XXVI. fig. 4.), presents, in its transverse contour, two convex pulleys,
_a_ and _b_, and an intermediate concavity, forming one continuous
articular surface. The external or fibular trochlea (_a_) is strictly
speaking convex only at its posterior part, the upper surface gradually
narrowing to a ridge, as it advances forwards from which, the inner and
outer parts slope away at an angle of 35°.
The tibial[55] convexity (_b_) is more regular and less elevated, it has
only half the antero-posterior extent of the outer pulley; its marginal
contour forms an obtuse angle at the inner side.
In the Megatherium the upper articular surface of the astragalus is also
divided into two trochleæ, of which the one on the fibular side (fig. 3,
_a_), is of much greater relative size and extent than the tibial one
(_b_), and is raised nearly four inches above the level of the latter,
although in the oblique position in which the bone is naturally placed
in the skeleton, the highest part of each convexity is on the same
level. The fibular trochlea differs also from that in the Scelidothere
in being regularly convex in the transverse as well as the
antero-posterior direction. The tibial convexity resembles that in the
Scelidothere, save in its smaller relative size; its internal margin
likewise forms an angular projection below the internal malleolus.
The upper surface of the astragalus of the Mylodon, or Megalonyx(?) (Pl.
XXVIII. fig. 5.),[56] differs from that in the Megatherium in having a
narrower fibular trochlear ridge.
The astragalus of the Ai (_Bradypus tridactylus_) differs widely from
that of either the Megathere, Mylodon(?) or Scelidothere in having a
conical cavity on the upper surface, in place of the fibular convexity,
in which concavity the distal end of the fibula rotates like a pivot.
This mechanism is closely related to the scansorial uses of the inwardly
inflected foot of the Sloth.
If the astragalus of an Armadillo[57] were placed side by side with that
of the Megathere, it would be very difficult to determine the analogous
parts, especially of the upper surface, unless guided by the
intermediate structure presented by the Scelidothere. The upper surface
of this bone, in the Armadillo, is, however, divided into two
transversely convex trochleæ, separated by a much wider transversely
concave surface. The fibular trochlea resembles that of the Scelidothere
in having its upper and outer facets sloping away at an acute angle, but
without meeting at a ridge anteriorly; this surface is not more raised
above the tibial trochlea than in the Scelidothere.
The inner trochlea differs from that of the Scelidothere in having a
greater relative antero-posterior extent, and in forming, in place of an
uniform convex surface, a trochlea similar in structure to that on the
outer side. The extent of rough surface on the upper part of the
astragalus intervening between the articular surface for the bones of
the leg, and that for the scaphoides is extremely small in the Megathere
and Mylodon(?); it is relatively greater in the Scelidothere; it is
still more extensive in the Armadillo; but is the longest in the Sloth.
The anterior extremity of the astragalus which is entirely occupied by
the scaphoid articular surface is very peculiar in the Scelidothere (Pl.
XXVI. fig. 2.): it presents one convex and two concave facets, which,
however, form part of one continuous articular surface: the convex facet
forms the internal part of the surface, and presents a rhomboidal form
with the long axis vertical. The concave facets (_c_ and _d_) are
extended transversely and placed one above the other; they are slightly
concave in the transverse, and nearly flat in the vertical directions.
In the Megatherium (fig. 1.) the scaphoid surface of the astragalus is
divided only into one concave and one convex portion, both continuous
with each other: the concave facet (_c_) corresponds with the upper
concavity in the Scelidothere, but is a pretty uniform subcircular
depression, fourteen lines in depth: the convex facet, _d_, is continued
across the whole breadth of the under part of the scaphoid surface and
corresponds with both the inner convex, and lower concave surfaces of
the scaphoid articulation in the Scelidothere.
In the Mylodon(?) (Pl. XXVIII. fig. 3.), the articular facet,
corresponding with that marked (_c_) in the astragali of the Megathere
and Scelidothere, is simply flattened, instead of being concave; the
rest of the scaphoid surface corresponds with that in the Megatherium.
In the Armadillo the scaphoid articular surface is undivided and wholly
convex: in this part of the astragalus, therefore, we find the
Scelidothere deviating from the Armadillo further than does the
Megathere; while the Mylodon or Megalonyx(?) most resembles the
Armadillo in the configuration of this part of the astragalus.
