MEGALONYX. 41 



Tlio specimen of an astragalus described and referred by Professor Owen to the 

 Megalonyx Jeffersanii^ most probably also belongs to the OnaiJiopsis Oircni, indicated 

 on page 14 of this monograph. 



Os Calcis. — (PL XII, Figs. 5, G.) The calcaneum is remarkable for its extraor- 

 dinary expansion, which gives to the bone very much the appearance of an os ilium 

 of some smaller animal. Its articular extremity is formed by an abrupt widening 

 and decrease in depth of the anterior part, where it supports three facets (Fig. G), 

 separated by a trifurcate groove. The largest facet is supero-internal and trilate- 

 rally ovoidal (a), and articulates with the astragalus. Of the other two facets, 

 which are nearly equal in size and irregularly ovoidal, that inferiorly {]>) articulates 

 with the astragalus, and that externally (c) with the cuboid bone. 



Posterior to the articular extremity, the calcaneum (Fig. 5) forms a large plate 

 nine inches in depth, and only a fourth of an inch in thickness towards the centre. 

 Its posterior margin is thick, strong, rugged, and convex, and measures fifteen 

 inches in its curvature. The extremities of this margin form convex tuberosities, 

 of which that inferiorly is much the larger. Above the inferior one, externally, is 

 a third tuberosity, which is strong and prominent. 



The outer surface of the calcaneum is concave antero-posteriorly, but in the 

 vertical direction is convex below and concave above. The inner surface is concave 

 both antero-posteriorly and vertically. 



The superior margin of the calcaneum is acute, and curves forward and out- 

 ward, to terminate above the cuboid articulation in an obtuse prominence. The 

 inferior margin is thick and convex; expands forward; and antero-posteriorly 

 forms a deep concavity. The greatest antero-posterior diameter of the calcaneum 

 is nine and three quarter inches. 



Os Guboides. — (PL XIII, Figs. 4, 5, 6.) The cuboid bone is irregularly cuboidal, 

 and it presents large, quadrilateral dorsal (Fig. 4), and plantar surfaces for liga- 

 mentous attachment. Posteriorly it has a convex facet for articulating with the 

 OS calcis, and anteriorly it is perforated and roughened for ligamentous attach- 

 ment. Internally a large concave facet (Fig. 6) exists for articulation with the 

 astragalus, continuous with a second facet upon the antero-external margin of the 

 bone, for articulating with the scaphoides. By the prolongation of this articular 

 surface upon the supero-external angle anteriorly, a third facet is formed to join 

 the cuneiforme externum. Externally the cuboides presents a large, trilateral, 

 slightly convex surface (Fig. 5), the half of which posteriorly articulates with the 

 fifth metatarsal bone, the remaining portion with the fourth. 



Inches. 



Greatest antero-posterior diameter of the cuboid bone 2^ 



Greatest transverse diameter ... ....... 2 



Greatest height .........■■• 2i 



Os Scaphoides. — (PL XIII, Figs. 7, 8.) The scaphoid bone is a thick quadri- 

 lateral plate with convex margins. Its posterior surface (Fig. 7) is occupied with 



Zool. Voy. Beagle, Pt. I, 94, 98, 99 ; Mem. ou the Mylodou, 133, VI .wiii, Figs. 5 and C. 

 6 



