generic relationship between the Glossothere and Scelidothere: now it
may first be remarked that the styloid articular depression is
relatively much larger and much deeper in the Glossothere than in the
Scelidothere; in the former its diameter equals, as we have seen, one
inch; in the Scelidothere it measures only a third of an inch, the whole
cranium being about two-fifths smaller; if we turn next to the anterior
condyloid foramina, which in the Scelidothere are double on each side,
we obtain from them evidence that the muscular nerve of the tongue could
only have been one-third the size of that of the Glossothere. These
proofs of the superior relative development of the tongue in the
Glossothere indicate a difference of habits, and a modification,
probably, of the structure of the locomotive extremities; and when we
associate these deviations from the Scelidothere, with the known
difference in the position of the occipital plane, which in the
Glossothere corresponds with that in the _Myrmecophaga_ and _Bradypus_,
we shall be justified in continuing to regard them, until evidence to
the contrary be obtained, as belonging to distinct genera.
The parietal bones present an oblong regular quadrate figure, the
sagittal suture running parallel with the squamous, and the frontal with
the lambdoidal suture; there is scarcely any trace of denticulations in
the sagittal suture; the bones are of remarkable thickness, varying, at
this suture, from six to nine lines, and their opposed surfaces are
locked together by narrow ridges, which slightly radiate from the lower
to the upper part of the uniting surface: the substance of the bone
consists of an uniform and pretty dense diploë; and there are no sinuses
developed in it. We can hardly regard the extraordinary air-cells which
occupy the interspace of the two tables of the skull in the parietal and
occipital bones of the Glossothere (Pl. XVI., fig. 3) as a difference
depending merely on age.
The frontal and æthmoid bones are broken away in the present cranium.
The sphenoid commences two inches in front of the foramen occipitale;
the fractured state of the skull does not allow its anterior or lateral
limits to be accurately defined; its body is occupied with large
air-sinuses; the only part, indeed, of this bone which is exposed to
observation is that which forms part of the floor of the cranium; and
this we shall now proceed to describe, in connexion with the other
peculiarities of the cranial cavity, (fig. 1. Pl. XXIII.) The body of
the sphenoid is impressed on its cranial surface with a broad and
shallow sella turcica (_a_), bounded by two grooves, (_b_ _b_,) leading
forwards and inwards from the carotid foramina (_c_); the line of suture
between the sphenoid and occipital bones runs along a slight transverse
elevation (_d_), which bounds the sella posteriorly; this suture is
partially obliterated: a slight median protuberance (_e_) bounds the
sella turcica anteriorly; there are neither anterior nor posterior
clinoid processes. External to the carotid channel there is a wide
groove (_f_) leading to the foramen ovale (_g_); this foramen is about
one-third smaller than in the Glossothere, and therefore, as compared
with the anterior condyloid foramina, indicates that the tongue was
endowed with a greater proportion of sensitive than motive power in the
Scelidothere: but in reasoning on the size of this nerve, it must be
remembered that in both animals certain branches, both of the second and
third divisions of the fifth pair of nerves, are to be associated with
the persistence of large dental pulps, of which they regulate the
secreting power. Anterior to the foramen ovale, and at the termination
of the same large common groove, lodging the trunk of the fifth pair of
nerves is the foramen rotundum (_h_); this leads to a very long canal,
the diameter of which is five lines, being somewhat less than that for
the third division of the fifth pair. The anterior sphenoid is broken
away, so that no observation can be made on the optic foramina.
The basilar process of the occipital bone is perforated at its middle by
two small foramina (_i_) on the same transverse line, about half an inch
apart.
In the Armadillo these foramina do not exist: in the Orycterope they are
present, but open beneath an overhanging ridge, which is continued from
them to the upper part of the anterior condyloid foramen on each side.
The sella turcica of the Orycterope is deeper and narrower than in the
Scelidothere; and is separated from the basilar occipital process by a
transverse ridge, which sends forward two short clinoid processes; two
smaller anterior clinoid processes project backwards from the angle of
the anterior boundary of the sella turcica. The foramina ovalia and
rotunda open in the same continuous groove, as in the Glossothere and
Scelidothere, but they are relatively wider apart; and the canal for the
third division of the fifth pair is shorter, and runs more directly
outwards.