If we compare the outer surfaces of the astragalus in these quadrupeds,
we shall find, however, that the Scelidothere and Armadillo closely
agree: the outer facet of the fibular trochleæ, above described, is
continued in the Scelidothere (Pl. XXVIII. fig. 2.), upon the fibular
side of the astragalus reaching nearly half-way down the posterior part,
and down nearly the whole of its anterior.
In the Armadillo, it extends over the whole of the anterior part of the
outer side of the astragalus. In both animals the lower boundary of this
articular surface describes a strong sigmoid curve.
In the Megatherium (Pl. XXVIII. fig. 1), the corresponding surface for
the fibular malleolus on the outer side of the astragalus is formed by a
comparatively very small semicircular flattened facet, which by its
roughness indicates that the end of the fibula was attached to it by
ligamentous substance, and that the synovial bag was not continued upon
that surface as in the Scelidothere and Armadillo.
In the Mylodon(?) (Pl. XXVIII. fig. 4), even this rough facet is wanting
and the fibular trochlea is bounded by the angle which divides the upper
from the outer surface of the astragalus.
Turning now our attention to the under surface of the astragalus, we
observe that it presents in the Scelidothere (Pl. XXVI. fig. 6), an
irregular quadrate form, having the outer side occupied by an elongated
subovate articular facet, _e_, for the calcaneum, bounded externally by
a sharp edge, with its long axis and its greatest concavity in the
antero-posterior direction, and slightly convex from side to side: a
second calcaneal articular surface (_f_) is situated at the inner and
anterior angle; it is oblong and nearly flat; is continuous with the
inferior concave facet of the scaphoid articulation, but is divided from
the convex facet by a groove: the two calcaneal articulations are
separated by a deep and rough depression, traversing the under surface
of the astragalus diagonally, and increasing in breadth towards the
posterior and internal angle. The inner side of the astragalus presents
a convex protuberance.
The correspondence between the astragalus of the Scelidothere and
Megathere is best seen at the under surface of the bone: in both the two
calcaneal articulations are separated by the diagonal depression, and
the internal and anterior surface is continuous with the scaphoid
articulation. In the Megathere, however, in consequence of the absence
of the inferior concavity which characterizes the Scelidothere, the
anterior calcaneal facet (_f_) appears as a more direct backward
continuation of the scaphoidal surface; but they are divided by a more
marked angle than is represented in the figure (fig. 5, Pl. XXVI.). 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
Scelidothere. The protuberance from the inner surface of the astragalus
is more compressed laterally in the Megathere than in the Scelidothere.
The correspondence between the astragali of the Mylodon(?) (Pl. XXVIII.
fig. 6) and Megathere in the conformation of the under surface is so
close, that the few differences which exist will be sufficiently
appreciated by an inspection of the figures.
In the Armadillo the astragalus, in consequence of the greater
production of its anterior part, presents more of an angular than a
quadrate figure; and the scaphoid articular surface, being
proportionally carried forwards, is altogether separated from the
anterior calcaneal surface. The posterior and inner calcaneal surface
resembles that in the Scelidothere, but is less inclined upwards; and is
continuous with the posterior part of the tibial articular surface.
Thus the astragalus in the structure of its two most important
articulations, viz. that which receives the superincumbent weight from
the leg, and that which transmits it to the heel, presents a closer
correspondence in the Scelidothere with that of the Dasypus, than with
that of the Megathere or Mylodon.
The ungueal phalanx of the Scelidothere before alluded to, is
represented of the natural size in Pl. XXVII. The side view, fig. 3.
shows the position of the articular surface on the proximal end, sloping
obliquely towards the under surface, and overtopped by an obtuse
protuberance, calculated to impede any upward retraction of the claw:
the present joint, in fact, illustrates in every particular the argument
by which Cuvier established the true affinities of the allied extinct
genus Megalonyx.[58]
The present phalanx is, however, less compressed, and less incurved than
those of the Megalonyx, which have been hitherto described; but it more
resembles in these proportions one of the smaller, and presumed hinder,
ungueal phalanges of the Megatherium. The upper and lateral parts of the
bone are rounded, and it gradually tapers to the apex, which is broken
off. The osseous sheath for the claw is developed only at the under part
of the bone: it presents the form of a thick flat plate of bone, with
the margin very regularly and obliquely bevelled off, and having a
vertical process of bone attached lengthwise to the middle of its under
surface. This process must have served for the insertion of a very
powerful flexor tendon. The figures of this bone preclude the necessity
of any further verbal description.
