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Show 52 ZOOLOGY OF TilE VOYAGE OF TilE BEAGLE. h t f t111·s boile . its anterior articular surface, for example, is simple, c arac crs o ' . · · · h th and not divided into a double trochlea by a vertical ndge: lastly, It IS wit e a~- tragalus of the Tapir and Palreothere that it presents the cl~sest correspondenc.e 1 ~ the general form and the minor details of structnrc, an~ With the~e ~ac~~~~Ims, therefore, I shall chiefly limit the comparison of the Macr~uchema, m lC aid to tlJe bone in questiou. lf the npper or tibial articular surface (fig. 5, Pl. XIV.) be compared with that in the Palceotfterium, magnum (Ossem. Foss. Pl. LIV .. fig. 2,) it will be seen, that the general direction of that surface .is m.or~ parallel With. the axis of the bone in Macraucheuia. In the PalfBothenum It IS ~urn~d a httle towards the outer or fibular side, and in the Tapir the general du~ctl~n of .the same surface is placed still more obliquely. The anterior border of. tins arttct~lating surface is broken by a semicircular notch in the Palmothere; m the ~ap~I: it describes a gentle concave curve, and the Macrauchene r~sembles .the 1 apn in this respect. The chief difference between the astruga~us of the Ta~n· and the Palreothcre, when viewed from above, obtains in the relative length of th~ bon~, anterior to the tibial articulating surface: the Nlacrauchene presents, m tlus. respect, an intermediate structure, but differ::; from both in the greater extent of the tibial side of this part of the astragalus. If we next direct attention to the anterior or scaphoid articular surface, (fig. 3, Pl. XIV.) and compare it with that of the Palceolfterium magnwrn, (fig. 4, Pl. liv, Ossem. Foss.) it will be seen, that it presents in the Macrauchenia an oval, and in the Palreotherium an irregular quadrangular form: in the Macranchenia, this surface is uniform or undivided, and is gently convex, except at its lower part; while in the Palreothere it is divided by an oblique ridge into a bwad internal facet for the scaphoid bone, and a nanow internal surface for articnlation with the os cuboides; the larger surface is also concave transversely, and slightly convex vertically: in the Tapir, the anterior surface of the astragalus deviate still further from that of the Macrauchenia, both in general form, and in the proportion of the cuboidal facet. Jn the didactyle Anoplotherium, Camel, and true Ruminants, where the cuboides presents a large relative size, a still greater proportion of the anterior surface of the astragalus is devoted to the articulation with this bone, and is separated from the scaphoid surface by a well-developed vertical ridge. The Macrauchenia presents, therefore, the extreme variation from this type ;-and should the entire tarsus hereafter be discovered, it will doubtless be found, that the os cuboides is articulated posteriorly to the os calci exclusively. The extemal surface of the astragalus of the Macrauchene, (fig. 1. Pl. XIV,) is longer in proportion to its vertical extent than in the Tapir or Palreotlwre: the articular surface for the fibular malleolus is less curved. Between this surface and the anterior facet the bone is excavated by a deep notch, both in the Tapir and Palreothere; but in the Macrauchenia by a gentle concavity. Beneath the FOSSIL MAMMALIA. 53 malleolar articular smooth surface in the Palreothere there is a deep pit; in the Tapir a shallow one; but in the Macrauchenia we observe only a smooth and slightly convex triangular surface. If we compare the inner surface of the astragalus in these three animals, we shall find the existing Tapir again forming a transition between the two extinct genera. In the Palreothere, a round protuberance projects from the anterior part of this surface: in the Tapir, we observe a gentle rising of the bone in the same part, while in the Macrauchene (fig. 2) the surface of the bone is level at this part. The margin of the tibial malleolar articular surface, which is very slightly raised in the Macrauchene, is more developed in the Tapir, and still more so in the Paheothere, where it forms a ridge, overhanging the rough outer side of the bone. Near the lower part of this surface we observe a small but deep depression in the Palreothere; there is a shallower one in the corresponding part in the Tapir; and the depression is still wider and shallower in the Macrauchenia. In the Palreothere the astragalus articulates by three surfaces with the os calcis, posteriol'ly by a large concave snrface, externally by a lon()"itudinal sub-elliptic surface, and anteriorly by a thin transverse facet: in the Macrauchene (fig.4) two only of these surfaces are present, viz. the concave and the longitudinal one, the anterior transverse surface being wanting: in the Tapir, the transverse surface is present, but is confluent with the ]ongitudinal one. The posterior surface is relatively larger and deeper in the Macrauchene than in the Palmothere, and approaches nearer to the triangular than the oval form: the longitudinal surface is placed more obliquely, and is truncated anteriorly. In the Tapir this surface is confluent with the scaphoid articular surface, but it is separated therefrom by a narrow strip of bone in both the Palreotherc and Macrauchene. It is satisfactory to find in the bone, which marks most strongly the affinity of Macraucltenia to Palceothe- 1·ium, so many easily recognizable differences, because the structure of the cervical vertebrm in the latter genus is too imperfectly known, to allow us to predicate confidently a distinction between it and JJfacraucltenia in that particnlar; the difference, however, which they present in the condition of the bones of th fore-arm ami leg, forbids their being considered as generically related. There remains to be noticed only a single fractured metatarsal bone (fiO'. I. Pl. XV.) This, from its bent and unsymmetrical figure, is evidently not a middle one, and having the side of the proximal end, which was articulated to the adjoining metatarsal in a nearly perfect state, it enables us to refer it with certainty to the hindfoot, since it does not agree with any of the corresponding surfaces at the proximal extremities of the metacarpal bones. It remaius then to be determined, whether it is an external metatarsal of the right-foot, or an internal one of the left-foot, the general curvature of these being in the same direction. With neither of these bone .jn th Tapir docs our metatarsal agree, since it has but one articular facet on the latera] |