1903.] Brown, A Neiv Genus of Ground Sloth. 58 I 



flattened, slightly convex and rugose, the posterior border thin 

 and rounded. The upper end of the inner surface is excavated 

 with a rugose border. The proximal end is truncated ob- 

 liquely, presenting two facets: the outer and smaller for the 

 articulation of a sesamoid; the inner (a very long surface 

 convex in its narrowest dimension) for articulation with the 

 tibia. About one half of the proximal end of the fibula on 

 the outer side presents a rugose border and excavated surface 

 for the attachment of the peroneus longus. The tibial facet 

 is prolonged backward in a hook-like process. The distal end 

 is expanded in an irregular quadrilateral form on the inner 

 surface and a convex, very rugose, pitted obtuse point. The 

 inner surface presents a deeply excavated concavity and two 

 articular facets; the lower part of the concavity is occupied 

 by the fiat, oblique, tibial facet, which continues uninter- 

 ruptedly into the vertical plane of the astragalar facet. The 

 posterior distal third of the malleolus is rugose. In M. 

 rohustus the tibial and astragalar facets are separated by a 

 transverse concavity . 



Astragalus. 

 The astragalus, aside from being much larger than that of 

 M. rohustus, agrees with it in form and proportion with the 

 following exceptions : The navicular facet is deeply excavated 

 in the middle, with convex edges continuing uninterruptedly 

 into the cuboidal facet. In M. rohustus it is flat on its upper 

 half and convex on the lower. The calcaneal facet is elongate 

 and triangular to a greater extent than in M. rohustus, with 

 scarcely a perceptible constriction in the middle. 



Calcaneum. 

 The base of the calcaneum forms a nearly straight plane, 

 concave on its posterior triangular surface, and of greater 

 length proportionally than in M. rohustus, with a broadly ex- 

 panded posterior end. The astragalar facet occupies a more 

 anterior position than in M. rohustus, while the large tendi- 

 nous grooves on the outer side of the bone are similar, but 

 the tuberosities marking their boundaries are more prominent. 

 In other respects it agrees with M. rohustus. 



