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condj'le. Tlie distal articulation presents as marked a distinguishing character 

 as the proximal one ; the rotular surface is formed by the anterior continuation 

 and expansion of the articulation upon the outer condyle, and is divided by a 

 rough isthmus two inches broad from the inner condyle : in the Mylodon the 

 rotular articular surface is continuous with those of both the condyles. 



From the cast of the distal epiphysis of a femur in the Museum of the College, 

 which is as much compressed in the antero-posterior direction as in the Me- 

 gatherium, the genus Megalonyx would appear to offer a third modification of 

 the knee-joint, the rotular surface being distinct from those of both condyles. 

 Thus the knee-joint of the Mylodon must have consisted of one large and 

 complex synovial sac, that of the Megatherium of two, that of the Megalonyx 

 of three distinct sacs. The distal end of a femur of a Megatherioid animal, trans- 

 mitted with the bones of the Mylodon rohustus, was at once distinguished and set 

 apart by presenting the same modification of the distal articular surface as in the 

 North American Megalonyx, but in its transverse and antero-posterior diameters 

 it approaches nearer to the Mylodontal proportions ; it consequently differs from 

 the above-mentioned Megalonyx in the narrower interspace of the condyles ; 

 and the inner condyloid surface has a peculiar excavation at its anterior termi- 

 nation. The fragment of the femur here compared, which may indicate a second 

 species of true Megalonyx, differs from the femur of the Mylodon robustus in the 

 less angular production of the outer margin of the femur above the condyle, and 

 in the deeper excavation at the back of the femur near that margin, but more 

 especially by the existence of a well-defined but small medullary cavity, with a 

 strongly reticulated surface. 



The remains of the Scelidotherium in the Museum of the College include a 

 perfect os femoris*. This is intermediate in its proportions between those of 

 the Megatherium and Mylodon, but as compared with the other parts of the 

 skeleton, its breadth is more remarkable than in either of those genera: 

 compared with its antero-posterior diameter, the transverse diameter exceeds 

 that in the Megatherium, but, compared with the length of the femur, it does 

 not surpass the transverse diameter of that bone in the Megatherium. The 

 head of the femur of the Scelidotherium is as deeply and extensively 

 impressed by the ligamentum rotundum as in the Mylodon ; it is not so 

 much produced inwards as in the Mylodon or Megatherium : the small tro- 

 * ^'03•age of the Beagle, ' Fossil Mammalia,' 4to, pi. 25, fig. 5. 



