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Show 96 ZOOLOGY OF TilE VOYAGE OF Till!; BEAGLE. h ll fi d h that the Scelidothere and Armadillo closely agt:ee : the we s a n , owever, · d · th S 1 d th ·e outer facet of the fibular trochlere, above described, is contmue m e ~e ' o er (Pl XXVIII. fiO'. 2.) upon the fibular side of the astragalus .reachm~ nearly · 0 half-wa down the pos' ten. or part, anc1 d own nem· 1 Y the whol.e of tts antenor. ln rhe Armadillo, it extends over the whole of the antenor p~rt of. the outer side of the astragalus. In both animals the lower boundary of tlus articular sur-face describes a strong sigmoid curve. . I-n tl1 e M egath en·u m (Pl . XXVIII · fig · 1) • the c.o l'l'espondmg surface .f or. the fibular malleolus on the outer side of the astragalus JS formed b! a_ compamtlvely very sma1 1 semiC· ll·' CU 1a i. flattened c1.<1 cet , which by its roughness mdtcates that the end of the fibula was attached to it by ligamentous substance, and t~1 at the syno-vial bag was not continued upon that surface as in the Scehdothere anti Armadillo. . In the Mylodon (1) (Pl. XXVIII. fig. 4), even this rough facet is wantmg and the fibular trochlea is bounded by the angle which divides the upper from the outer surface of the astragalus. Turning now our attention to the under surface of the astr~galus, we observe that it presents in the Scelidothere (Pl. XXVI. fig. 6), an ureg~lar qt~adrate form, having the outer side occupied by an elongated sub-ovate arttcula.r facet,. e, for the calcaneum, bounded externally by a sharp edge, with its long ax1s and tts greatest concavity in the antero-posterior direction, and slightly convex _from side to side : a second calcaneal articular surface (f) is situated at the mner and anterior angle; it is oblong and nearly fiat; is continuous with the inferior concave facet of the scaphoid articulation, but is divided from the convex facet by a groove: the two calcaneal articulations are separated by a deep and rough depression, traversing the under surface of the astragalus diagonally, and increasing in breadth towards the posterior and internal angle. The inner side of the astragalus presents a convex protuberance. The correspondence between the astragalus of the Scelidothere and Mega there is best seen at the under surface of the bone: in both the two calcaneal articulations are separated by the diagonal depression, and the internal and anterior surface is continuous with the scaphoid articulation. In the Megathere, however, in consequence of the absence of the inferior concavity which characterizes the Scelidothere, the anterior calcaneal facet (f) appears as a more direct backward continuation of the scaphoidal surface; but they are divided by a more marked angle than is represented in the figure (fig. 5, Pl. XXVI.). The posterior and outer calcaneal surface in the Megathere (e) is broader in proportion to its length, continued further tlpwards upon the outward surface, is consequently more convex in the transverse direction, and is not bounded externally by so sharp and prominent a ridge as in the FOS IL MAMJ\'TAUA. 97 Scelidothere. The p~·otuberance fl'om the inner surface of the astragalus is mo1·e compressed laterally m the Megathcre than in the Scelidothere. The correspon~ enee between t~e astragali of the Mylodon (?)(Pl. XXVIII. fig. o) and Megathere 111 the conformatiOn of the undet· surface is so close, that the few differences which exist will be sufficiently appreciated by an inspection of the figures. . In t~w Armadillo the astragalus, in consequence of the greater production of 1t ant~r1or p.art, presents more of an angular than a quadrate figure; and the scaphoid articular surface, being proportionally carried forwards, is altogether separated from the anterior calcaneal surface. The posterior and inner calcaneal surface resembles that in the Scelidothere, but is less inclined upwards; and is continuous with the posterior part of the tibial articular surface. Thus the astragalus in the sti'Ucture of its two most important articuh1tions viz. that whiclt receives the superincumbent weight from the leg, and that which transmits it to the heel, pre ents a closer correspondence in the Scelidothere with that of the Dasypus, than with that of the Megathere or Mylodon. The uno·ueal phalanx of the Scelidothcre before alluded to, is represented of the natnral size in Pl. XXVIT. The side-view, fig. 3. shows tbe position of the articnlar surface on the proximal end, sloping obliquely towarus the under urface, and overtopped by an obtuse protuberance, calculated to impede any upward retraction of the claw: the present joint, in fact, illustrates in every particular the argument by which Cuvier established the true affinities of the allicu extinct genus Mcgalonyx.* The present phalanx is, however, less compressed, and less incurved than those of the Megalonyx, which have beeu hitherto described; but it more resembles in these proportions one of the smaller, :md presumed hinder, ungueal phalanges of the Megatherium. The upper and lateral parts of the bone arc rounded, and it gradually tapers to the apex, which is broken off. The o seou · sheath for the claw is developed only at the under part of the bone: it presents the form of a tltick flat plate of bone, with the margin very rcgulnrly and obli()uely bevelled off, and having a vertical process of bone attached lengthwise to the mid1llc of its under surface . This process must have served fot· the insertion of a very powerful flexor tendon. The figures of this bone preclude the necessity of any further verbal description. M. Lund lays most stress upon the argument founded on the inward inflection of the sole of the foot in the Megalonyx, and appeals with greatest confidence to this structure in support of his hypothesis of the scansorial habits of that extinct Edental.t * Ossemens Fossiles, vol. v. part i. p. 163. t For the translation of the following passage, and of others alluded to in tho presrnt work, from the original Danish Memoir of M. Lund, loc. cit., I am much indebted to tho Rev. W. Bilton, M.A. &c. &c.:- 0 |