40 . MEGALONYX. 



measures four and a half inches transversely by two in depth at the middle. The 

 outer surface (Fig. 5) of the bone is slightly convex and roughened with longitu- 

 dinal ridges. 



Inches. 



Length of the patella . . . 5 



Breadth of its base * 



Tarsus. — Of the tarsal bones Dr. Owen's collection contains the astragalus and 

 calcaneum of both sides, and the left scaphoid, and the right cuboid and external 

 cuneiform bones. 



Astragalus. — (PI. XII, Figs. 7, 8, 9, 10.) The astragalus of Megalomjx bears 

 much more resemblance to that of the recent sloths than of any other of the 

 extinct ones. It may be described as consisting of two portions, one of which is 

 quadrate and articulates with the bones of the leg, while the other is an ovoidal 

 apophysis projecting antero-internally as in the Ai, and Unau, but relatively to a 

 much less degree. 



The quadrate portion of the astragalus its whole extent superiorly supports the 

 tibial articular surface (Fig. 7), which antero-posteriorly is convex and transversely 

 nearly a plane ; and it measures four inches and three-quarters in the curve and 

 three inches and a quarter in breadth. The anterior border presents a deep angu- 

 lar emargination verging upon a concave fossa, which accommodates a corresponding 

 process of the tibia. The posterior border is narrow, and forms also the limit of 

 the inferior surface of the bone. The outer side is a demi-crescentic, vertical, 

 slightly convex, articular surface (Fig. 10, a) for the fibula. This surface is con- 

 tinuous at right angles with that for the tibia ; and it measures three and a half 

 inches antero-posteriorly and two inches at its deepest part, which is anteriorly. 

 The inner side of the astragalus presents an uneven surface for ligamentous attach- 

 ment. The under side of the quadrate portion supports the larger calcanean 

 articular facet (Fig. 8), which is trilateral, slightly concave transversely, convex 

 antero-posteriorly and internally, and nearly a plane in the same direction externally. 



The ovoidal apophysis forms anteriorly a deep, concave, articular surface (Fig. 9), 

 which extends upon the wide convex border beneath, for conjunction with the 

 scaphoid bone. The outer and broader continuation of the convex border of the 

 apophysis articulates with the cuboid bone (Fig. 10, 6) ; and the under side of the 

 apophysis presents a quadrilateral surface (Fig. 8), which is nearly a plane, for 

 articulation with the calcaneum. 



The two calcanean articular surfaces are separated by a deep interarticular 

 gutter (Fig. 8), expanding on the inner and outer sides of the astragalus. The 

 posterior surface is about three times the area of the other, and it measures about 

 three inches transversely by two antero-posteriorly where broadest. 



Inches. 



Greatest antero-posterior diameter of astragalus ^\ 



Greatest transverse diameter . . . . . . . . - ■ ^% 



Greatest depth externally . . . . . . ■ ■ ■ ■ ■ '^t 



