PEOrESSOE OWEN ON THE MEGATHEEIIJM. 
821 
co-adaptation, the same relation of the osseous masonry to the transference of pressure 
from the leg to the outer border of the foot will be appreciated, as has been pointed 
out in the fore limb. The broad surface of the astragalus~the great keystone of the 
tarso-metatarsal arch (Plate XLI. tig. 1, a )- — transmits the superincumbent weight in two 
chief directions, backward upon the massive heel-bone (i), forward upon the metatarsus. 
By the naviculare (c) it is transmitted through the ectocuneiforme (e) and the produced 
outer angle of the base of the mid-metatarsal (m 3 ) to the fourth (m 4 ), and thence to 
the fifth metatarsal (m s). The cuboides (d), receiving the weight from both astragalus 
and naviculare, transmits it by its produced fore part to the base of the fourth meta- 
tarsal ; and partly by that medium, but chiefly by direct articulation, to the side of the 
base of the fifth metatarsal. The tendency in the cuboides to yield under this pressure 
and slip back, is resisted by the abutment of the calcaneum (d, t) against its back part. 
Comparison of the Bones of the Hind Foot. 
No knoum recent Mammal offers in its astragalus any repetition of the peculiarities, 
of that bone in the Megatherium. In the Anteaters and Armadillos the upper surface 
of the astragalus has the usual configuration, and is received into a deep tibio-peroneal 
mortice. In the Sloths the outer part of the astragalus is excavated by a deep cell, in 
which the pivot-shaped end of the fibula rotates: the inner side is applied, as usual, 
against a malleolar process of the tibia. There is an analogy in the pivoted part of the 
articulation, but the process and the cavity are on reverse parts of the ankle-joint, as. 
compared with the Megatherium. 
The convex protuberance or pivot on the inner half of the tibial surface of the astra- 
galus is common to all the extinct Megatherioids hitherto discovered, but it is asso- 
ciated with well-marked modifications of the bone in each genus, and with minor 
differences in difierent species. In the Mylodon* the calcaneal surface is single, and is 
continuous with the navicular one ; no part is insulated by a bisecting groove, as in 
the Megatherium : the middle division [a) of the upper articular surface is less convex ; 
the upper half of the navicular surface is flat, instead of being concave. 
The astragalus of the Scelidotheriumf agrees with that of the Mylodon in the less 
depth of the middle division of the upper surface, and the more open angle at which it 
joins the inner convexity ; it agrees with that of the Megatherium in the division of 
the calcaneal surface ; but it differs from both in the presence of two deep concavities 
upon the naviculo-cuboid surface, the portion to which the cuboides articulates being 
concave instead of convex. 
As the modification of the calcaneal surface of the astragalus governs that of the 
co-adapted surface in the calcaneum, this bone, in the Mylodon, is distinguished by the 
uninterrupted continuity of the articulation presented to the astragalus, with which 
* “Description of tlie Skeleton of the Mylodon rolustus," 4to, 1842, p. 117. pis. 21, 22, & 23. 
figs. 1 & 2. 
t “ Ecssil Mammalia,” Voyage of the Beagle, 4to, pi. 26. 
