striated concentric tract on the grinding surface of the teeth; it is of
a yellowish-white colour in the fossil, and stands out, as an obtuse
ridge, from that surface: both these conditions depend on the large
proportion of the mineral to the animal constituent in this substance of
the tooth. The external layer of the cement presents in the fossil the
same yellowish-brown tint as the bone itself, which it so closely
resembles, both in intimate structure and in chemical composition; the
internal layer next the dense ivory is jet black, indicating the great
proportion of animal matter originally present in this part. The central
pillar of coarse ivory, which, from its more yielding texture, has been
worn down into a hollow at the triturating surface of the tooth, also
presents, as a consequence of the less proportion of the hardening
phosphates, a darker brown colour than the external layer of the cement,
or the bone itself.
The teeth are implanted in very deep sockets; about one-sixth only of
the last molar projects above the alveolus; the proportion of the
exposed part of the tooth increases as they are placed further forwards.
The implanted part of each tooth is simple; preserving the same size and
form as the projecting crown, and presenting a large conical cavity at
the base, indicative of the original persistent pulp, and perpetual
growth of these teeth.
The extent of the whole four alveoli is four inches, eight lines; the
length of the jaw from the angle to the broken end of the symphysis is
seventeen inches and a half;[42] from the figures it will be seen that
only a small proportion of the anterior part of the jaw is lost, so that
we may regard the dentigerous part of the jaw as being limited to about
one-fourth of its entire length; the alveoli being nearly equidistant
from the two extremities. The first and second teeth, counting
backwards, are separated by an interspace of rather more than three
lines; that between the second and third is one line less; the third and
fourth are rather more than a line apart: from the oblique position,
however, of the three hinder teeth the intervals between them appear in
a side view, as in fig. 1, Pl. XIX., to be less than in reality, and the
third and fourth teeth seem to touch each other.
Each tooth has a form and size peculiar to itself, and different from
the rest, but corresponds of course with its fellow on the opposite
side. The same may be observed, but in a less degree, in the teeth of
the Megatherium itself; hence, it is obviously hazardous to found a
generic distinction upon a single tooth, unless, as in the case of the
_Glyptodon_,[43] the modification of form happens to be extremely
well-marked. The whole series of teeth, or their sockets, at least of
one of the jaws, should be known for the purpose of making a
satisfactory comparison with the previously established Edentate genera.
The first molar in the present jaw is the smallest and simplest of the
series: its transverse section is ellipsoid, or subovate, narrowest in
front, and somewhat more convex on the outer than on the inner side: the
long diameter of the ellipse is nine lines, the short or transverse
diameter six lines: the length of the tooth may be about three inches,
but I have not deemed it necessary to fracture the alveolus in order to
ascertain precisely this point.
The second tooth presents in transverse section a more irregular and
wider oval figure than the first: the line of the outer side is convex,
but that of the inner side slightly concave, in consequence of the tooth
being traversed longitudinally by a broad and shallow channel or
impression; the longitudinal diameter of the transverse section is one
inch; the transverse diameter at the widest part nine lines. There is a
slight difference in the size of this tooth on the two sides of the jaw,
the right one, from which the above dimensions are taken, being the
largest.
The transverse section of the third tooth has a trapezoidal or
rhomboidal form; the angles are rounded off; the posterior one is most
produced; the anterior and posterior surfaces are flattened, the latter
slightly concave in the middle; the external and internal sides are
concave in the middle, especially the inner side, where the concavity
approaches to the form of an entering notch. The longest diameter of the
transverse section of this tooth is thirteen lines, the shortest seven
lines and a half: in the tooth on the right side the external surface is
nearly flat; this slight difference is not indicated in the figure (Pl.
XVIII.)
