FOSSIL MAMMALIA. 95 



Megathere, it would be very difficult to determine the analogous parts, espe- 

 cially of the upper surface, unless guided by the intermediate structure presented 

 by the Scelidothere. The upper surface of this bone, in the Armadillo, is. 

 however, divided into two transversely convex trochlea:, separated by a much wider 

 transversely concave surface. The fibular trochlea resembles that of the Scelido- 

 there in having its upper and outer facets sloping away at an acute angle, but 

 without meeting at a ridge anteriorly ; this surface is not more raised above the 

 tibial trochlea than in the Scelidothere. 



The inner trochlea differs from that of the Scelidothere in having a greater 

 relative antero-posterior extent, and in forming, in place of an uniform convex sur- 

 face, a trochlea similar in structure to that on the outer side. The extent of rough 

 surface on the upper part of the astragalus intervening between the articular sur- 

 face for the bones of the leg, and that for the scaphoides is extremely small in the 

 Megathere and Mylodon(?) ; it is relatively greater in the Scelidothere ; it is still 

 more extensive in the Armadillo ; but is the longest in the Sloth. The anterior 

 extremity of the astragalus which is entirely occupied by the scaphoid articular 

 surface is very peculiar in the Scelidothere (PL XXVI. fig. 2.) : it presents one 

 convex and two concave facets, which, however, form part of one continuous arti- 

 cular surface : the convex facet forms the internal part of the surface, and pre- 

 sents a rhomboidal form with the long axis vertical. The concave facets (c and d) 

 are extended transversely and placed one above the other ; they are slightly con- 

 cave in the transverse, and nearly fiat in the vertical directions. 



In the Megatherium (fig. 1.) the scaphoid surface of the astragalus is divided 

 only into one concave and one convex portion, both continuous with each other : the 

 concave facet (c) corresponds with the upper concavity in the Scelidothere, but is 

 a pretty uniform subcircular depression, fourteen lines in depth : the convex facet, 

 d, is continued across the whole breadth of the under part of the scaphoid surface 

 and corresponds with both the inner convex, and lower concave surfaces of the 

 scaphoid articulation in the Scelidothere. 



In the Mylodon (?) (PL XXVIII. fig. 3.), the articular facet, corresponding 

 with that marked (e) in the astragali of the Megathere and Scelidothere, is simply 

 flattened, instead of being concave ; the rest of the scaphoid surface corresponds 

 with that in the Megatherium. 



In the Armadillo the scaphoid articular surface is undivided and wholly con- 

 vex: in this part of the astragalus, therefore, we find the Scelidothere deviating 

 from the Armadillo further than does the Megathere ; while the Mylodon or 

 Megalonyx (?) most resembles the Armadillo in the configuration of this part of 

 the astragalus. 



If we compare the outer surfaces of the astragalus in these quadrupeds, 



