o f the astragalus o f the Scelidothere from th e Megatherioid to the Dasypodoid
type o f structure. For if the Megatherioid type o f structure had really been one
suitable to the ex ig encies o f climbing quadrupeds, it miglit have been expec ted to
have exhibited the scansorial modifications more decidedly, as the species
diminished in stature; but as regards the instructive bone o f the hind-foot,
the modifications o f which we have ju st been considering, this is b y no means
the case.
DESCRIPTION OF A MUTILATED LOWER JAW OF T IIE
M EG ALO NY X J E F F E R SO N I I .
ItJ the preceding section an astralagus w'as described, which was regarded as
belonging possibly to the same Edentate spe cie s as the jaw figured and described,
p. 69, PI. X V I I I . and X IX ., under the name o f Mylodon D a rw in ii; but the same
correspoiideiice, — that o f relative size, — renders it equally possible tiiat this
astragalus may belong to the spe cie s o f Megalonyx to which the lower jaw now
under consideration appertains. There could be no doubt, from its structure,
that it was the astragalus o f a g igantic spe cie s o f the order B ru ta , and o f the
Megatherioid family, and more nearly allied to the Megathere than is the S c e lid o there,
but sufficiently distinct from both.
T h e lower jaw, figured in PI. X X IX ., is the only fossil brought home by
Mr. Darwin that could be confidently referred to the genus M eg a lon yx ; but the
form o f the tooth in place on the right side o f the jaw fully justifies this determination.
The jaw its e lf is deeply and firmly imbedded in the matrix, so that
only the upper or alveolar border is visible. Th e coronoid and condyloid proce
sses are broken away, and the texture o f the remaining part o f the jaw was
too friable, and adhered too firmly to the surrounding matrix to admit o f more of
its form being ascertained than is figured.
There were four molars on each side o f this jaw ; the large oblique perfora-
tion near the fractured symphysis is the anterior extremity o f the wide dental
canal. The forms o f the alveoli are b est preserved in the right ramus; the first
is the smallest, and seems to have contained a tooth, o f which the transverse
section must have been simply e llip tic a l: the second tooth is likewise laterally
compressed, but the transverse section is ovate, the great end being turned
forwards : the third so cket presents a corresponding form, but a larger size ; the
fourth socket is too much mutilated to allow o f a correct opinion being formed as
to the shape o f the tooth which it once contained. The natural size o f the tooth
0 2