PEOEESSOE OWEN ON THE J^IEOATHEEIOI. 
816 
inner calcaneal facet from the naviculo-cuboid surface. Both this channel and the 
calcaneal one are perforated by conspicuous vascular canals. 
The calcaneum (Plates XL. & XLI. fig. 1, h) is a long irregular pyramidal bone, 
with an obtuse apex : the base is obliquely truncated, forms the fore part of the bone, and 
supports two articular surfaces (Plate XLI. fig. 2): the upper and outer surface {as) is 
ovate, concave where that on the astragalus is convex, and %ice versa: the oblique 
channel which divides this astragalar surface from the astragalo-cuboidal surface {os') is 
as deep as in the astragalus, and, like it, is perforated by vascular canals. The slightly 
concave triangular inner astragalar facet (os') is continued below, at an obtuse angle, 
into the semielliptic slightly concave surface {cl) for the os cuboides. External to the 
astragalar facet a strong vertically extended tuberosity (Plate XLI. fig. 1, t) forms the 
fore part of a wide and deep tendinal canal {%(,) ; a narrow and feeble ridge bounds it 
behind ; a second and more shallow groove {v) succeeds ; and, behind this, is a wide and 
deep vascular perforation {w). The under part of the calcaneum is i er} rugged and flat. 
The obtuse hinder extremity of the heel-bone shows by its sculp tming and the out- 
standing osseous spiculse, the force with which the attached cable-like ‘ tendo Achilhs 
must have acted on so unusually produced a calcaneal lever. 
The os naviculare (Plates XL. & XLI. fig. 1, <?) is a transversely oblong bone, com- 
pressed from before backward. Its posterior surface is occupied by the articulation for 
the astragalus, which is equally divided into an inner concaidty and an outer convexity, 
the latter approaching the conical form. From the lower part of this is continued at a 
right angle a small flat triangular surface for the cuboides (Plate XL. fig. o, cl"). The 
upper non-articular surface is narrowest at its middle, and is developed into a low oblong 
tuberosity on each side. On the fore part of the bone (Plate XL. fig. 3) is the articular 
surface, divided into the narrow oblong tract {cm), with two slight convexities, for the 
mesocuneiforme {il. fig. 1,/), and into the almost flat triangular tract (fig. 3, ci) for the 
ectocuneiforme (fig. 1, e). The latter surface (fig. 3, ci) is divided by a vmy naiTow non- 
articular tract from the cuboidal surface (fig. 3, cl"). The inner (tibia!) part of the 
anterior surface is non-articular, and extends beyond the mesocuneiform articulation to 
where the entocuneiforme would have been, had that bone existed in the tarsus of the 
Megatherium. 
The mesocuneiforme (Plates XL. & XLI. fig. 1, /) is a laterally compressed reniform 
bone, with a thick rough convex inferior border : the inner (tibial) surface (Plate XL. 
fig. 1, y*) is irregular and flattened: the similarly modified outer (fibular) surfiice 
(Plate XL. fig. 2,y) is varied by a flat elliptic articular facet {in) near its upper part, 
for a similar surface on the side of the metatarsal of the ‘ digitus medius. The back 
part of the bone is chiefly occupied by the narrow surface for the naviculare. The fore 
part of the bone is obtuse and rough. Not a vestige of the toe (digitus secundus), 
usually supported by the mesocuneiforme, is developed in the Megatherium. 
The ectocuneiforme (Plates XL. & XLI. fig. 1, e) presents the normal wedge-hke 
figure of the tarsal cuneiform bones. It is flattened from before backward; ■with its 
