94 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



the same time the peculiarities of the astragalus of the Sloth, Megatherium and 



Armadillo.* 



The upper articular surface of the astragalus of the Scelidotherium (PI. 



XXVI. fig. 4.), presents, in its transverse contour, two convex pulleys, a and b, 

 and an intermediate concavity, forming one continuous articular surface. The 



external or fibular trochlea (a) is strictly speaking convex only at its posterior 



part, the upper surface gradually narrowing to a ridge, as it advances forwards 



from which, the inner and outer parts slope away at an angle of 35°. 



The tibial f convexity (b) is more regular and less elevated, it has only half the 

 antero-posterior extent of the outer pulley ; its marginal contour forms an obtuse 

 angle at the inner side. 



In the Megatherium the upper articular surface of the astragalus is also 

 divided into two trochlea, of which the one on the fibular side (fig. 3, a), is of much 

 greater relative size and extent than the tibial one (&), and is raised nearly four 

 inches above the level of the latter, although in the oblique position in which the 

 bone is .naturally placed in the skeleton, the highest part of each convexity is on 

 the same level. The fibular trochlea differs also from that in the Scelidothere in 

 being regularly convex in the transverse as well as the antero-posterior direction. 

 The tibial convexity resembles that in the Scelidothere, save in its smaller 

 relative size ; its internal margin likewise forms an angular projection below the 



■ 



internal malleolus. 



The upper surface of the astragalus of the Mylodon, or Megalonyx (?) (PL 

 XXVIII. fig. 5.), % differs from that in the Megatherium in having a narrower 



fibular trochlear ridge. 



The astragalus of the Ai (Bradypus tridactylus) differs widely from that of 

 either the Megathere, Mylodon (?) or Scelidothere in having a conical cavity 

 on the upper surface, in place of the fibular convexity, in which concavity the 

 distal end of the fibula rotates like a pivot. This mechanism is closely related to 









the scansorial uses of the inwardly inflected foot of the Sloth. 



If the astragalus of an Armadillo^ were placed side by side with that of the 



* Dasypus Q-cinctus, L., is the species of which I have the astragalus separate, so as to be able to follow out 



- 



the comparison. 



t In distinguishing these trochlea as fibular and tibial, it is to be understood that the terms relate only to 

 aspects corresponding to the position of those bones, and not that the fibula is articulated to the whole of the 

 trochlea so called : it probably rested only upon the outer facet in the Scelidothere. 



% This astragalus was found at Santa Fe, in Entre Rios, associated with the remains of the Mastodon and Tox- 

 odon ; but from its size and form I entertain little doubt that it belonged to a Megatherioid quadruped as large as 

 the Mylodon or Megalonyx. The brief allusion to the astragalus of the Megalonyx in M. Lund's Memoir does 

 not afford the means of determining with certainty this point. 



§ See the figures of this bone, given by Cuvier in PI. x. and xi. Ossemens Fossiles, vol. v. part i. 