M. Lund lays most stress upon the argument founded on the inward
inflection of the sole of the foot in the Megalonyx, and appeals with
greatest confidence to this structure in support of his hypothesis of
the scansorial habits of that extinct Edental.[59]
It is quite true that the Quadrumana derive advantage from this position
of the foot in climbing trees, and that it is carried to excess in the
Sloths, which can only apply the outer edge of the foot to the ground.
But we may ask, was the inversion of the sole of the foot actually
carried to such an extent in the _Megalonyx_? And, admitting its
existence in an inferior degree, is it then conclusive as to the
scansorial habits of that species?
M. Lund expressly states that it is produced by a different structure
and arrangement of the tarsal bones, from that which exists in the
Sloth, but he does not specify the nature of this difference.
If the astragalus, which I have referred with doubt to the _Megalonyx_,
do not actually belong to that genus, it is evidently part of a very
closely allied species. Now this astragalus, as we have seen, resembles
most closely that of the Megatherium; and since we may infer that the
calcaneum, scaphoides, and cuboides had a like correspondence, the
inclination of the sole of the foot inwards must have been very slight,
as I have determined from examination of the structure and co-adaptation
of those bones in the incomplete skeleton of the Megatherium in the
London College of Surgeons. Such an inclination of the foot may be
conceived to have facilitated the bending of the long claws upon the
sole, during the ordinary progressive movements of the animal, but it is
quite insufficient to justify the conclusion, that it related to an
application of the hind-feet for the purposes of climbing.
It is not without interest again to call to mind the deviation of the
structure of the astragalus of the Scelidothere from the Megatherioid to
the Dasypodoid type of structure. For if the Megatherioid type of
structure had really been one suitable to the exigencies of climbing
quadrupeds, it might have been expected to have exhibited the scansorial
modifications more decidedly, as the species diminished in stature; but
as regards the instructive bone of the hind-foot, the modifications of
which we have just been considering, this is by no means the case.
DESCRIPTION OF A MUTILATED LOWER JAW OF THE
MEGALONYX JEFFERSONII.
In the preceding section an astralagus was described, which was regarded
as belonging possibly to the same Edentate species as the jaw figured
and described, p. 69, Pl. XVIII. and XIX., under the name of _Mylodon
Darwinii_; but the same correspondence,—that of relative size,—renders
it equally possible that this astragalus may belong to the species of
_Megalonyx_ to which the lower jaw now under consideration appertains.
There could be no doubt, from its structure, that it was the astragalus
of a gigantic species of the order _Bruta_, and of the _Megatherioid_
family, and more nearly allied to the Megathere than is the
Scelidothere, but sufficiently distinct from both.
The lower jaw, figured in Pl. XXIX., is the only fossil brought home by
Mr. Darwin that could be confidently referred to the genus _Megalonyx_;
but the form of the tooth in place on the right side of the jaw fully
justifies this determination. The jaw itself is deeply and firmly
imbedded in the matrix, so that only the upper or alveolar border is
visible. The coronoid and condyloid processes are broken away, and the
texture of the remaining part of the jaw was too friable, and adhered
too firmly to the surrounding matrix to admit of more of its form being
ascertained than is figured.
There were four molars on each side of this jaw; the large oblique
perforation near the fractured symphysis is the anterior extremity of
the wide dental canal. The forms of the alveoli are best preserved in
the right ramus: the first is the smallest, and seems to have contained
a tooth, of which the transverse section must have been simply
elliptical: the second tooth is likewise laterally compressed, but the
transverse section is ovate, the great end being turned forwards: the
third socket presents a corresponding form, but a larger size: the
fourth socket is too much mutilated to allow of a correct opinion being
formed as to the shape of the tooth which it once contained. The natural
size of the tooth _in situ_, and of the adjoining socket, is given in
Pl. XXIX., fig. 2. The difference of form which the jaw of the Megalonyx
presents, as compared with that of the Mylodon, especially in the
greater recedence of the two horizontal rami from each other, will be
appreciated by comparing Pl. XVIII. with Pl. XXIX.
