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Show 50 ZOOLOGY OF TilE VOYAGE OF TilE BEAGLE. . th Camel and the process above the inner condyles. is relatively] wi~~r ~:~ ~~eseerespect~ the Macrauchene inclines towards condyle IS more angu ar, 10 0 the Palreothere. f l b S of the leg of the Macrauchenia we find the In the structure o t 1e one . · f h . . l h . t 1 . his afforded bv the definable lumts o the anc y-same transitwna c atac er w HC ~ . • d d' f l .c In the Pachyderma the fibula IS an entll'e an IS- losed bones o t 1e 1ore-arm. d tm· ct b one. 1n the Rum1·nantia' with the exception of the small M.u sk-deer, an , m. an m· 1r el'l·O I• d egree, th e Elk ' the fibula appears only as a short contm·u o· us pIr ocehs s d f the under part of the external condyle of the tibia. n t e sent own rom . · ~ l · . · Camel tribe the only trace of the fibnla in the bones of the le.g, I:s ~ ns pw~ess m a still more rudimental state. In the Macrauchenia the fibula IS ~ntue, but IS confluent with the tibia through nearly its whole extent: the proximal pa~t .of the fibula is well defined; its bead is anchylosed to the outer condyle of the tibia, b~t the shaft is continued free for the extent of nearly two inche~, and then agam becomes confluent with the tibia, forming apparently the outer ndge of that bone. About five inches from the distal end of the tibia this outer ridge becomes flattened by being, as it were, pressed against the tibia, and the ~nterior and. p~sterior edges are raised above the level of the tibia ; beyond this part the h~Its of the fibula begin again to be defined by deep vascular grooves. The outer side of the distal end of the fibula is excavated by a broad tendinous groove. The fibula and tibia are distinct bones in both the Palreothere and Anoplothere, as in the Pachyderms. It is to the former genus, however, especially Pal. magnurn, .t~at the Macrauchene presents the nearest approach in the general form of the tibia, the principal bone of its leg: but in the Macrauchene the tib.ia is relat~v~ly shorter, and thicker, and is straighter and less expanded at Its extremities, especially the upper one, than in any of the Palreotheres. . . The mesial boundaries of the two superior articulating smfaces of the tibia are raised in the form of ridges, which are separated by a deep groove ; of these ridges the external is the highest, as in Pal. magnurn: but the articular surfaces in the Macrauchene slope away from these ridges more than in the Palreotheres. The rotular or anterior tuberosity of the tibia is more produced, and rises higher than in the Palreotheres ; the ridge continued downwards from this process is more marked in the Macrauchene, and its limits are better defined : the shaft of the tibia below the ridge is also more flattened in the antero-posterior direction than in the Palreothere. The configuration of the back part of both proximal and distal extremities of the tibia are so clearly and accurately given in figures 2 and 3, Pl. XIII., as to render verbal description unnecessary. Neither the text nor the figures in the 'Ossemens Fossiles' afford the means of pursuing the comparison between the Macrauchene and Palreothere in these particulars; and I proceed, FOSSIL MAMMALIA. 51 therefore, to the consideration of the inferior articulating surface of the bones of the leg (fig. 4, Pl. XIII.) Since, of the hind foot, we possess in the present collection only a single tarsal and metatarsal bone, the structure of the distal articular surface of the tibia is attended with peculiar interest, because we are taught by Cuvier that it reveals to us in the Ungulate animals the didactyle or tridactyle structure of the foot. In the Ruminants this articular surface is nearly square, and extended transversely between two perpendicular malleoli, while in the Pachyderms with three toes to the hind-foot the articular surface of the tibia is oblique, and is divided into two facets between the perpendicular malleolar boundaries. Now in the Macrauchenia, although the two bones of the leg are anchylosed together, the extent of that part of the tarsal articular surface which is due to the tibia is indicated, as in the case of the radius in the joint of the fore-arm, by a groove ; and we are thus able to compare this surface with the distal articular surface of the tibia in the Palreothere and Anoplothere. It presents in the Macrauchenia a very close resemblance with that of the Palceotlteriwn rnagnum,* being divided into two facets by a convex rising, which traverses the joint from behind forwards; but the ridge is narrower, the internal facet somewhat deeper, and the external oblique surface rather flatter than in the three-toed Palreothere. In the portion of the tarsal articular surface due to the fibula, we find, however, a more marked deviation from the Palreothere, and an interesting correspondence with the Anoplothere, in the inferior truncation and horizontal articular surface which is continued upon the lower extremity of the fibnla, at right angles with the vertical malleolar facet which forms the outer boundary of the trochlea of the astragalus: this articular surface uneningly indicates a corresponding articular projection in the calcaneum, which, therefore, although the bone itself does not form part of the present collection, we may conclude to differ from the calcaneum of the Palreotherium, and to resemble that of the Anoplotherium, in this particular at least. The valuable indication which the distal articular surfaces of the anchylosed tibia and fibula have given of the correspondence of the hind-foot with the forefoot of the Macrauchenia, in regard to the number of the toes, receives ample confirmation from the astragalus, which, of all the bones in the foot, is the one that an anatomist would have chosen, had his choice been so limited, and which most fortunately has been secured by Mr. Darwin, in a very perfect state, in the present instance. I have compared this astragalus with that of the Giraffe, and other Ruminants, the Camel, the Anoplothere, the Horse, the Hog, the Hippopotamus, Rhinoceros, Tapir, and Palreothere: it is with the Pachyderms having three toes to the hind-foot, that the Macrauchenia agrees in the main distinguishing • See Osscm. Foss. iii. Pl. XXVI. fig. 5. |