaneum, are developed to a great 

 size. 



The astragalus (Plates XXV. & XXVI. fig. 1, a) is of a peculiarly irregular form : if 

 the foot be placed with the sole flat on the ground, as in Plate XXV., the chief articular 

 surface (a) for the tibia looks inward, and the small fibular facet, at right angles there- 

 with, is uppermost. The extensive surface by which it articulates with the bones of the 

 leg is divided into three parts, the planes of which are at right angles to each other. 

 The middle and largest division (a), answering to the outer ridge of the trochlear 

 surface in the common form of astragalus, is here expanded into a broad reniform 

 smooth tract, horizontal in the ordinary position of the Megatherium's foot (Plate XXVI. 

 fig. 1, a), almost flat from before backward, convex from side to side. This surface is 

 continued over the outer edge upon the outer side of the bone, in a triangular form, with 

 the apex rounded off, to form the facet (o) for the fibula. The surface answering to the 

 main part of the trochlea and its inner malleolar facet in ordinary astragali, is here 

 reduced to a small triangular convexity (ib. & Plate XXV. fig. 1, i), forming the third 

 and internal division of the surface, and supported on what appears to be an obtuse 

 pyramidal process from the inner and lower part of the bone. This convexity is wedged 



• ' Os cuneiforme internum' of S<EMMEBBi2fG. 



