

96 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



we shall find, however, that the Scelidothere and Armadillo closely agree : the 

 outer facet of the fibular trochlese, above described, is continued in the Scelidothere 

 (PL XXVIII. fig. 2.), upon the fibular side of the astragalus reaching nearly 

 half-way down the posterior part, and down nearly the whole of its anterior. 



In the Armadillo, it extends over the whole of the anterior part of the outer 

 side of the astragalus. In both animals the lower boundary of this articular sur- 

 face describes a strong sigmoid curve. 



In the Megatherium (PL XXVIII. fig. 1), the corresponding surface for the 

 fibular malleolus on the outer side of the astragalus is formed by a comparatively 

 very small semicircular flattened facet, which by its roughness indicates that the 

 end of the fibula was attached to it by ligamentous substance, and that the syno- 

 vial bag was not continued upon that surface as in the Scelidothere and 

 Armadillo. 



In the Mylodon (?) (PL XXVIII. fig. 4), even this rough facet is wanting and 

 the fibular trochlea is bounded by the angle which divides the upper from the 

 outer surface of the astragalus. 



Turning now our attention to the under surface of the astragalus, we observe 

 that it presents in the Scelidothere (PL XXVI. fig. 6), an irregular quadrate 

 form, having the outer side occupied by an elongated sub-ovate articular facet, e, 

 for the calcaneum, bounded externally by a sharp edge, with its long axis and its 

 greatest concavity in the antero-posterior direction, and slightly convex from side 

 to side : a second calcaneal articular surface (f) is situated at the inner and 

 anterior angle ; it is oblong and nearly flat ; is continuous with the inferior con- 

 cave facet of the scaphoid articulation, but is divided from the convex facet by a 

 groove : the two calcaneal articulations are separated by a deep and rough de- 

 pression, traversing the under surface of the astragalus diagonally, and increasing 

 in breadth towards the posterior and internal angle. The inner side of the 

 astragalus presents a convex protuberance. 



The correspondence between the astragalus of the Scelidothere and Megathere 

 is best seen at the under surface of the bone : in both the two calcaneal articulations 

 are separated by the diagonal depression, and the internal and anterior surface is con- 

 tinuous with the scaphoid articulation. In the Megathere, however, in consequence 

 of the absence of the inferior concavity which characterizes the Scelidothere, the 

 anterior calcaneal facet (/) appears as a more direct backward continuation of the 

 scaphoidal surface; but they are divided by a more marked angle than is repre- 

 sented in the figure (fig. 5, PL XXVI.). The posterior and outer calcaneal sur- 

 face in the Megathere (e) is broader in proportion to its length, continued further 

 upwards upon the, outward surface, is consequently more convex in the transverse 

 direction, and is not bounded externally by so sharp and prominent a ridge as in the 





