75 



against a malleolar process of the tibia. There is an analogy to the megatherian 

 structure in the pivoted part of the articulation, but the process and the cavity are on 

 reverse parts of the ankle-joint. 



The convex protuberance or pivot on the inner half of the tibial surface of the astra- 

 galus is common to all the extinct Megatherioids hitherto discovered, but it is asso- 

 ciated with well-marked modifications of the bone in each genus, and with minor 

 differences in different species. In the Mylodon * the calcaneal surface is single, and is 

 continuous with the navicular one; no part is insulated by a bisecting groove, as in 

 the Megatherium : the middle division (a) of the upper articular surface is less convex ; 

 the upper half of the navicular surface is flat, instead of being concave. 



The astragalus of the Scelidotheriumf agrees with that of the Mylodon in the less 

 depth of the middle division of the upper surface, and the more open angle at which it 

 joins the inner convexity ; it agrees with that of the Megatherium in the division of 

 the calcaneal surface ; but it differs from both in the presence of two deep concavities 

 upon the naviculo-cuboid surface, the portion to which the cuboides articulates being 

 concave instead of convex. 



As the modification of the calcaneal surface of the astragalus governs that of the 

 co-adapted surface in the calcaneum, this bone, in the Mylodon, is distinguished by the 

 uninterrupted continuity of the articulation presented to the astragalus, with which 

 that for the cuboides is continuous ; the posterior part of the astragalar surface is less 

 convex than in the Megatherium : the posterior prolongation of the calcaneum is rela- 

 tively shorter and is less pointed : the posterior wall of the great outer tendinal groove 

 is more developed in the Mylodon. In the Scelidotherium the posterior termination of 

 the calcaneum is broader, and terminated by a less angular convexity than in the Mylo- 

 don ; 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 

 Bruta, most resemble the Megatherioids in the posterior prolongation of the calca- 

 neum, and more especially the Megalonyx in the compression and distal expansion of 

 that lever ; but, in the Megalonyx, this expansion in the vertical direction is extreme, 

 and gives the heel-bone more the form of an os ilium than of a tarsal bone. 



In the os cuboides of the Mylodon the two surfaces for the fourth and fifth metatarsals 

 are nearly on the same plane; in the Megatherium they are nearly at a right angle. 

 In the Scelidotherium the surface by which the cuboides joins the astragalus is convex, 

 instead of being concave, as it is in the Megatherium and Mylodon. In the latter genus 

 the surface on the os naviculare for the mesocuneiform bone is relatively broader than 

 in the Megatherium ; and the upper division of the astragalar surface is flat, instead of 

 being convex. 



With respect to the digits, it is significant of the true affinities of the Megatherium 



• " Description of the Skeleton of the Mylodon robuttus," 4to, 1842, p. 117. pis. 21, 22, & 23. 

 figs. 1 & 2. 

 t "Fossil Mammalia," Voyage of the Beagle, 4to, pi. 26. 



