Mr. Owen on the Glyptodon clavipes. 91 



the gradation from the Armadillo to the Sloth. In the astragalus of the Glyptodon 

 a rough surface encroaches in the form of a concave notch upon the anterior 

 margin of the tibial articular surface, as in the Dasypus gigas : the astragalus, 

 anterior to this part, is extended forwards in the form of a rough convex protube- 

 rance, which overhangs the scaphoid articular surface (PI. XI. fig. 4, a) ; the aspect 

 of this articular surface is obliquely downwards and forwards, so that the whole of 

 it is visible when we look upon the inferior surface of the astragalus : in the antero- 

 posterior direction the scaphoid articular surface is nearly flat, very slightly con- 

 cave anteriorly, and as slightly convex posteriorly ; in the transverse direction it 

 is regularly convex. Such a structure and disposition of the scaphoid articulation 

 does not exist in any Armadillo ; and the deviation on the part of Glyptodon from 

 Dasypus in this respect is greater than any of the specific differences observable in 

 the several Dasypi. The peculiar transverse extension of the articular surface for 

 the scaphoid is well shown when the astragalus of the Glyptodon is viewed from 

 the inner or tibial side, as in fig. 5, where this surface, surmounted by the tubero- 

 sity above described, is directed towards the right hand. The tibial articular 

 surface encroaches only a slight way upon this side of the astragalus. The outer 

 or fibular surface of the astragalus presents, in Glyptodon, a remarkably regular 

 triangular figure, of which the upper and longest side is slightly convex, while the 

 two lower sides are straight and equal ; the fibular articular surface encroaches about 

 half-way down this surface, but further posteriorly than anteriorly : the reverse of 

 this is seen in the Armadillos, the astragalus of which also never presents this tri- 

 angular form. The astragalus, indeed, of the Scelidothere, which in other respects 

 more nearly resembles that of the Megathere, resembles that of the Armadillos in 

 the form and condition of its outer facet more than does the astragalus of the 

 Glyptodon. On the inferior surface of the astragalus of Glyptodon we perceive 

 that the internal facet for the articulation with the calcaneum (PI. XI. fig. 4, b) is 

 separated by a rough groove, as in the Armadillos, from the scaphoid articular 

 surface (a). In the Armadillos, however, the inner calcaneal surface is flat, or 

 slightly convex ; in the Glyptodon it is remarkably concave in the antero-posterior 

 direction. The outer calcaneal facet {ib. fig. 4, c) is almost perfectly flat in Gly- 

 ptodon ; it is concave in all the Armadillos : both these differences may be regarded 

 as something more than merely specific. 



Os calcis*. — In the calcaneum we of course perceive a corresponding difference 

 between Glyptodon and Dasypus ; the inner articular surface being flat in the for- 

 mer and convex in the latter, while the outer articular surface presents the reverse 

 conditions. The inner surface of the posterior projecting part of the calcaneum 

 is more concave in Glyptodon than in Dasypus ; the under surface is broader and 



* PI. XIII. figs. 4 and 5. 

 N 2 



