slopes forward, at a less obtuse angle with the basal plane, to the
first named ridge which separates the occipital from the coronal or
superior surface of the skull. The contour of this surface runs
forwards, as far as the fragment extends, in an almost straight line:
the extent of surface between the temporal muscular ridges must have
been about five inches posteriorly, but it decreases gradually as it
extends forwards: all that part which is preserved is quite smooth. The
attachment of the fasciculi of the temporal muscle, and the convergence
of their fibres as they passed through the zygoma are well-marked on the
sculptured surface of the bone. The zygomatic process is relatively
stouter than in _Orycteropus_: it is prismatic: the external facet is
nearly plane: the superior is concave, and increases in breadth
anteriorly: the inferior surface offers a slight convexity behind the
flattened articular surface for the lower jaw. The margin of the zygoma
formed by the meeting of the upper and lower facets presents a
semicircular curve, extended transversely from the cranium, and directed
forwards.
The anterior extremity is obliquely truncated from below upwards and
forwards, and presents a flattened triangular surface indicative of its
junction with an os malæ: the space between this extremity and the side
of the cranium measures one inch and nine lines across, and thus gives
us the thickness of the temporal muscle. The distance from the origin of
the zygoma to the occipital plane is relatively greater than in
_Orycteropus_; _Glossotherium_ is in this respect more similar to
_Myrmecophaga_ and _Manis_.
The sphenoid bone forms a somewhat smooth protuberance below and behind
the base of the _zygoma_. The tympanic bone is wedged in between this
protuberance in front, and the mastoid process behind. The chief
peculiarity of the broad mastoid is the regular semicircular cavity at
its under part for the articulation of the styloid bone of the tongue.
This depression is separated below by a broad rough protuberance from
the foramen jugulare, (_f_, fig. 2, Pl. XVI,) which is immediately
external to, and slightly in advance of the great foramen condyloideum,
_c_. A small rugged portion of the os petrosum separates the jugular
from the carotid canal, which arches upwards and directly inwards to the
side of the shallow sella turcica, (the external and internal orifices
of the carotid canal are shown at _g_, figs. 2 and 3). The chief
protuberance on the basis cranii is a large and rugged one, serving for
the attachment of muscles, and due chiefly to the expansion of a great
sinus in the body of the sphenoid. This protuberance is separated from
the smaller sphenoid protuberance before mentioned by a large groove
continued downwards and forwards from the tympanic cavity, and
containing the Eustachian tube, which does not traverse a complete
osseous canal. Immediately internal to the glenoid cavity is the large
orifice of the canal transmitting the third division of the fifth pair
of nerves, the principal branch of which endows the tongue with
sensibility; this foramen (_h_, fig. 2) is rather less than that for the
muscular nerve of the tongue.
The internal surface of the present cranial fragment affords a very
satisfactory idea of the size and shape of the brain of the extinct
species to which it belongs. It is evident that, as in other Bruta, the
cerebellum must have been almost entirely exposed behind the cerebrum;
and that the latter was of small relative size, not exceeding that of
the Ass; and chiefly remarkable, as in the Orycterope, Ant-eater, and
Armadillo for the great development of the olfactory ganglia. The
antero-posterior extent of the cribriform plate, as exposed in this
fragment, is three inches, and the complication of the œthmoid olfactory
lamellæ which radiate from it into the nasal cavity is equal to that
which exists in the smaller Edentata (fig. 3, Pl. XVI). The nasal cavity
is complicated in _Glossotherium_ by the great number and capacious size
of the air-cells which are in communication with it: these extend over
all the upper, lateral, and back parts of the cranial cavity, as far
even as the upper boundary of the foramen magnum: they also occupy the
anterior two-thirds of the basis cranii. The external configuration of
the skull would, therefore, afford a very inadequate or rather deceptive
notion of the capacity of the cerebral cavity, were not the existence
and magnitude of these sinuses known. The interspace of the outer and
inner tables of the cranium are separated above the origins of the
olfactory ganglia for the extent of three inches: above the middle of
the cerebrum they are an inch and a half apart; at the sides of the
cranium the interposed air-cells are from one to two inches across; at
the back part of the cranium about one inch. The sinuses have generally
a rounded form.
The foramen rotundum, (through which in figure 3 a probe is represented
as passing), and the foramen ovale are situated close together, within a
common transversely oblong depression (_i_). The carotid canal (_g_)
opens into the outer side of the commencement of this wide channel,
which conducts the great fifth pair of nerves to the outlets of its two
chief divisions.
The petrous bone projects into the cranial cavity, in the form of an
angular process with three facets: the foramen auditorium internum
(_k_), and the aqueductus vestibuli, are situated on the posterior
facet. Immediately behind the os petrosum is the foramen lacerum
jugulare (_l_), situated at the point of convergence of the vertical
groove of the lateral sinus, with a groove of similar size continued
forwards from above the anterior condyloid canal. The plane of the
internal opening of this canal (_c_, fig. 3) is directed obliquely
inwards and backwards, and the lateral wall of the foramen magnum behind
the foramen condyloideum slopes outwards to the edge of the condyle.
