104 



502. The proximal extremity of the left tibia and the left patella, cemented 



together by the calcareous matrix. 



Of the long bones of the leg only this end of the tibia is preserved ; 

 but it is valuable as showing another well-marked difference between the 

 Scelidothere and Megathere ; for whereas in the latter the fibula is an- 

 chylosed with the tibia, this bone in the Scelidothere presents a smooth, 

 flat, oval, articular surface below the outer part of the head ; from the 

 size and appearance of which I infer that the fibula would not have be- 

 come confluent with the tibia even in the mature and full-grown animal, 

 but would have remained separate as in the Mylodon. 



The patella is a thick, strong, oval bone, with the smaller end down- 

 wards, rough and convex externally, smooth on the internal surface, 

 which is concave in the vertical and convex in the transverse directions. 



503. The right astragalus, cemented by the calcareous matrix to the head of 



the femur. 



504. The left astragalus. 



The upper articular surface of this characteristic bone presents two 

 convex pulleys with an intermediate concavity: the outer or fibular tro- 

 chlea, though higher than the inner one, is much less elevated than in 

 the Megathere, and contracts to a ridge anteriorly. The anterior part of 

 the astragalus of the Mylodon which forms the articular surface for the 

 os naviculare, presents one convex and two concave facets, the outer one 

 receiving part of the os cuboides. In the Megathere the corresponding 

 part of the astragalus presents one convex and one concave surface, the 

 latter corresponding with the upper and inner concave surface in the 

 Scelidothere. In the Mylodon the part corresponding to the latter con- 

 cavity is flat. On the under surface of the astragalus the articulation 

 for the os calcis is divided from that for the naviculare by a deep rough 

 groove, as in the Megatherium, but the two surfaces are continuous in 

 the Mylodon robustus. 



505. The ungual phalanx of the longest or middle digit of the hind-foot. 



The articular pulley of this bone slopes towards the under surface and 