The petrous bone in the Scelidothere is relatively larger than in the
Glossothere, but this probably arises from the precocious development of
the organ of hearing in the present immature specimen in obedience to
the general law. The trunk of the fifth pair of nerves does not impress
it with so deep and well defined a groove as in the Glossothere; the
elliptic internal auditory foramen (_k_) is situated about the middle of
the posterior surface; behind this is the aqueductus vestibuli; and
immediately posterior to the petrous bone is the foramen jugulare (_l_):
the shape of the os petrosum agrees more with that of the Armadillo than
with that of the Orycterope. An accidental fracture of the right os
petrosum demonstrates its usual dense and brittle texture, and at the
same time has exposed the cochlea with part of its delicate and
beautiful lamina spiralis. The conservation of parts of the organs of
vision in certain fossils, has given rise to arguments which prove that
the laws of light were the same at remote epochs of the earth’s history
as now; and the structures I have just mentioned, in like manner,
demonstrate that the laws of acoustics have not changed, and that the
extinct giants of a former race of quadrupeds were endowed with the same
exquisite mechanism for appreciating the vibrations of sound as their
existing congeners enjoy at the present day.
The brain, being regulated in its development by laws analogous to those
which govern the early perfection of the organ of hearing, appears to
have been relatively larger in the Scelidothere than in the Glossothere:
it was certainly relatively longer; the fractured cranium gives us six
inches of the antero-posterior diameter of the brain, but the analogy of
the Orycterope would lead to the inference that it extended further into
the part which is broken away. The greatest transverse diameter of the
cranial cavity is four inches eight lines: these dimensions, however,
are sufficient to show that the brain was of very small relative size in
the Scelidothere; and, both in this respect, and in the relative
position of its principal masses, the brain of the extinct Edental
closely accords with the general character of this organ in the existing
species of the same Order. We perceive by the obtuse ridge continued
obliquely upwards from above the upper edge of the petrous bone, that
the cerebellum has been situated wholly behind the cerebrum, we learn
also from the same structure of the enduring parts that these perishable
masses were not divided, as in the Manis, by a bony septum, but by a
membranous tentorium, as in the Glossothere and Armadillos: in the
Orycteropus, as has been before remarked, there is a strong, sharp, bony
ridge extending into each side of the tentorium. The vertical diameter
of the cerebellum and medulla oblongata equals that of the cerebrum, and
is two inches three lines: the transverse diameter of the cerebellum was
about three inches nine lines; its antero-posterior extent about one
inch and a half. The sculpturing of the internal surface of the cranial
cavity bespeaks the high vascularity of the soft parts which it
contained, and there are evident indications that the upper and lateral
surfaces of the brain had been disposed in a few simple parallel
longitudinal convolutions. The two anterior condyloid foramina (_m_)
have the same relative position as the single corresponding foramen in
the Glossothere, Orycterope, and Armadillos, and the inner surface of
the skull slopes outwards from these foramina to the inner margin of the
occipital condyle.
Of the bones of the face there remain only portions of the malar,
lachrymal, palatine, and maxillaries. The chief peculiarities of the
malar bone have been already noticed: the breadth of the base of the
descending masseteric processes is two inches two lines; its termination
is broken off: the length of the ascending post-orbital process of the
malar cannot be determined from the same cause, but it is fortunate that
sufficient of this part of the cranium should have been preserved to
give this evidence of the affinities of the Scelidothere to the
Megathere. The malar bone is continued anteriorly, in a regular curve
forwards and upwards, to the lachrymal bone, and completes, with it, the
anterior boundary of the orbit: the size of the orbit is relatively
smaller than in the Orycterope, and still less than in the Ant-eaters:
here, however, we have merely an exemplification of the general law
which regulates the relative size of the eye to the body in the
mammalia. The malar bone does not extend so far forwards in front of the
orbit as in either the Orycterope or Armadillo; in the inclination,
however, with which the sides of the face converge forwards from the
orbits, the Scelidothere holds an intermediate place between the
Armadillos and Orycterope.
The lachrymal bone does not extend so far upon the face in the
Scelidothere as in the Orycterope; in which respect the Scelidothere
resembles more the Megathere. The foramen for the exit of the
infra-orbital nerve has the same situation near the orbit as in the
Megathere; its absolute distance from the anterior border of the orbit
is only half that in the Orycterope. The foramen is single in the
Scelidothere, as in the Orycterope; in the Megathere there are two or
three ant-orbital foramina. The vertical diameter of this foramen is
eight lines, the transverse diameter four lines. So much of the outer
surface of the superior maxillary bones as has been preserved, is smooth
and vertical. Each superior maxillary bone contains the sockets of five
teeth, occupying an antero-posterior extent of three inches seven lines,
(Pl. XXII. and XXIII. fig. 3). The posterior alveolus is situated just
behind the transverse line, extending across the anterior boundary of
the orbits; the remaining sockets of the molar series extend forwards
three inches in front of the orbits. In the Megatherium, the roots of
the five superior molars are all situated behind the anterior boundary
of the orbit: in the Orycteropus, on the contrary, the grinders are all
placed in advance of the orbit; so that the Scelidothere resembles that
species more than the Megathere in the relative location of the teeth.
