376 R. S. Lull — Pleistocene Ground Sloth, 



a great part of the articular surface for the tibia looks inwards ; 

 when articulated, therefore, to the leg, placed vertically above 

 it, the foot rests upon the ground by its outer edge, not by its 

 sole, and the peculiarities of the metatarsal structure relate to 

 this inversion of the foot." The 3fylodon astragalus is cer- 

 tainly one of the most oddly shaped elements in this most 

 uncouth animal. The upper surface bears a tibial articulation 

 divided into two confluent synovial areas, as in the distal end 

 of the tibia, the ascending portion is ovoid with one face dis- 

 tinctly flattened in the peculiar No. 10266, much less so in the 

 others. The surface from which it arises is convex in part, 

 this convexity being most pronounced in No. 10266, least so 

 in No. 10265. The surface loses its curved aspect toward the 

 inner side of the articular area, and in No. 10264 actually 

 becomes slightly concave. The ascending process in No. 10266 

 is much the stoutest, less so in 10265, and least in 10264, 

 though its height in all three remains approximately the same. 

 The horizontal surface is continued down over the antero- 

 lateral aspect of the bone in a slightly convex area, irregularly 

 quadrilateral, with slightly curved sides, three convex and one 

 concave. From this a narrow area passes backward, forming 

 the lateral margin of the tibial facet. This area articulates 

 with the malleolar process of the flbula. 



Below and anteriorly there is a long continuous area of 

 articulation with the calcaneum, cuboid, and navicular. That 

 which meets the calcaneum is the most posterior and is roughly 

 triangular, narrowest and most convex at the hindermost 

 extremity, growing gradually flatter and wider anteriorly. 

 The antero-posterior axis, on the other hand, is first gently 

 convex, then concave in a long, sweeping curve, giving to the 

 entire surface a saddle-like aspect. Anteriorly the margin is 

 cleft by an entrant angle which is the posterior limitation of 

 the depression bounded by the calcaneal, navicular, cuboidal, 

 tibial and malleolar facets — a depression, like certain others 

 on the bone, irregular and deeply pitted, the pits being the 

 orifices of the vascular canals. The remaining part of the 

 anterior limit of the calcaneal facet adjoins the cuboidal, the 

 line of juncture forming a full curve, almost the segment of a 

 circle, and passing through an arc of about 130°. The exten- 

 sion of this curve toward the inner side of the astragalus gives 

 an area to the calcaneal facet which is apparently much wider 

 than the complementary facet on the calcaneum itself, though 

 of this I have to judge from a very imperfect bone. If this 

 be true, it implies a considerable range of movement between 

 astragalus and calcaneum. 



The cuboidal facet of the astragalus is a cylindrical though 

 somewhat warped curve and is continuous with that of the 



