we shall find, however, that the Scelidothere and Armadillo closely agree : the
outer facet o f the fibular trochleae, above described-, is continued in the Scelidothere
(PI. X X V I I I . fig. 2 .), upon the fibular sid e o f the astragalus reaching nearly
half-way down the posterior part, and down nearly the whole o f its anterior.
In the Armadillo, it extends over the whole o f the anterior part o f the outer
side o f the astragalus. In both animals the lower boundary o f this articular surface
describes a strong sigmoid curve.
In the Megatherium (PI. X X V I I I . fig. 1), the corresponding surface for the
fibular malleolus on the outer side o f the astragalus is formed by a comparatively
very small semicircular flattened facet, which by its roughness indicates that the
end o f the fibula was attached to it by ligamentous substance, and that the syn o vial
bag was not continued upon that surface as in the Scelidothere and
Armadillo.
In the Mylodon (?) (PI. X X V I I I . fig. 4), even this rough facet is wanting and
the fibular trochlea is bounded b y the angle which divides the upper from the
outer surface o f the astragalus.
Turning now our attention to the under surface o f the astragalus, we observe
that it presents in the Scelidothere (PI. X X V I . fig. 6 ), an irregular quadrate
form, having the outer side occupied b y an elongated sub-ovate articular facet, e,
for the calcaneum, bounded externally b y a sharp edge, with its long ax is and its
greatest concavity in the antero-posterior direction, and sligh tly convex from side
to side : a second calcaneal articular surface ( f ) is situated at the inner and
anterior a n g le ; it is oblong and nearly f la t ; is continuous with the inferior concave
facet o f the scaphoid articulation, but is divided from the convex facet by a
gro o v e: the two calcaneal articulations are separated by a deep and rough depression,
traversing the under surface o f the astragalus diagonally, and increasing
in breadth towards the posterior and internal angle. The inner side o f the
astragalus presents a convex protuberance.
The correspondence between the astragalus o f the Scelidothere and Megathere
is best seen at the under surface o f the b o n e : in both the two calcaneal articulations
are separated b y the diagonal depression, and the internal and anterior surface is continuous
with the scaphoid articulation. In the Megathere, however, in consequence
o f the absence o f the inferior concavity which characterizes the Scelidothere, the
anterior calcaneal facet ( / ) appears as a more direct backward continuation o f the
scaphoidal surface; but they are divided by a more marked angle than is represented
in the figure (fig. 5, PI. X X V I .). The posterior and outer calcaneal surface
in the Megathere (e) is broader in proportion to its length, continued further
upwards upon the outward surface, is consequently more convex in the transverse
direction, and is not bounded externally by so sharp and prominent a ridge as in the