The palatal interspace between the roots of the last molar tooth of each
series is eleven lines; the palate gradually though slightly widens, as
it advances forwards: the posterior margin of the palate is terminated
by an acute-angled notch. In the breadth of the bony palate the
Scelidothere is intermediate between the Megathere and Orycterope.
The anterior of the upper molars is represented at fig. 3, 4, and 5, Pl.
XXI., and at 1, fig. 3, Pl. XXIII.; it corresponds closely in form and
size with the opposite molar below; the base of the triangle given by
its transverse section is turned inwards and obliquely forwards.
The second molar of the upper jaw, also presents in transverse section a
triangular form, with the angles rounded off; but the inner side of the
tooth is traversed by a longitudinal groove. The largest diameter of the
transverse section, which is placed obliquely as regards the axis of the
skull, measures ten lines and a half; the opposite diameter of the tooth
is six lines.
The third and fourth molars present the same form and size, and relative
position as the second.
The fifth molar is the smallest of the series; its transverse section
gives an inequilateral triangle, with the corners rounded off; the
broadest side is turned outwards, and is slightly concave; the
antero-posterior diameter of this tooth is seven lines; the transverse
four lines. The length of the teeth in the upper jaw is about two inches
and a half.
It is almost superfluous to observe that the teeth of the Scelidothere,
as in other _Bruta_, are without fangs, and have their inserted base
excavated by large conical cavities, for the lodgment of a persistent
pulp. The tooth is composed of a small central body of coarse ivory or
‘dentine,’ traversed by medullary canals, which at the periphery of the
coarse dentine anastomose by loops, from the convexity of which the
calcigerous tubes are given off which form the fine dentine: the layer
of this substance, which immediately surrounds the coarse dentine, is
about one line and a half in thickness, and the whole is invested with a
very thin coating of cement. The teeth of the Scelidothere thus present
a more resisting structure than do those of the Mylodon; having a larger
proportion of the dense ivory composed of the minute calcigerous tubes,
and a much smaller proportion of the softer external cæmentum; in this
respect the Scelidothere recedes farther from Megathere, and approaches
nearer the Armadillos than does the Mylodon.
The lower jaw resembles, in the general form of the posterior moiety
which is here preserved, that of the Sloth and Mylodon more than that of
any other Edentate species. Its deep posterior angle is produced
backwards, and a broad coronoid process rises and nearly fills the
zygomatic space; the condyle is flat, as the glenoid surface has already
indicated; its transverse diameter is an inch and eight lines; its
antero-posterior diameter seven lines: it is principally extended
inwards beyond the vertical line of the ascending ramus. The lower
contour of the jaw describes an undulating line; which, commencing from
the posterior angle, is at first gently convex, then slightly concave,
then again convex, below the alveoli of the teeth, where it is rounded
and expanded, as in the Orycterope. The fractured condition of the right
ramus of this part fortunately exposed the roots of the four grinding
teeth, which constitute the dental series on each side of the lower jaw.
The length of the jaw occupied by these four alveoli is three inches ten
lines, which exceeds a little that of the opposed five grinders above;
the ramus of 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 of the alveoli, whence it gradually inclines to the
outer wall of the jaw.
The whole ascending ramus of the jaw consists of a very thin plate of
bone; it is slightly concave on the inner side, and the inferior margin
of 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 of the part. There
is a foramen on the outer side of the ramus at the anterior part of the
base of the coronoid process corresponding with that in the lower jaw of
the Mylodon, but the longitudinal channel which runs along the outer
side of the alveolar processes is wanting, and the expansion at the base
of those processes is more sudden and relatively greater; the general
correspondence, however, between these lower jaws is such as would lead
to the idea that they belonged to animals of the same genus, were it not
that the teeth present modifications of form in the Scelidothere, as
distinct from those of the Mylodon, as are any of the minor dental
differences on which genera or subgenera of existing Mammalia are
founded in the present state of Zoological Classification.
To make this distinction more readily intelligible, I have given a view
of the transverse section of the teeth in the right ramus of the lower
jaw (fig. 4, Pl. XXIII.), corresponding with that of the _Mylodon
Darwinii_, (Pl. XVII., fig. 5). In the present subgenus the
antero-posterior extent of the four alveoli of 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 of 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 sides are also
slightly concave.
The second molar is placed more obliquely in the jaw; the long axis of
its transverse section intersects at an acute angle that of the jaw
itself; the transverse section presents a compressed or oblong form,
with the larger end next the outer side, and the smaller end next the
inner side of the jaw; this end is simply rounded, but the outer end
presents a sinuosity, corresponding to a broad groove which traverses
the whole length of the outer side of the tooth; the anterior, which
corresponds to the internal side or base of the transverse section of
the preceding molar, is slightly concave.