Immediately internal to the foramen condyloideum is a small vascular
foramen conducting a branch of the basilar artery into the condyloid
canal, for the nourishment, doubtless, of the great lingual nerve.
In the relations of the plane of the internal orifice of the anterior
condyloid foramen with that of the foramen magnum, we search in vain for
a corresponding structure in any of the Mammiferous orders, save the
Edentata:[30] and among these the Orycterope comes nearest the
Glossothere in this respect. In the degree of development of the
internal osseous ridge giving attachment to the tentorium cerebelli, the
Ant-eaters and Armadillos more resemble the Glossothere than does the
Orycterope; in which a continuous bony plate arches across the cranial
cavity: in the Manis a still greater proportion of the tentorium is
ossified, and it consequently recedes the furthest amongst the Edentata,
in this, as in most other particulars of the cranial organization, from
the Glossothere. The chief distinctive peculiarity in the cranium of the
Glossothere, so far as it can be studied in the present fragment, and
compared with that of other Edentata, is the deep, well-marked,
semicircular styloid depression, above described.
A question may arise after perusing the preceding evidence, upon which
the present fossil is referred to a great Edentate species nearly allied
to the _Orycteropus_, whether one or other of the lower jaws,
subsequently to be described, and in like manner referable, from their
dentition, either to the _Orycteropodoid_ or _Dasypodoid_ families of
Edentata, may not have belonged to the same species as does the present
mutilated cranium. I can only answer, that those jaws were discovered by
Mr. Darwin in a different and very remote locality,—that no fragments or
teeth referable to them were found associated with the present fossil;
and that, as it would be, therefore, impossible to determine from the
evidence we have now before us, which of the two lower jaws should be
associated with _Glossotherium_; and as both may with equal if not
greater probability belong to a totally distinct genus, it appears to me
to be preferable, both in regard to the advancement of our knowledge of
these most interesting Edentata of an ancient world, as well as for the
convenience of their description, to assign to them, for the present,
distinct generic appellations.
The figures in Plate XVI. preclude the necessity of a table of
admeasurements of the cranial fragment of _Glossotherium_.
DESCRIPTION OF A MUTILATED LOWER JAW AND TEETH, ON WHICH IS FOUNDED A
SUBGENUS OF MEGATHERIOID EDENTATA, UNDER THE NAME OF
MYLODON.
The genus _Megalonyx_, as is well known, owes its name and the discovery
of the fossil remains on which it was founded, to the celebrated
Jefferson,[31] formerly President of the United States. Cuvier, from an
examination of a single tooth, and the casts of certain bones of the
extremities, especially the terminal ones, determined the ordinal
affinities of this remarkable extinct quadruped.[32] But while he
retained, the name of _Megalonyx_, and used it in a generic sense,
Cuvier offered no characters whereby other fossil remains might be
generically either distinguished from, or identified with the _Megalonyx
Jeffersonii_, unless, among such remains there happened to be a tooth,
or a claw exactly corresponding with the descriptions and figures in the
_Ossemens Fossiles_; and when, of course, a specific identity, and not
merely a generic relationship would be established.
The greater part of Cuvier’s chapter on _Megalonyx_ is devoted to the
beautiful and justly celebrated reasoning on the ungueal phalanx,
whereby it is proved to belong, not to a gigantic Carnivore of the
Lion-kind, as Jefferson supposed, but to the less formidable order of
Edentate quadrupeds; and Cuvier, in reference to the tooth,—the part on
which alone a generic character could have been founded,—merely observes
that it resembles at least as much the teeth of one of the great
Armadillos, as it does those of the Sloths.[33]
In the last edition of the _Rêgne Animal_, Cuvier introduces the
_Megatherium_ and _Megalonyx_, between the Sloths and Armadillos; but
alludes to no other difference between the two genera than that of
size,—“l’autre, le _Megalonyx_, est un peu moindre.” (p. 226.) Some
systematic naturalists, as Desmarest, and Fischer, have, therefore,
suppressed the genus, and made the _Megalonyx_ a species of
_Megatherium_ under the name of _Megatherium Jeffersonii_. The dental
characters of the genus _Megatherium_ are laid down by Fischer[34] as
follows:—“_Dent. prim, et lan. ⁰⁄₀., molar es ⁴⁄₄⁴⁄₄, obducti, tritores,
coronide nunc planâ transversim sulcatâ nunc medio excavatâ marginibus
prominulis._” That _Megalonyx_ had the same number of molares as
_Megatherium_, (supposing that number in the Megathere to be correctly
stated, which it is not,) is here assumed from analogy, for neither
Jefferson, Wistar, nor Cuvier,—the authorities for _Megalonyx_ quoted by
Fischer—possessed other means of knowing the dentition of that animal
than were afforded by the fragment of a single tooth.
