52 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 





characters of this bone ; its anterior articular surface, for example, is simple, 

 and not divided into a double trochlea by a vertical ridge : lastly, it is with the as- 

 tragalus of the Tapir and Palseothere that it presents the closest correspondence in 

 the general form and the minor details of structure, and with these Pachyderms, 

 therefore, I shall chiefly limit the comparison of the Macrauchenia, in regard to 

 the bone in question. If the upper or tibial articular surface (fig. 5, PI. XIV.) 

 be compared with that in the Palceotherium magnum (Ossem. Foss. PI. LIV. fig. 2,) 

 it will be seen, that the general direction of that surface is more parallel with the 

 axis of the bone in Macrauchenia. In the Palseotherium it is turned a little 

 towards the outer or fibular side, and in the Tapir the general direction of the 

 same surface is placed still more obliquely. The anterior border of this articu- 

 lating surface is broken by a semicircular notch in the Palseothere ; in the Tapir 

 it describes a gentle concave curve, and the Macrauchene resembles the Tapir 

 in this respect. The chief difference between the astragalus of the Tapir and the 

 Palseothere, when viewed from above, obtains in the relative length of the bone, 

 anterior to the tibial articulating surface : the Macrauchene presents, in this 

 respect, an intermediate structure, but differs 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, 

 PI. XIV.) and compare it with that of the Palceotherium magnum, (fig. 4, PI. liv, 

 Ossem. Foss.) it will be seen, that it presents in the Macrauchenia an oval, and 

 in the Palseotherium an irregular quadrangular form : in the Macrauchenia, this 

 surface is uniform or undivided, and is gently convex, except at its lower part ; 

 while in the Palseothere it is divided by an oblique ridge into a broad internal 

 facet for the scaphoid bone, and a narrow internal surface for articulation with 

 the os cuboides ; the larger surface is also concave transversely, and slightly 

 convex vertically : in the Tapir, the anterior surface of the astragalus deviates 

 still further from that of the Macrauchenia, both in general form, and in the pro- 

 portion of the cuboidal facet. In 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 calcis exclusively. 



The external surface of the astragalus of the Macrauchene, (fig. 1. PI. XIV,) is 

 longer in proportion to its vertical extent than in the Tapir or Palseothere : 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 Palseothere ; but in the Macrauchenia by a gentle concavity. Beneath the 



