PEOFESSOE OWEJT ON THE MEOATHEEIFM. 
819 
ponderous quadruped, and have alleviated the effect of the enormous pressure on the 
two gradatorial toes. The whole outer and under part of the non-articular surface of 
the fifth metatarsal is strongly sculptured by irregular ridges, tubercles, grooves, and 
foiamina, indicative of the hoof-like callosity of the outer border of the sole in which it 
was chiefly imbedded. The upper and inner non-articular surfaces are comparatively 
smooth. The inner side is flat, and is traversed by an oblique shallow (tendinal 1) groove. 
On the distal surface are two small articular facets ; the inner one, w^hich is the best 
marked, is subcircular, about 8 lines in diameter. 
Phalanges. The bones of this class are unusually reduced in number in the foot of 
the Megatherium, even admitting the accui’acy of the figures of the hind foot of the 
Madrid skeleton, in which two stunted phalanges appear to terminate both the fourth 
and fifth toes. For, as the great unguiculate toe, like that of the fore-foot, has only 
two moveable phalanges, the total number of these bones is but six, not exceeding that 
of the tarsal bones of the same foot. 
The phalanx (Plate XLI. figs. 1 & 3, i& 2 ) of the innermost toe (m), answering to the 
third of the pentadactyle foot, represents, as in the corresponding digit of the fore foot, 
the proximal and middle phalanges connate. The compound bone {ih. fig. 3, 1 & 2 ) is 
shaped like a wedge with an oblique edge, deeply notched. It has articular surfaces 
not only on its proximal and distal ends, but upon its upper or dorsal surface. The two 
former surfaces, which are the ‘ ends ’ of the bone in ordinary phalanges, here form the 
‘ sides of the wedge : the sides of the phalanx are the ‘ margins ’ of the wedge : the 
dorsal surface fonns the base, the plantar or under surface is represented by the two 
processes of the cleft apex or edge of the wedge. The proximal articular surface presents 
a longitudinal channel, convex vertically, concave transversely, from which a flat surface 
extends from nearly the whole of the tibial side, and a convex surface from the upper 
part of the fibular side ; the whole articular surface being the counterpart of that on 
the metatarsal (ib. m), with which the present remarkable bone is, by this interlocking 
jomt, firmly united. The lower prominence of the median rising of the distal joint of 
the metatarsal {jp) protrudes through the lower notch in the phalanx {n). A yielding, 
elastic, slightly-sliding movement was all that could take place between these bones. 
The complex distal articulation is adapted to an equally restricted junction with the 
enormous terminal phalanx (3); it consists of four distinct articular surfaces, two on the 
anterior and two on the upper part of the connate phalanges (1 & 2 ). The internal of 
the tTvo distal surfaces is the largest and is slightly concave, the external one, near the 
upper and outer angle of the bone, is shghtly convex ; they are divided by a rough tract, 
indicating strong ligamentous union with the claw-phalanx, of half an inch in breadth,* 
and they cover the smaller proportion of the distal surface of the bone. Below the 
outer articular sui'face there is a protuberance, with a smooth but non-synovial surface 
in the piesent bone, which is adapted to a definite smooth surface upon the claw- 
phalanx. The two upper surfaces are subcircular, each about half an inch in diameter, 
very slightly convex, and about 5 lines apart. 
5 p 2 