The last molar, which is generally the most characteristic in the fossil
_Bruta_, presents in an exaggerated degree the peculiarities of the
preceding tooth; the longitudinal channels on both the outer and inner
surfaces encroach so far upon the substance of the tooth, that the
central coarse ivory substance is as it were squeezed out of the
interspace, and the elevated ridge of the dense ivory describes an
hour-glass figure upon the triturating surface, the connecting isthmus
being but half the breadth of the rest of the tract; the external
cæmentum preserves nearly an equal thickness throughout. Of the two
lobes into which this tooth is divided by the transverse constriction,
the anterior is the largest; their proportions and oblique position are
pretty accurately given in the figure. The longitudinal diameter of the
transverse section of this tooth is one inch, seven lines, its greatest
lateral or transverse diameter is ten lines, its least diameter at the
constricted part is three lines, the length of the entire tooth is four
inches. Judging from the form of the jaw, the length of the other teeth
decreases in a regular ratio to the anterior one. The posterior tooth is
slightly curved, as shown in fig. 2, Pl. XIX., with the concavity
directed towards the outer side of the jaw.
The general form of the horizontal ramus of the jaw, is so well
illustrated in the figures Pl. XVIII. and XIX., that the description may
be brief.
The symphysis is completely anchylosed, about four inches in length, and
extended forward to the extremity of the jaw at a very slight angle with
the inferior border of the ramus: it is of great breadth, smooth and
gently concave internally, and suggests the idea of its adaptation for
the support and gliding movements forwards and backwards of the free
extremity of a long and well-developed tongue.
The exterior surface of the symphysis is characterized by the presence
of two oval mammilloid processes, situated on each side of the middle
line, and about half-way between the anterior and posterior extremes of
the symphysis. A front view of these processes, of the natural size, is
given in fig. 4, Pl. XIX.: a side view of the one on the right side
represented in the reduced figure.
Nearly four inches behind the anterior extremity of the above process is
the large anterior opening of the dental canal: it is five lines in
diameter, situated about one-third of the depth of the ramus of the jaw
from the upper margin. The magnitude of this foramen, which gives
passage to the nerve and artery of the lower lip, indicates that this
part was of large size; and the two symphyseal processes, which probably
were subservient to the attachment of large retractor muscles, denote
the free and extensive motions of such a lip, as we have presumed to
have existed from the size of the foramina destined for the transmission
of its nervous and nutrient organs.
The angle of the jaw is produced backwards, and ends in an obtuse point,
slightly bent upwards; a foramen, one-third less than the anterior one,
leads from near the commencement of the dental canal, to the outer
surface of the jaw, a little below and behind the last molar tooth; this
foramen presents the same size and relative position on both sides of
the jaw. I find no indication of a corresponding foramen, or of
symphyseal processes in the figures or descriptions of the lower jaw of
the Megatherium, nor in the lower jaw of the Sloths, Ant-eaters,
Armadillos, or Manises, which I have had the opportunity of examining
with a view to this comparison.
In the Megatherium the inferior contour of the lower jaw is peculiarly
remarkable, as Cuvier has observed, for the convex prominence or
enlargement which is developed downwards from its middle part. In the
Mylodon the corresponding convexity exists in a very slight degree, not
exceeding that which may be observed at the corresponding part of the
lower jaw of the Ai, or Orycterope. A broad and shallow furrow extends
along the outer side of the jaw, close to the alveolar margin, from the
beginning of the coronoid process to the anterior dental foramen.
The base of the coronoid process begins external and posterior to the
last grinder: the whole of the ascending ramus of the jaw, beneath the
coronoid process is excavated on its inner side by a wide and deep
concavity, bounded below by a well-marked ridge, which extends obliquely
backwards from the posterior part of the alveolus of the last grinder to
the inferior margin of the ascending ramus, which is bent inwards before
it reaches the angle of the jaw.
The large foramen or entry to the dental canal is situated in the
internal concavity of the ascending ramus of the jaw, two inches behind
the last molar, three inches from the lower margin of the ramus, and
nearly five inches from the elevated angle of the jaw: it measures nine
lines in the vertical diameter, and its magnitude indicates the large
size of the vessels which are destined to supply the materials for the
constant renewal of the dental substance,—a substance which from its
texture must be supposed to have been subject to rapid abrasion. About
an inch behind the dental foramen a deep vascular groove, about two
lines in breadth, is continued downwards to the ridge which
circumscribes the internal concavity of this part of the jaw, and
perforates the ridge, which thus arches over the canal: this structure
is present in both rami of the jaw. The mylo-hyoid ridge is distinctly
marked about an inch and a half below the alveolar margin. Other
muscular ridges and irregular eminences are present on the outer side of
the base of the ascending ramus, and near the angle of the jaw; as shown
in fig. 1, Pl. XIX.