DESCRIPTION OF A FRAGMENT OF THE SKULL AND OF THE TEETH OF THE
MEGATHERIUM CUVIERI.
Notwithstanding the full, accurate, and elaborate accounts of the
skeleton of the Megatherium given by Bru,[60] Cuvier,[61] Pander and
D’Alton,[62] and Mr. Clift,[63] the fragments of this most gigantic of
quadrupeds brought home by Mr. Darwin, possess much interest, and have
added, what could hardly have been anticipated, important information as
to the dental system, whereby an error in the generic character of the
Megatherium has been corrected.
The fragments here alluded to are portions of the skull of three
full-grown Megatheres: the most perfect part of which affords a view of
the posterior, and of part of the basal surface, which regions of the
cranium have not hitherto been elsewhere figured or described, (Pl.
XXX.)
The plane of the occipital foramen forms with that of the base of the
skull an angle of 140°, the plane of the posterior surface of the skull
forms with the basal plane an angle of 68°. The occipital condyles are
therefore terminal, or form the most posterior parts of the cranium. The
extent of their convex curvature in the antero-posterior direction,
which equals that of a semicircle, indicates that the Megatherium
possessed considerable freedom and extent of motion of the head. The
condyles are not extended in the lateral direction so far as in the
Toxodon; their axis is more oblique than in the Glossotherium, and their
internal surface is more parallel with the axis of the skull, the
foramen magnum not presenting that infundibuliform expansion which is so
characteristic of the Glossotherium. The occipital condyles resemble
most in form and position those of the Scelidotherium; but in the angle
of the occipital plane the Megatherium is intermediate between the
Scelidothere and Glossothere. The ex-occipitals terminate laterally and
inferiorly, each in a short, but strong obtuse process. The posterior
plane of the skull is traversed by a strong arched intermuscular crest,
which forms the upper boundary of a pretty deep fossa, which is divided
by a median vertical ridge, extending downwards to within an inch of the
upper margin of the foramen magnum. A second strong obtuse transversely
arched ridge curves over the first, and forms the upper boundary of the
posterior or occipital region of the skull: the interspace between the
two transverse ridges is very irregular, and indicates the firm
implantation of powerful nuchal muscles or ligaments, (Pl. XXX. fig. 1.)
In the configuration and angle of the occipital plane the Megatherium
indicates the same general correspondence with the Edentate type, which
has been pointed out in the descriptions of the crania of the
Glossothere and Scelidothere: and the resemblance to the Scelidothere is
not less striking in the small proportional size of the cranium in this
quadruped, which surpasses the rest of its class in so great a degree in
the colossal proportions of its hinder parts.
Having detected in the base of the skull of the Scelidothere an
articular semicircular pit for the head of the styloglossal bone,
similar to, but relatively smaller than, that remarkable one in the
skull of the Glossothere, it became a matter of interest to determine
whether this structure, which does not exist in any of the existing
Edentals, should likewise be present in the gigantic type of the
Megatherioid family. The result of a careful removal of the matrix from
the basal region of one of the cranial fragments of the Megatherium was
the detection of this articular cavity, in each temporal bone in the
same relative position as in the Glossothere and Scelidothere. The
styloid articular cavity is relatively smaller, and shallower, than in
the Glossothere, its proportions being much the same as those of the
Scelidothere. The cranial or posterior extremity of the stylo-hyoid bone
in the Scelidotherium is bent upwards at an obtuse angle (Pl. XXI.), and
terminates in an articular ball which rotates in this cavity. The size
of this bone, and its mode of articulation, indicates great power and
muscularity of tongue in the Megatherioids, and calls to mind the
importance of that organ in the Giraffe, which subsists on the same kind
of food as that which I have supposed to have supported the
Megatherioids, although the general organization of these animals and
the mode in which the foliage was brought within reach of the tongue are
as opposite as can well be imagined.
