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and well-proportioned to the weight they had to sustain, they appear weak and 

 slender when placed by the side of the femur of the Megatherium or Mylodon. 

 The Rhinoceros, which has the thigh-bone relatively broader and flatter than in 

 the Proboscidian or other Pachyderms, differs more markedly from the Mylo- 

 don and Megatherium in the presence of the third trochanter. 



The flattened form of femur is common to all the Edentata ; but the Oryc- 

 terope and Armadillos, in which this character is conspicuous, diifer, like the 

 Rhinoceros, from the Megatherioids in having the third trochanter. This pro- 

 cess is not present in the Sloths, Pangolins and Ant-eaters ; but in all these the 

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

 sreat Ant-eater offering the nearest resemblance as to form amongst the Eden- 

 tata ; while the Sloths alone, amongst the existing terrestrial MammaUa, repeat 

 the remarkable Mylodontal character of the absence of a medullary cavity in the 

 shaft of the femur. 



The general characters of the femur of the Mylodon are present in all the 

 other species of Megatherioids in which this bone has been discovered. In the 

 great Megatherium they are even exaggerated, and instead of there being any 

 closer approximation to the form of the femur in the Mastodon or Elephant, the 

 shaft of the bone is relatively shorter and broader than in the Mylodon. The 

 only character in which the Megatherium offers a resemblance to the great 

 Proboscidian Pachyderms, not found in the smaller Megatherioids, is the absence 

 of the depression in the head of the bone for the ligamentum teres, but in this 

 respect it equally agrees with the Sloths. We may, however, connect the absence 

 of the ligament in the Megatherium, unless a slight emargination at the posterior 

 circumference of the head indicate a vestige of the hgament, with the same ver- 

 tical position of the head of the bone under the nearly horizontal acetabulum, 

 which seems to govern the like deficiency in the Elephant and Mastodon. The 

 Sloths may probably, like the Orang-utan, derive some advantage from the 

 absence of the ligamentum teres by the greater freedom thus allowed to the 

 lateral movements of the hinder-limbs in the act of climbing. The concavity 

 between the head and great trochanter at the proximal end, which can scarcely 

 be termed the neck of the femur, is deeper, while the trochanterian depression 

 at the back of the bone is much shallower in the Megatherium than in the 

 Mylodon. The distal expansion of the femur is relatively greater in the Mega- 

 there, but is characterized by the same great angxilar projection above the outer 



