129 



chanter is more developed, attaining the vertical hne dropped from the inner 

 surface of the head. The distal articulation of the femur is relatively smaller 

 than in the Mylodon or Megatherium : the rotular surface is continuous with 

 both condyles as in the Mylodon. The angle above the outer condyle in the 

 femur of the Scelidotherium is not formed by a projection of the bone, but by 

 the truncation of the lower and outer angle, the outer vertical margin of the 

 femur being thus continued by an oblique line to the margin of the condyle. 

 The prominence above the inner condyle is more marked in the Scelidothere 

 than in the Mylodon. In all these modifications of the thigh-bone the Scelido- 

 therium Cuvieri* agrees with the Scelidotherium leptocephalum, but some slight 

 specific differences may be discerned in the sculpturing of the back of the femur, 

 and in the form of the outer condyle. 



In the great extent of the trochanter major, and in the strong development 

 of the ridges and inequalities which are at once the result and indication of the 

 muscular forces, the femur in all the Megatherioids surpasses that bor^e in any 

 known recent or extinct mammal. These striking characters harmonize with 

 our conceptions of the nature of the support required by the vast pelvis, and 

 bespeak the strength of the powerful muscles, which, in the peculiar exertions 

 put forth by the living animal, must have acted and reacted upon the trunk and 

 the hind-extremities. 



The femur of the Mylodon is distinguished from that of any existing mam- 

 mal by its great breadth, but the bones of the leg are more remarkable for the 

 extent to which they surpass any recent type in their peculiarly massive pro- 

 portions, being as strong but relatively shorter even than in the Megatherium. 

 Nevertheless it is in this segment of the hind-extremity of the small Sloths that 

 we have the first recognisable evidence of a resemblance to that of the Mega- 

 therioids : it is manifested by the unusual breadth of the leg, produced however 

 by the mutual divarication of the tibia and fibula by outward curvaturesf, 

 leaving an unusually wide interosseous space, which, in the heavy Mylodon, is 

 reduced by the ossification of the broad tibia to a small compass. The Mylodon 

 agrees with both Sloths and Ant-eaters in the persistent articulations between 

 the tibia and fibula: these bones coalesce at the proximal extremity in the 

 Orycterope, and at both extremities in the Armadillos. 



* Lund, toe. cit. pi. 4, fig. 1, 'Megalonyx Cuvieri.' t Cuvier, loc. cit. p. 360. 



R 



