The external surface of the astragalus of the Macrauchene, (fig. 1. Pl. [XIV],) is longer in proportion to its vertical extent than in the Tapir or Palæothere: 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 Palæothere; but in the Macrauchenia by a gentle concavity. Beneath the malleolar articular smooth surface in the Palæothere 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 Palæothere, 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 Palæothere, 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 Palæothere; 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 Palæothere the astragalus articulates by three surfaces with the os calcis, posteriorly by a large concave surface, externally by a longitudinal 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 longitudinal one. The posterior surface is relatively larger and deeper in the Macrauchene than in the Palæothere, 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 Palæothere and Macrauchene. It is satisfactory to find in the bone, which marks most strongly the affinity of Macrauchenia to Palæotherium, so many easily recognizable differences, because the structure of the cervical vertebræ in the latter genus is too imperfectly known, to allow us to predicate confidently a distinction between it and Macrauchenia in that particular; the difference, however, which they present in the condition of the bones of the fore-arm and leg, forbids their being considered as generically related.
There remains to be noticed only a single fractured metatarsal bone (fig. 1. 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 hind-foot, since it does not agree with any of the corresponding surfaces at the proximal extremities of the metacarpal bones. It remains 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 bones in the Tapir does our metatarsal agree, since it has but one articular facet on the lateral surface of its proximal end, while the outer metatarsal of the right-foot of the Tapir, with which, in other respects, it most closely corresponds, has two articular surfaces. In the cast of a hind-foot of a Palæothere, I find that the outer metatarsal bone closely agrees with this metatarsal bone of the Macrauchene, in the structure just alluded to: the articulation with the middle metatarsal being by a single sub-oval facet, which stands out a little way from the surface of the bone: the articular surface in the Macrauchene presents a similar form and condition, and is similarly situated to that in the Palæothere, being at the posterior 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 having 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 Palæothere this anterior part of the ridge is wholly suppressed, and the posterior is much more feebly developed than in the Macrauchenia. 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 Pachydermatous quadruped, not proboscidian, 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 fixed immoveably in the prone position. Now in the Ungulated series there are but two known genera,—the Rhinoceros and Palæotherium,—which, like the quadruped in question, have only three toes on the fore-foot. Again, in referring the Macrauchenia to the Tridactyle family of Pachyderms, we find, towards the close of our analysis, and by a detailed comparison of individual bones, that the Macrauchenia has the closest affinity to the Palæotherium.
But the Palæotherium, 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 vertebræ 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 Palæotherium.
Most interesting, indeed will be the knowledge, whenever the means of obtaining it may arrive, of the structure of the skull and teeth in the Macrauchenia. 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 vertebræ 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 particulars the Camelidæ, without losing the essential characters of Ruminantia, manifested a tendency to the Pachydermatous type; and the evidence which the lost genera, Macrauchenia and Anoplotherium, bear to a reciprocal transition from the Pachyderms to the Ruminants, through the Camelidæ, 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, (Tapirus,) at that time, regarded as exclusively South American. Still greater surprise was excited when a species of the genus Didelphys was discovered to have co-existed in Europe with the Palæotherium.
Now, on the other hand, we find in South America, besides the Tapir, which is closely allied to the Palæothere,—and the Llama, to which the Anoplothere offers many traces of affinity,—the remains of an extinct Pachyderm, nearly akin to the European genus Palæotherium: 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.
| ADMEASUREMENTS OF THE BONES OF THE MACRAUCHENIA. | ||
|---|---|---|
| Inches. | Lines. | |
| Length of third (?) cervical vertebra | 7 | 9 |
| Vertical diameter of ditto | 4 | 0 |
| Vertical diameter of body of ditto | 2 | 3 |
| Transverse diameter of ditto | 3 | 3 |
| Vertical diameter of spinal canal | 1 | |
| Length of fourth lumbar vertebra | 5 | 5 |
| Vertical diameter of body of ditto | 2 | 9 |
| Transverse diameter of ditto | 2 | 10 |
| Vertical diameter of spinal canal | 1 | 1 |
| Transverse ditto ditto[[26]] | 1 | 6 |
| Transverse diameter of last lumbar vertebra | 9 | |
| Transverse diameter of body of ditto | 2 | 2 |
| Vertical diameter of ditto | 1 | 3 |
| Entire length of lumbar region of vertebral column | 20 | |
| Vertical diameter of glenoid cavity of scapula | 3 | |
| Transverse ditto ditto ditto | 2 | 10 |
| Elevation of spine of scapula | 3 | 5 |
| Vertical diameter of proximal articular surface of fore-arm | 3 | 6 |
| Transverse ditto ditto ditto | 3 | 5 |
| Height of olecranon | 5 | 3 |
| Greatest diameter of its base | 2 | |
| Circumference of proximal end of anchylosed radius and ulna | 11 | 10 |
| Entire length of inner toe of fore-foot, inclusive of metacarpal bone | 13 | |
| Breadth of proximal end of metacarpus | 3 | 8 |
| Breadth of distal end of ditto | 5 | 4 |
| Length of inner metacarpal bone | 7 | 6 |
| Length of middle ditto | 8 | |
| Length of outer ditto | 7 | |
| Length of inner proximal phalanx | 3 | 6 |
| Length of middle ditto | 2 | 10 |
| Length of outer ditto | 3 | 4 |
| Length of inner middle phalanx | 2 | |
| Length of middle ditto | 2 | 3 |
| Length of inner distal phalanx[[27]] | 1 | |
| Length of the femur | 24 | |
| Diameter of base of articular surface of the head of ditto | 3 | 6 |
| Greatest diameter of proximal end | 7 | |
| Greatest diameter of distal end | 6 | 3 |
| Circumference of middle of shaft | 8 | |
| Length of tibia | 18 | |
| Greatest diameter of proximal end | 5 | 7 |
| Greatest diameter of distal end, including fibula | 4 | 4 |
| Circumference of middle of shaft | 9 | |
| Length of metatarsal bone[[28]] | 7 | 4 |
Errata.—The reader is requested to substitute the word ‘right’ for ‘left’ in the last line of p. 35, before the words ‘radius,’ ‘fore-foot,’ and ‘femur,’ and in the first line of p. 36, before the words ‘tibia,’ and ‘hind-foot.’