The third molar has nearly the same form and relative position as the
preceding; the long diameter of 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 of the
Mylodon is well-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 of the same kind;
the transverse section gives an irregular oblong figure with its axis
nearly parallel with that of 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 of the tooth; the latter groove
is much wider and shallower 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 of convex as in
the Mylodon; the posterior one is more compressed; the longitudinal or
antero-posterior diameter of the transverse section of this tooth is one
inch five lines; the greatest transverse diameter is nine lines; the
diameter of the isthmus joining the lobes is three lines and a half; the
entire length of this tooth is three inches three lines.[46]
VERTEBRAL COLUMN.
Of this part of the skeleton of the Scelidothere, Mr. Darwin’s specimen
includes, as is represented in Plate XX., the cervical, part of the
dorsal, and the sacral series of vertebræ in a more or less perfect
condition.
The cervical vertebræ present the ordinary mammalian number, seven, and
are free, or so articulated as to have permitted reciprocal movement
upon each other. Their transverse processes are perforated as usual for
the vertebral arteries. These processes in the atlas are remarkable for
their great breadth, length, and thickness; and indicate the muscular
forces which must have worked the head upon the spine to have been very
powerful. The axis is provided with a robust ‘processus dentatus,’
having a base equal in breadth to the body of the axis itself; and a
smooth articular convexity on the side of the apex on which the ring of
the atlas rotated. The line of union between the axis and its
characteristic process, which here resembles the body of an abortive
vertebra, is very distinct. The transverse processes of the vertebra
dentata are comparatively feeble, but this condition is amply
compensated for by the great development of the spinous process. (Pl.
XXIV. fig. 1.) This process is bent backwards at nearly a right angle,
overlaps with its reflected extremity the spine of the third cervical
vertebra, and rests by its base, on the under part of which are the
posterior articular surfaces, upon the broad and strong anterior oblique
processes of the third vertebra.
The third, fourth, fifth, and sixth cervical vertebræ have moderately
developed and pointed spinous processes: their transverse processes are
broad, and extend obliquely backwards, and slightly overlap each other.
On the under part of the transverse process of the sixth cervical
vertebra there is the fractured base of what I conjecture to have been
an expanded aliform plate, analogous to that observable in the
corresponding vertebra of the Orycterope. The seventh cervical vertebra
has part of the articular depression for the head of the first rib upon
each side of its body: the transverse process is feebly developed, but
the spine is double the height and size of those of the preceding
vertebræ.
The spinous process of the first dorsal vertebra in like manner rises to
twice the height of the preceding spine of the seventh cervical, and
preserves an equal antero-posterior diameter from its base to its
summit, which is thick and slightly bent backwards: four or five
succeeding dorsal vertebræ give evidence of having been surmounted by
spines of equal height and strength. The transverse processes of these
dorsal vertebræ present bold concavities on their inferior part for the
reception of the tubercles of the ribs, and they gradually ascend upon
the base of the spines as the vertebræ are placed further back, so as to
increase the expansiveness of the chest. The state of the fossil did not
afford further information as to the condition of this part of the
vertebral column, but the parts which have been preserved are precisely
those from which the most interesting inferences as to the affinities
and habits of the extinct quadruped can be deduced.
Whether the Megatherium be most nearly allied to the tribes of the Sloth
or Armadillo has been a question under recent discussion, and, as a
corollary of this problem, whether its habits were those of a scansorial
or of a fossorial quadruped. For, strange as it may appear at first
sight, there have not been wanting arguments, and those urged by an
anatomist to whom we owe much novel and interesting information
respecting the extinct Edentata, in support of the belief that the
Megatherium, gigantic and ponderous as must have been its frame,
actually climbed trees like a Sloth, and had claws and feet organised
for prehensile actions, and not in accordance with that type by which
they are usually adapted for digging up the soil.[47]
Now, in whatever degree the Megatherium may be involved in this
question, the smaller Megatherioid species at present under
consideration must be at least equally implicated in it. In the
adaptation of the frame of a mammiferous quadruped for especial and
peculiar actions and modes of life, such as for climbing and living in
trees, or for burrowing and seeking concealment in the earth, not only
the immediate instruments, as the feet, are modified, but the whole of
the osseous and muscular fabric is more or less impressed with
corresponding adaptations, whilst at the same time these special
adjustments are invariably subordinated to the type of organization
which characterizes the group.
The type of the order _Bruta_ or _Edentata_ is well-marked; one or more
claws of unusual length and strength, characterize the fore-feet and
sometimes the hind-feet in every genus, and the term ‘Macronykia’ would
more aptly designate them than the term which Cuvier substituted for the
good old Linnean appellation. The uniform absence of true roots to the
teeth, where these are present, is another general character; the
skeleton exhibits many well-marked peculiarities common to the whole
order; while at the same time it is modified in various modes and
degrees in accordance with the peculiar habits and exigencies of the
species.