The anterior condyloid foramen presents scarcely one-half the absolute
size of that of the Glossothere, whence we may infer a correspondingly
inferior development of the tongue in the Megathere. The fractured
parietes of the cranial cavity of the Megatherium every where exhibit
evidences of the great extent of the air-cells or sinuses continued from
the nasal cavity: on the basilar aspect of the cranium they extend as
far back as the jugular foramina: the whole of the basi-sphenoid being
thus excavated, and permeable to air, derived from the sphenoid sinuses,
(Pl. XXX. fig. 2.). The vertical diameter of the cranial cavity is four
inches, eight lines; its transverse diameter, which is greatest in the
posterior third part of the cavity, corresponding with the posterior
part of the cerebrum is six inches: from the indications afforded by the
remains of the cranial cavity in Mr. Darwin’s specimens, I conclude that
the brain of the Megatherium was more depressed, and upon the whole,
smaller by nearly one-half than that of the Elephant; but with the
cerebellum relatively larger, and situated more posteriorly with
relation to the cerebral hemispheres: whence it may be concluded that
the Megatherium was a creature of less intelligence, and with the
command of fewer resources, or a less varied instinct than the Elephant.
It has been usual to characterize the Megatherium, in conformity with
the concurrent descriptions of Bru, Cuvier, and D’Alton, by the dental
formula of _molares_ ⁴⁄₄ ⁴⁄₄, i. e. by the presence of four grinding
teeth on each side of the upper, as of the lower jaw. It was the
agreement of the excellent authorities above cited in this statement,
which induced Mr. Clift and myself to regard a single detached tooth,
which formed part of the valuable collection of remains of the
Megatherium deposited in the Hunterian Museum by Sir Woodbine Parish, as
being, from its comparatively small size, the tooth of either a younger
individual or of a smaller species of Megatherium. Upon clearing away
the matrix from the palatal and alveolar surface of one of the cranial
fragments of the Megatherium in Mr. Darwin’s collection, I was gratified
by the detection of the crown of a fifth molar, corresponding in size
and form with the detached tooth, above alluded to: its small size, and
its position have doubtless occasioned its being over-looked in the
cranium of the great skeleton at Madrid.
The anterior molar of the upper jaw presents a nearly semicircular
transverse section, with the angles rounded off; the three succeeding
teeth are four-sided, with the transverse somewhat exceeding the
antero-posterior diameter: they are rather longer and larger than the
first: the last molar is likewise four-sided, but presents a sudden
diminution of diameter, and is relatively broader. The following are the
respective dimensions of the upper maxillary teeth.
First Second Third Fourth Fifth
Molar. Molar. Molar. Molar. Molar.
In. Lines. In. Lines. In. Lines. In. Lines. In. Lines.
Length 8 6 9 4 9 4 8 7 5 2
Transverse 1 9 2 4 2 3 2 0 1 4
diameter
Antero-posterior 1 5 2 0 2 0 1 11 0 10
diameter
Besides the differences in size, the upper molars vary as to their
curvature: this difference is exhibited in the vertical section of these
teeth figured in Pl. XXXI. The convexity of the curve of the first,
second and third molars is directed forwards; the fourth is straight,
its anterior surface only describing a slight convexity in the vertical
direction; the fifth tooth is curved, but in a contrary direction to the
others; and the bases of the five molars thus present a general
convergence towards a point a little way behind the middle of the
series.
The next peculiarity to be noticed in these remarkable teeth is the
great length of the pulp-cavity (_d_), the apex of which is parallel
with the alveolar margin of the jaw: a transverse fissure is continued
from this apex to the middle concavity of the working surface of the
tooth, which is thus divided into two parts. Each of these parts
consists of three distinct substances,—a central part analogous to the
body or bone of the tooth or ‘dentine,’ a peripheral and nearly equally
thick layer of _cæmentum_, and an intermediate thinner stratum of a
denser substance, which is described in Mr. Clift’s memoir on the
Megatherium as ‘enamel,’ and to which substance in the compound teeth of
the Elephant, it is analogous both in its relative situation, and
relative density to the other constituents.