pre,sent a more resisting stiaicture tlian do those o f the Mylodon ; having a larger
proportion o f the dense ivory composed o f the minute calcigerous tubes, and
a mucll smaller proportion o f the softer external cæmentum ; in tliis respect the
Scelidotliere recedes farther from Megathere, and approaches nearer tire Armadillos
than does the Mylodon.
The lower jaw resembles, in the general form o f the posterior moiety which
is here preserved, tliat o f the Sloth and Mylodon more tlian that o f any other Edentate
species. Its deep posterior angle is produced backwards, and a broad coronoid
process rises and nearly fills the zygomatic spac e ; the condyle is flat, as the
glenoid surface has already indicated; its transverse diameter is an inch and
eiglit lines ; its antero-posterior diameter seven lines : it is principally extended
inwards beyond the vertical line o f the ascending ramus. The lower contour o f
the jaw describes an iindiilating line ; which, commencing from the posterior
angle, is at first gently convex, tlien sligh tly concave, then again convex, below
the alveoli oi the teeth, where it is rounded and expanded, as in the Orycterope.
The fractured condition o f the right ramus o f this part fortunately exposed the
roots o f the four grinding teeth, wliich constitute the dental series on each side
o f the lower jaw. The length o f the jaw occupied by these four alveoli is three
inches ten lines, which exc eeds a little that o f the opposed five grinders above;
the ramus o f the jaw gradually diminishes in all its dimensions anterior to the
molar teeth ; the dental canal passes in a gentle curve below, and on the inner
side o f the alveoli, whence it gradually inclines to the outer wall of the jaw.
The whole ascending ramus o f the jaw consists o f a very thin plate o f bone ;
it is slightly concave on the inner side, and the inferior margin o f the produced
angle inclines inwards, as in the Mylodon and Sloth ; it is impressed on the outer
side with two shallow depressions, and two parallel ridges, both following the
gentle curvature o f the part. There is a foramen on the outer side o f the ramus
at the anterior part o f the base of the coronoid process corresponding with that
in the lower jaw o f the Mylodon, but the longitudinal channel which runs along
the outer side of the alveolar processes is wanting, and the expansion at the base
o f those processes is more sudden and relatively greater ; the general correspondence,
however, betw'een these lower jaw s is such as would lead to the idea that
they belonged to animals o f the same genus, were it not that the teeth present
modifications of form in the Scelidothere, as distinct from those o f the Mylodon, as
are any o f the minor dental differences on which genera or sub-genera o f existing
Mammalia are founded in the present sta te o f Z oological Classification.
To make this distinction more readily intelligible, I have given a view o f the
transverse section o f the teeth in the right ramus o f the lower jaw (fig. 4, PI. X X I I I.),
corresponding with that o ilh e Mylodoji DamaVm, (P l.X V I I ., fig .5 ). In the present
sub-genus the antero-posterior extent o f the four alveoli o f the lower jaw nearly
equals four inches, and is relatively greater than in the Mylodon, although the
teeth are placed closer together; this is owing to their greater relative size. The
first molar tooth presents the simplest form ; its transverse section is a compressed
inequilateral triangle with the angles rounded off ; the longest diameter o f this
section which is parallel with the inner alveolar border is eleven lines, the transverse
diameter almost six lines ; the base or broadest side of the triangle is turned
inwards, and is slightly concave ; the two smaller s id e s are also sligh tly concave.
The second molar is placed more obliquely in the jaw ; the long axis of its
transverse section intersects at an acute angle that o f th e jaw its e lf; the transverse
section presents a compressed or oblong form, with the larger end nex t the
outer side, and the smaller end next the inner side o f the jaw ; this end is simply
rounded, but the outer end presents a sinuosity, corresponding to a broad groove
which traverses the whole length o f the outer side o f the tooth ; the anterior,
which corresponds to the internal side or base of the transverse section o f the
preceding molar, is slightlj' concave.
The third molar has nearly the same form and relative position as the preceding;
the long diameter o f the transverse section is, in both, ten lines and a half;
the principal transverse diameter is, in the second molar five lines, in the third
nearly six ; the difference of form observable in these as compared with the two
middle grinders o f the Mylodon is w’ell marked ; in the latter these teeth are impressed
with a longitudinal groove on their inner sides ; in the Scelidothere they
have a similar impression along their outer but not along the inner side.
In the last molar the resemblance is much closer, and the modification of
form by which it differs from the preceding ones is o f the same kind ; the transverse
section gives an irregular oblong figure with its axis nearly parallel witli
that o f the jaw, and constricted at the middle by sinuosities produced by two wide
channels which traverse longitudinally, one the outer, the other the inner side o f
the tootli ; the latter groove is much wider and siiallower in the Scelidothere than
in the Mylodon. The two lobes produced by these grooves are more equal in
Scelidothere; the anterior one is concave on its anterior surface instead o f convex
as in the Mylodon; the posterior one is more compressed; the longitudinal or
antero-posterior diameter o f the transverse section o f this tooth is one inch
five lines ; the greatest transverse diameter is nine lin e s; the diameter o f the
isthmus joining the lobes is three lines and a half ; the entire length o f this tooth
is tliree inches three lines.*
• I t requires little stretch of imagination to conceive that this more complex posterior tooth (PI. X X I I I ,
fig. 4, 4) in the lower jaw is the representative of the two smaller posterior teeth (ib. fig. 3, 4, and 5) of tlie
upper jaw conjoined.
M 2