From the preceding descriptions it will be seen that the lower jaw of
the _Mylodon_ is very different from that of the _Megatherium_; with
that of the _Megalonyx_ we have at present no means of comparing it.
Among existing Edentata the Mylodon, in the form of the posterior part
and angle of the jaw, holds an intermediate place between the Ai and the
great Armadillo; in the form of the anchylosed symphysis of the lower
jaw it resembles most closely the Unau or two-toed Sloth; but in the
peculiar external configuration of the symphysis resulting from the
mammilloid processes above described, the Mylodon presents a character
which has not hitherto been observed in any other species of _Bruta_,
either recent or fossil.
In conclusion it may be stated, that the teeth and bones here described
offer all the conditions and appearances of those of a full grown
animal; and that they present a marked difference of size as compared
with those of the _Mylodon Harlani_, as will be evident by the following
admeasurements.
ADMEASUREMENTS OF THE LOWER JAW OF MYLODON DARWINII.
Inches. Lines.
Length (as far as complete) 17 6
Extreme width, from the outside of one ramus to that of the other 9 0
Depth of each ramus 4 9
Length of alveolar series 4 8
From first molar to broken end of symphysis 6 0
Breadth of symphysis 3 7
Longitudinal extent of symphysis 4 6
Circumference of narrowest part of each ramus 5 9
DESCRIPTION OF A CONSIDERABLE PART OF THE SKELETON OF A LARGE EDENTATE
MAMMAL, ALLIED TO THE MEGATHERIUM AND ORYCTEROPUS, AND FOR WHICH IS
PROPOSED THE NAME OF
SCELIDOTHERIUM[44] LEPTOCEPHALUM.
Of the large Edentate quadrupeds that once existed in the New World,
sufficient of the osseous remains of the gigantic Megatherium alone has
been transmitted to Europe to give a satisfactory idea of the general
form and proportions of the extinct animal.
Different bones of the Megalonyx, Mylodon, and Glyptodon have been
described, but not sufficient of the remains of any individual of these
subgenera has, hitherto, reached Europe, or been so described as to
enable us to form a comparison between them and the Megatherium, or any
of the existing Edentata, in regard to the general construction and
proportions of the entire skeleton.
This state of our knowledge of the osteology of the singular giants of
the Edentate Order renders the remains of the present animal peculiarly
interesting, since, although the extremities are too imperfect to enable
us to reconstruct the entire skeleton, a sufficient proportion of it has
been preserved in the natural position to give a very satisfactory idea
of its affinities to other Edentata, whose osteology is more completely
known.
The fossil remains here described were discovered by Mr. Darwin in the
same bed of partly consolidated gravel at Punta Alta, Northern
Patagonia, as that in which the lower jaws of the _Toxodon_ and
_Mylodon_ were imbedded. The parts of the skeleton about to be described
were discovered in their natural relative position, as represented at
Pl. XX., indicating, Mr. Darwin observes, that the sublittoral formation
in which they had been originally deposited had been subject to little
disturbance.[45] They include the cranium, nearly entire, with the teeth
and part of the os hyoides; the seven cervical, eight of the dorsal, and
five of the sacral vertebræ, the two scapulæ, left humerus, radius and
ulna, two carpal bones, and an ungueal phalanx; both femora, the
proximal extremities of the left tibia and fibula, and the left
astragalus.
The principal parts of the cranium which are deficient are the anterior
extremities of both the upper and lower jaws, the os frontis, æthmoid
bone, and the whole upper part of the facial division of the skull; but
sufficient remains to show that the general form of the skull resembled
an elongated, slender, subcompressed cone, commencing behind by a
flattened vertical base, slightly expanding to the zygomatic region, and
thence gradually contracting in all its dimensions to the anterior
extremity.
