PEOEESSOE OWEN ON THE MEOATHEEIEM. 815 
shaped and adjusted, however, are the tarsal bones in the Megatherium, that the tibia 
articulates with the side instead of the summit of the tarsus, so that the whole foot is 
turned inward and rests upon its outer edge instead of its sole (Plate XLI. fig. 1, and 
Plate XVII. cl^ Philosophical Transactions, 1855). 
The number of tarsal bones is reduced to six, through the absence of the entocunei- 
form * , but the astragalus, and especially the calcaneum, are developed to a great 
size. 
The astragalus (Plates XT. & XLI. fig. 1, (z) is of a peculiarly irregular form i if the 
foot be placed with the sole flat on the ground, as in Plate XL., the chief articular 
surface (a) for the tibia looks inward, and the small fibular facet, at right angles there- 
with, is uppermost. The extensive surface by which it articulates with the bones of the 
leg is divided into three parts, the planes of which are at right angles to each other. 
The middle and largest dmsion (a), answering to the outer ridge of the trochlear 
sm-face in the common form of astragalus, is here expanded into a broad reniform 
smooth tract, horizontal in the ordinary position of the Megatherium’s foot (Plate XLI. 
fig. 1, (z), almost flat from before backward, convex from> side to side. This surface is 
continued over the outer edge upon the outer side of the bone, in a triangular form, with 
the apex rounded off, to form the facet (o) for the fibula. The surface answering to the 
main part of the trochlea and its inner malleolar facet in ordinary astragali, is here 
reduced to a small triangular convexity {ib. & Plate XL. fig. I, i), forming the third and 
internal division of the surface, and supported on what appears to be an obtuse pyramidal 
process from the inner and lower part of the bone. This convexity is wedged into the 
deeper excavation on the inner part of the tibial articular surface, and forms a kind of 
pivot on which the foot worked. 
The under or calcaneal side of the astragalus slopes from behind downward and 
forward, and is divided by an oblique groove, about an inch broad, into two facets ; the 
outer and posterior one is for the calcaneum exclusively ; the inner and anterior one is 
for the calcaneum, but is continuous with the surface for the navdculare. The outer 
calcaneal surface is ovate, concave lengthwise, convex across its posterior broader end, 
nearly flat at its anterior end. The inner calcaneal surface is of much less extent and 
is nearly flat ; its anterior end suddenly bends forward and upward to be continued into 
the outer convex part of the navicular surface, which surface is divided into this convex 
portion and a contracted subcircular concavity. The lower part of the convex facet rests 
upon an articular concave surface on the cuboides. 
Only a small proportion of the outer surface of the astragalus is non-articular, and this 
is chiefly on the outer, or, in the megatherian position of the foot, the upper surface 
(Plate XLI. fig. I). The groove dividing the calcaneal surfaces begins at their lower 
part and passes backward and inward, with shght terminal bends, which give it the form 
of the italic yi It is divided by a ridge from a wider channel, excavating the under part 
of the inner process ; from which a third channel extends backward, dividing part of the 
* ‘ Os cuneiforme internum’ of Scbmmeuiiixg. 
