characters of this bone ; its anterior articular surface, for example, is simple,
and not divided into a double trochlea b y a vertical ridge : la stly , it is with the a stragalus
o f the Tapir and Palæothere that it presents the close st correspondence in
the general form and the minor details o f structure, and with these Pachyderms,
therefore, I shall chiefly limit the comparison o f the Macrauchenia, in regard to
the bone in question. I f the upper or tibial articular surface (fig. 5, PI. X IV .)
be compared with that in the Paloeotherium magnum (Ossem. Foss. PI. L IV. lig. 2,)
it will be seen, that the general direction o f that surface is more parallel with the
axis o f the bone in Macrauchenia. In the Palæotherium it is turned a little
towards the outer or fibular side, and in the Tapir the general direction o f the
same surface is placed still more obliquely. The anterior border o f this articula
ting surface is broken b y a semicircular notch in the Palæothere ; in the Tapir
it describes a gentle concave curve, and the Macrauchene resembles the Tapir
in this respect. The ch ie f diiference between the astragalus o f the Tapir and the
Palæothere, when viewed from above, obtains in the relative length o f the bone,
anterior to the tibial articulating surface : the Macrauchene presents, in this
respect, an intermediate structure, but diflers from both in the greater extent o f
the tibial side o f this part o f the astragalus.
I f we nex t direct attention to the anterior or scaphoid articular surface, (fig. 3,
PI. X IV .) and compare it with that o f the Paloeotherium magnum, (fig. 4, PI. liv,
Ossem. F o ss.) it will be seen, that it presents in the Macrauchenia an oval, and
in the Palæotherium an irregular quadrangular form : in the Macrauchenia, this
surface is uniform or undivided, and is gently convex, exc ept at its lower part ;
while in the Palæothere it is divided by an oblique ridge into a broad internal
facet for the scaplioid 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 o f the astragalus deviates
still farther from that o f the Macrauchenia, both in general form, and in the proportion
o f the ciiboidal facet. In the didactyle Anoplotherium, Camel, and true
Ruminants, where tlie cuboides presents a large relative size, a still greater
proportion o f the anterior surface o f the astragalus is devoted to the articulation
with this bone, and is separated from the scaphoid surface by a well-developed
vertical ridge. Th e Macrauchenia presents, therefore, the extreme variation from
this ty p e ;—and should the entire tarsus hereafter be discovered, it will doubtless
be found, that the os cuboides is articulated posteriorly to the os calcis exc lusively.
The external surface o f the astragalus o f the Macrauchene, (fig. 1. PI. X IV ,) is
longer in proportion to its vertical extent than in the Tapir or Palæothere : the
articular surface for the fibular malleolus is le ss 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