The Cape Ant-eater (_Orycteropus_), of all Edentata, most nearly
resembles the present fossil in the form of its cranium, and next in
this comparison the great Armadillo (_Dasypus gigas_, Cuv.) may be
cited: on the supposition, therefore, that the correspondence with the
above existing Edentals observable in the parts of the fossil cranium
which do exist, was carried out through those which are defective, the
length of the skull of the Scelidothere must have been not less than two
feet. If now the reader will turn to Pl. XX. he will see that this
cranium is singularly small and slender in proportion to the rest of the
skeleton, especially the bulky pelvis and femur, of which bones the
latter has a length of seventeen inches, and a breadth of not less than
nine inches; the astragalus, again, exceeds in bulk that of the largest
Hippopotamus or Rhinoceros; yet the condition of the epiphyseal
extremities of the long bones proves the present fossils to have
belonged to an immature animal. Hence, although the Scelidothere, like
most other Edentals, was of low stature, and, like the Megatherium,
presented a disproportionate development of the hinder parts, it is
probable, that, bulk for bulk, it equalled, when alive, the largest
existing pachyderms, not proboscidian. There is no evidence that it
possessed a tesselated osseous coat of mail.
I shall commence the description of the present skeleton with the
cranium. The condyles of the occiput (See Pl. XXI. fig. 2,) are wide
apart, sub-elliptic, very similar in position, form, and relative size
to those in _Orycteropus_. The foramen occipitale is transversely oval,
its plane slopes from above downwards and forwards at an angle of 40°
with that of the occipital region of the skull. This region, as before
stated, is vertical in position (see fig. 1, Pl. XXI.), of a
sub-semicircular form, the breadth being nearly one-third more than the
height; it is bounded above and laterally by a pretty regular curve; but
the superior margin is not produced so far backwards as in
_Orycteropus_. The occipital plane is bisected by a mesial vertical
ridge; there is a less developed transverse curved intermuscular crest
which runs parallel with and about half an inch below the marginal
ridge: the surface of the occipital plane on the interspaces of these
ridges is irregularly pitted with the impression of the insertion of
powerful muscles. The corresponding surface is smooth in the Orycterope
and Armadillos; in the great extinct Glossothere it resembles in
character that of the Scelidothere; but in the forward slope of the
occipital plane the Glossothere differs in a marked degree from the
present animal.
The upper surface of the cranium is smooth and regularly convex. The
extent of the origin of the temporal muscles is defined by a
slightly-raised broad commencement of a ridge, which, in the older
animal, might become more developed. There is no trace of this ridge in
the Orycterope; but it exists in the Armadillos, in which the teeth are
of a denser texture, and better organized for mastication, and
consequently are associated with better developed masticatory muscles.
It will be subsequently shown that the Scelidothere resembles the
Armadillos in so far as it possesses a greater proportion of the dense
ivory to the external cæmentum in its teeth, than does the Megatherium;
while it differs widely from the Orycterope, in the structure of its
teeth. The teeth, however, are fewer in the Scelidothere than in any
Armadillo, and relatively smaller than in most of the species of that
family. Accordingly we find that the zygomatic arches are relatively
weaker; and in this particular the _Scelidothere_ corresponds with the
Orycterope. The zygomatic process of the temporal commences posteriorly
about an inch and a half from the occipital plane, its origin or base is
extended forwards in a horizontal line fully four inches, where it
terminates as usual in a thin concave edge, as shown on the right side
in Pl. XXII. The free portion of the zygoma, continued forwards from the
outer part of this edge, is a slender sub compressed process, half an
inch in the longest or vertical diameter, and less than three lines in
the transverse; the extremity of this process is broken off; the
opposite extremity of the malar portion of the zygoma is entire, and
obtusely rounded. The bony arch may have been completed by the extension
of the temporal process to the malar one, but the two parts undoubtedly
were not connected together by so extensive a surface as in the
Orycterope. On the other hand, if the zygomatic arch be naturally
incomplete in the Scelidothere, the interspace between the malar and
temporal portions must be relatively much less than in the Sloth or
Ant-eater; for the broken end of the temporal part is separated from the
obtusely rounded apex of the malar process in the present specimen by an
interval of only one inch.
