133 



governs a corresponding structure in that of the bone to which it is adapted, the 

 OS calcis of the Mjdodon might be distinguished from that of the other known 

 Megatherian genera by the uninterrupted continuity of the articulation presented 

 to the astragalus, with which that for the cuboides is continuous. In the Mega- 

 therium the small anterior concave astragalar surface is continuous with the cu- 

 boidal facet : the insulated posterior surface for the astragalus is more convex 

 than the corresponding part in the Mylodon : the posterior prolongation of the 

 heel-bone is relatively longer and more pointed in the Megatherium than in the 

 Mylodon : the posterior wall of the wide and deep outer groove of the calcaneum 

 is less developed in the Megatherium. In the Scelidotherium the posterior ter- 

 mination of the calcaneum is broader, and terminated by a less angular convexity 

 than in the Mylodon ; but in the separation of the posterior from the anterior 

 part of the astragalar surface it agrees with the Megatherium and with the Sloths. 

 These, of all recent Edentata, most resemble the Megatherioids in the posterior 

 prolongation of the calcaneum, but it is slender and compressed. The thickness 

 and strength of the great lever for the extensors of the foot are among the most 

 striking pecuUarities of the skeleton of the extinct Megatherians. 



The OS cuboides of the Megatherium, in addition to its size, differs from that 

 of the Mylodon in having the surface for the tifth metatarsal placed at right 

 angles to that for the fourth metatarsal, while they are in nearly the same plane 

 in the Mylodon. The surface of the cuboid which joins the astragalus is concave 

 in the Mylodon as in the Megatherium, and most probably also in the Mega- 

 lonyx ; but it must be convex in the Scelidotherium, since the corresponding 

 surface of the astragalus forms the second concavity on its anterior prominence 

 already mentioned. 



The anterior surface of the cuboides gives the number and size of the external 

 metatarsals, and shows that these two bones must have been of nearly the same 

 relative size in the Megatherium as in the Mylodon, but that the external or fifth 

 toe was directed more outwards, and the figure of the hind-foot of the Mega- 

 therium* agrees with these indications. As the os cuboides teaches the relative 

 size and position of the two outer metatarsals, so the os naviculare indicates the 

 number of the remaining toes, or at least of the cuneiform bones, which respect- 

 ively support them. 



• Pander and D'Alton, loc. cit. tab. 4, fig. 36. 



