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Show 94 ZOOLOGY OF TilE VOYAGE OF TilE BEAGLE. the same time the peculiarities of the astragalus of the Sloth, Megatherium and Armadillo.* s r 1 tl · (Pl The upper articular surface of the astragalus of the ce tc o Ienum · XXVI. fig. 4.), presents, in its trans:erse contom:, two co~~ex P~1lleys, a and b, and an intermediate concavity, fonmng one contmuous attlculai s~rface. ~he external or fibular trochlea (a) is strictly speaking convex only at tts postenor part, the upper surface gradual l y nar.r .o w·m g t o a 1:1d ge • as it advao nces forwards • from which, the inner and outer parts slope away at an angle o~ 35 · The tibial"! convexity (b) is more regular and less elevated, 1t has only half the antero-posterior extent of the outer pulley; its marginal contour forms an obtuse angle at the inner side. . In the Megatherium the upper articular surface of the astragalus 1s also divided into two trochlere, of which the one on the fibular side (fig. 3, a), is of much greater relative size and extent than the tibial one (b!, and is .r~ise~ near~y four inches above the level of the latter, although in the obhque pos1t10n m whiCh the bone is naturally placed in the skeleton, the highest part of each convexity is on the same level The fibular trochlea differs also from that in the Scelidothere in being regularly convex in the transverse as well as the antero-posterior direction. The tibial convexity resembles that in the Scelidothere, save in its smaller relative size; its internal margin likewise forms an angular projection below the internal malleolus. The upper surface of the astragalus of the Mylodon, o1· Megalonyx (?) (Pl. XXVIII. fig. 5. ), :t differs from that in the Megatherium in having a narrower fibular trochlear ridge. The astt:agalus of the Ai (Bradypus tridactylus) differs widely from that of either the Mega there, Mylodon (?) or Scelidothere in having a conical cavity on the upper surface, in place of the fibular convexity, in which concavity the distal end of the fibula rotates like a pivot. This mechanism is closely related to the scansorial uses of the inwardly inflected foot of the Sloth. If the astragalus of an Armadillo§ were placed side by side with that of the • Dasypus 6-cinctus, L., is the species of which I have tho astragalus separate, so as to be able to follow out the corn parison. t In distinguishing these trochleoo as fibular and tibial, it is to be understood that the terms relate only to aspects corresponding to the position of those bonce, and not that the fibula is articulated to the whole of the trochlea so called : it probably rested only upon the outer facet in the Scelidothere. t This astragalus was found at Sa uta Fe, in Entre Rios, associated with tho remains of the Mastodon and Toxodon; but from its size and form I entertain little doubt that it belonged to a l'tfegatherioid quadruped as large as the Mylodon or Mcgalonyx. The brief allusion to the astragalus of the Megalonyx in M. Lund's Memoir does not afford the means of determining with certainty this point. § See the figures of this bone, given by Cuvier in Pl. x. and xi. Osscmens Fossiles, vol. v. part i. FOSSIL MAMMALIA. 95 ~egathere, it would be very difficult to determine the analogous parts, especially of the ~pper surface, unless guided by the intermediate structure presented by the Scehdothere. The upper surface of this bone, in the Armadillo, is, however, divided into two transversely convex trochlere, separated by a much wider transversely concave surface. The fibular trochlea resembles that of the Scelidoth. ere in havi.ng its up~er and o~ter facets sloping away at an acute angle, but without meetmg at a r1dge antenorly; this surface is not more raised above the tibial trochlea than in the Scelidothere. The inner trochlea differs from that of the Scelidothere in having a greater relative antero-posterior extent, and in forming, in place of an uniform convex surface, a trochlea similar in structure to that on the outer side. The extent of rough surface on the upper part of the astragalus intervening between the articular surface for the bones of the leg, and that for the scaphoides is extremely small in the Megathere and Mylodon(?); it is relatively gr·eater in the Scelidothere; it is still more extensive in the Armadillo ; but is the longest in the Sloth. The anterior extremity of the astragalus which is entirely occupied by the scaphoid articular surface is very peculiar in the Scelidothere (Pl. XXVI. fig. 2. ) : it presents one convex and two concave facets, which, however, form part of one continuous articular surface: the convex facet forms the internal part of the surface, and presents a rhomboidal form with the long axis vertical. The concave facets (c and d) are e~tended transversely and placed one above the other; they are slightly concave m the ,transverse, and nearly flat in the vertical directions. In the Megatherium (fig. 1.) the scaphoid surface of the astragalus is divided only into one concave and one convex portion, both continuous with each other: the concave facet (c) corresponds with the upper concavity in the Scelid~there, but is a pretty uniform subcircular depression, fourteen lines in depth: the convex facet, d, is continued across the whole breadth of the under part of the scaphoid surface and corresponds with both the inner convex, and lower concave surfaces of the scaphoid articulation in the Scclidothere. . In the Mylodon (?) (Pl. XXVIII. fig. 3.), the articular facet, corresponding wtth that marked (c) in the astragali of the Megathere and Scelidothere, is simply flattened, instead of being concave ; the rest of the scaphoid surface corresponds with that in the Megatherium. In the Armadillo the scaphoid articular surface is undivided and wholly convex: in this part of the astragalus, therefore, we find the Scelidothere deviating from the Armadillo further than does the Megathere; while the Mylodon or Megalonyx (?) most resembles the Armadillo in the configuration of this part of the astragalus. If we compare the outer surfaces of the astragalus in these CJ,Uadrupeds, |