130 



The most peculiar feature in the tibia of the Mylodon is the form of the 

 distal articular surface. In no existing Edentate quadruped does this part of 

 the tibia agree with the extinct type. Neither the Sloths nor any other Edentates, 

 nor indeed any known existing mammal, present the hemispherical excavation 

 on the fore and inner part of the distal joint of the tibia : only in the Mega- 

 therium and Mylodon has this peculiarity been observed, and in the extinct 

 congeners only of these gigantic leaf-devourers can such character be inferred 

 from the structure of the astragalus*. This unusually secure interlocking of 

 the foot to the leg bespeaks some singular habits of the lost species to which it 

 is peculiar, and is evidently connected with the requirement of unusual resistance 

 in the foot to the forces acting upon it from the leg and thigh. 



In the Megatherium the hemispherical excavation is nearer the inner or tibial 

 side of the bone than in the Mylodon, and the broader articular depression ex- 

 ternal to it is deeper, and is separated by a more extended and marked convex 

 ridge in the Megatherium. The leading difference, however, between the 

 Mylodon and Megatherium, in the structure of the bones of the leg, arises from 

 the tibia and fibula being anchylosed together at both extremities in the more 

 gigantic species. The shaft of the fibula of the Megatherium is relatively more 

 slender than that of the Mylodon. 



From a cast of the tibia of the Megalonyx Jeffersoni, it would seem that the 

 fibula was detached in that species as in the Mylodon ; but as the specimen 

 compared has belonged to a young animal, and has lost the distal epiphysis, it 

 may possibly acquire the character of the tibia of the Megatherium by age. The 

 cast demonstrates that the tibia of the Megalonyx, like that of the Megatherium, 

 is relatively longer than in the Mylodon, and is more slender than in either of 

 those genera. At the proximal articular surface the division for the outer 

 condyle is slightly convex in the Megalonyx, but sUghtly concave in the 

 Mylodon : in the Megatherium the same surface is more convex than in the 

 Megalonyx, and is continued by a regular curve into the surface for the popliteal 

 sesamoid, which surface, in the Mylodon, meets the condyloid surface for the 

 femur at an open angle. 



The tibia of the Mylodon Harlani is longer in proportion to its breadth than 

 in the Mylodon robustus, but is shorter than in the Megalonyx : the proximal 



• See the description of the astragalus in the Scelidotherium, Me^jaloryx and Megatherium in the 

 Fossil Mammalia of the Beagle, p. 94, plates 26 and 28. 