The articular surface (Pl. XXIII., fig. 2) beneath the zygoma for the
lower jaw is flat and even, with the outer and inner margin slightly
bent down, but having no definable anterior or posterior limits; its
breadth is two inches. It differs from the corresponding surface in the
Orycterope in being separated by a relatively wider interval from the
tympanic bone, and in wanting consequently the support which the bony
meatus auditorius gives in the Orycterope to the back part of the
mandibular joint. The Armadillos differ still more from the Scelidothere
in this important part of the cranial organization, inasmuch as the
glenoid cavity is not only protected behind by the descending os
tympanicum, but also in front by a corresponding vertical downward
extension of the os malæ. The Scelidothere in the general form and
relative position of the surface for the articulation of the lower jaw
resembles the Glossothere more closely than any other Edentate animal
with which I have been able to compare it.
The malar bone of the Megatherium presents, as is well known, two
characters, in which it conspicuously differs from that of the
Orycterope and Armadillos, and approximates in an equally marked degree
to the Sloths; these characters consist in a process ascending as if to
complete the posterior circumference of the orbit, and another process
descending outside the lower jaw to give advantageous and augmented
surface of attachment to the masseteric muscle, in its character of a
protractor of the jaw. Now both these modifications of the malar bone
are present in the Scelidothere, and are the chief if not the sole marks
of the affinity to the Megatherium which the structure of the cranium
affords. They are, however, the more interesting, perhaps, on that
account, and because they are associated with other and more numerous
characters approximating the species in question to the ordinary
terrestrial as distinguished from the arboreal Edentata. For if the
Scelidothere, instead of the Megathere, had been discovered half a
century ago, and if its true nature and affinities had been in like
manner elucidated by the genius and science of a Cuvier; and supposing
on the other hand that the Megatherium instead of the Scelidothere had
been one of the novel and interesting fruits of Mr. Darwin’s recent
exploration of the coast of South America, then the affinities of the
Megathere with the Sloths would undoubtedly have been viewed from a
truer point than at the time when,—the Scelidothere, and analogous
transitional forms, being unknown,—it was regarded as a gigantic Sloth.
Having indicated the principal characters of the cranium of the
Scelidothere, which determine its affinities amongst the _Edentata_,
there next remains to be considered the relative position, extent, and
connections, of the different bones composing the cranium.
The occipital bone constitutes the whole of the posterior, the usual
proportion of the inferior, and a small part of the upper and lateral
portions of the cranial cavity: there is a small descending ex-occipital
process immediately exterior to the condyle: above this part the
occipital bone is articulated to the mastoid process of the temporal,
and the supra-occipital plate is joined by a complex dentated lambdoidal
suture to the two parietals, without the intervention of interparietal
or Wormian bones; the course and form of the lambdoidal suture is shown
in Pl. XXII.; it has the same relative position as in the Orycterope; in
the Armadillos, the suture runs along the angle between the posterior
and superior surfaces of the skull. The thickness of the occipital bone,
at this angle, in the Scelidothere, exceeds an inch, and its texture
consists of a close massive diploë, between the dense outer and inner
tables, (Pl. XXIII. fig. 1.)
The squamous portion of the temporal bone has a very slight elevation,
not extending upon the side of the cranium more than half an inch above
the zygoma; it is thus relatively lower than in the _Orycteropus_; but
is similarly bounded above by an almost straight line, (Pl. XXI., fig.
1.). The mastoid process is small, compressed, with a rounded contour;
immediately internal to it is a very deep depression, corresponding to
that for the digastric muscle. But the most interesting features in this
region of the temporal bone consist in the free condition of the
tympanic bones, and the presence of a semicircular pit, immediately
behind the tympanic bone for the articulation of the styloid element of
the hyoid or tongue-bone: in these points we trace a most remarkable
correspondence with the Glossothere, and in the separate tympanic bone
the same affinity to the Orycteropus, as has been already noticed in the
more bulky extinct Edental.