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Show 51 ZOOLOGY OF TilE VOYAGE OF THE BEAGLE. surface of its proximal end, while the outer metatarsal of the right-foot ~f the Tapir, with which, in other respects, it most closely corresponds, has two a•:t1Cular surfaces. In the cast of a hind-foot of a Palreothere, I find that the oute1 .metatarsal bone closely agrees with this metatarsal bone ~f the Macrauchen~, m the structure just alluded to: the articulation with the tmddle metatarsal bemg by a single sub-oval facet, which stanus out a little way fro~ ~he surface of the ~o.ne : the articular surface in the Macrauchene presents a similar form and condition, and is similarly situated to that in the Palreothere: being at the p~sterior part of the lateral surface, and a little below the superiOr or tarsal articular surface. The bone expands towards its distal end, which corresponds in structure with those of the two lateral metatarsals in the fore-foot, in being completely divided into two trochlear surfaces by a well-developed median ridge, and in havi.ng the posterior half of this ridge suddenly produced, so as to project about two lines further from the trochlear surface than the anterior part of the same ridge. In both the Tapir and Palreothere this anterior part of the ridge is wholly suppresseu, and the posterior is much more feebly developed than in theM acrauchenia. The metatarsal bone here described is of exactly the same length with the internal metacarpal bone, and proves, in conjunction with the proportions of the astralagus, that the fore and hind feet of the Macrauchenia were of equal size. Thus then we obtain evidence, from a few mutilated bones of the trunk and extremities of a single representative of its race, that there once existed in South America a Pachyuermatous quadruped, not probosciuian, which equalled in stature the Rhinoceroses and Hippopotamuses of the old world. But this, though an interesting and hitherto unsuspected fact, is far from being the sum of the information which is yielded by these fossils. We have seen that the single ungueal phalanx bespeaks a quadruped of the great series of Ungulata, and this indication is corroborated by the condition of the radius and ulna, which are fixeu immoveably in the prone position. Now in the Ungulated series there are but two known genera,-the Rhinoceros and Palreotherium,-which, like the quauruped in question, have only three toes on the fore-foot. A~ain, in referring the Macrauchenia to the Tridactyle family of Pachyderms, we find, towarus the close of ~ur analysis, and by a detailed comparison of individual bones, that the Macrauchenia has the closest affinity to the Palreotherium. But the Palreotherium, like the Rhinoceros and Tapir, has the ulna distinct from the radius, and the fibula from the tibia; so that even if the Parisian Pachyderm had actually presented the same peculiarities of the cervical vertebra:! as the Patagonian one, it would have been hazardous, to say the least, while ignorant of the dentition of the latter, to refer it to the genus Palceotlterium. Most interesting, inueed will be the knowledge, whenever the means of obtaining it may arrive, of the structure of the skull and teeth in the Macrauchenia. FOSSIL M:AM:M:ALIA. 55 Meanwhile, we cannot but recognise, in the anchylosed and confluent state of the bones of the fore-arm and leg, a marked tendency in it towards the Ruminant Order, and the singular modifications of the cervical vertebrre have enabled us to point out the precise family of that order, with which the Macrauchenia is more immediately allied. In first demonstrating this relationship, it was shown in how many particular the Camelidce, without losing the essential characters of Ruminantia, manifested a tendency to the Pachydermatous type; and the evidence which the lost genera, Macmucltenia and Anoplotlterium, bear to a reciprocal transition from the Pachyderms to the Ruminants, through the Camelidce, cannot but be viewed with extreme interest by the Zoologist engaged in the study of the natural affinities of the Animal Kingdom. The Macrauchenia is not less valuable to the Geologist, in reference to the geographical distribution of animal forms. It is well known how unlooked-for and unlikely was the announcement of the existence of an extinct quadruped entombed in the Paris Basin, whose closest affinities were to a genus, (Tapints,) at that time, regarded as exclusively South American. Still greater surprise was excited when a species of the genus .Didelplzys was discovered to have co-existed in Europe with the Palmotlte1·ium. Now, on the other hand, we find in South America, besides the Tapir, which is closely allied to the Palreothere, -and the Llama, to which the Anoplothere offers many traces of affinity,- the remains of an extinct Pachyderm, nearly akin to the European genus Palceot!teriurn: and, lastly, this Macrauchenia is itself in a remarkable degree a transitional form, and manifests characters which connect it both with the Tapir and the Llama. ADJ\1EASUREMENTS OF TUE DONES OF TUE MA.CRAUOUENIA. Length of third(?) cervical vertebra V crtical diameter of ditto Do. do. of body of ditto Transverse diameter of ditto V crtical diameter of spinal canal Length of fourth lumbar vertebra V crtical diameter of body of ditto Transverse diameter of ditto V crtical diameter of spinal canal Transverse ditto ditto* Inches. 7 4 2 a 1 5 2 2 Lines. 9 0 3 a 5 9 10 G * This diameter increases rapidly in the posterior lumbar vertcbrre, in correspondence with the enlargement of the spinal chord, which gives off the great nerves of the hinder extremities. |