Now the almost entire lower jaw about to be described offers, in so far
as respects the general form and structure of the teeth, the same kind
and degree of correspondence with the _Megatherium_, as does the
_Megalonyx Jeffersonii_ of Cuvier: and, what is only probable in that
species, is here certain, viz., an agreement with the Megatherium in the
class, viz. _molares_, to which the teeth exclusively belong. The
question, therefore, on which I find myself, in the outset, called upon
to come to a decision is, as to the preference of the mode of viewing
the subject of the generic relationship of the _Megalonyx_ adopted by
Desmarest, Fischer, &c., or of that, on which Cuvier, and after him Dr.
Harlan, have practically acted: whether, in short, the genus
_Megatherium_ is to rest upon the more comprehensive characters of kind
and general structure of the teeth, or upon the more restricted ones, of
form and such modifications in the disposition and proportions of the
component textures of the tooth, as give rise to the characteristic
appearances of the triturating surface of the crown.
With respect to existing Mammalia, most naturalists of the present day
seem to be unanimous as to the convenience at least of founding a
generic or subgeneric distinction on well-marked modifications in the
form and structure of the teeth, although they may correspond in number
and kind, in proof of which it needs only to peruse the pages of a
_Systema Mammalium_ which relate to the distribution of the Rodent
Order. According to this mode of viewing the logical abstractions under
which species are grouped together, the extinct Edentate Mammal
discovered by Jefferson must be referred to a genus distinct from
_Megatherium_, and for which the term _Megalonyx_ should be retained.
This will be sufficiently evident by comparing the descriptions given by
Cuvier of one of the teeth of the _Megalonyx Jeffersonii_, and by Dr.
Harlan of a tooth of his _Megalonyx laqueatus_, with those of the
_Megatherium_ which have been published by Mr. Clift. The fragment of
the molar tooth of the _Megalonyx Jeffersonii_, described and figured in
the _Ossemens Fossiles_, seems to have been implanted in the jaw, like
the teeth of the _Megatherium_, by a simple hollow base similar in form
and size to the protruded crown: its structure Cuvier describes as
consisting of a central cylinder of bone enveloped in a sheath of
enamel.[35] The transverse section of this tooth presents an irregular
elliptical form, the external contour being gently and uniformly convex,
the internal one, undulating; convex in the middle, and slightly concave
on each side, arising from the tooth being traversed longitudinally on
its inner side by two wide and shallow depressions.
The imperfect tooth of the species called by Dr. Harlan _Megalonyx
laqueatus_, and of which a cast was presented by that able and
industrious naturalist to the Museum of the Royal College of Surgeons,
resembles in general form, and especially in the characteristic double
longitudinal groove on the inner side, the tooth of the _Megalonyx
Jeffersonii_. It is thus described by Dr. Harlan:
“The fractured molar tooth appears to have belonged to the inferior
maxilla on the right side; the crown is destroyed; a part of the cavity
of the root remains. The body is compressed transversely, and presents a
double curvature, which renders its anterior and exterior aspects
slightly convex; the posterior and interior gently concave; these
surfaces are all uniform, with the exception of the interior or mesial
aspect, which presents a longitudinal rib or ridge, one-half the
thickness of the long diameter of the tooth; with a broad, not profound
longitudinal groove or channel along each of its borders. It is from
this resemblance to a portion of a fluted column, that the animal takes
its specific appellation (_Meg^x. laqueatus_).
“The crown would resemble an irregular ellipsis widest at the anterior
portion. The tooth consists of a central pillar of bone surrounded with
enamel, the former of a dead white, the latter of a ferruginous brown
colour: the transverse diameter is more than two-thirds less than its
length, whilst that of _Meg^x. Jeffersonii_ is only one-third less—the
antero-posterior diameter is one-half its length in the former, and
two-thirds less in the latter. The proportions of this tooth are
consequently totally at variance with that of its kindred species.”
[Vide Pl. XII. fig. 7, 8, 9.][36]
Dr. Harlan describes also two claws of the fore-foot, a radius, humerus,
scapula, one rib, an os calcis, a metacarpal bone, certain vertebræ, a
femur, and tibia, of the same _Megalonyx_; these parts of the skeleton,
together with the tooth, which so fortunately served to establish the
generic relationship of the species with the _Megalonyx_ of Jefferson
and Cuvier, were discovered in Big-bone-cave, Tennessee, United States.
Dr. Harlan does not enter into the question of the generic characters of
_Megalonyx_, but it would seem that he felt them to rest not entirely on
dental modifications, for he observes that “a minute examination of the
tooth and knee-joint renders it not improbable, supposing the last named
character to be peculiar to it, that if the whole frame should hereafter
be discovered, it may even claim a generic distinction, in which case,
either Aulaxodon, or PLEURODON, would not be an inappropriate name.”[37]
There can be no doubt, as it appears to me, with respect to a fossil jaw
presenting teeth in the same number, and of the same general structure,
as in the _Megatherium_, and with individual modifications of form, as
well-marked as those which distinguish _Megatherium_ from _Megalonyx_,
that the Palæontologist has no other choice than to refer it, either as
Fischer has done with _Megalonyx_, to a distinct species of the genus
_Megatherium_, or to regard it as the type of a subgenus distinct from
both. With reference, however, to the _Pleurodon_ of Dr. Harlan, after a
detailed comparison of the cast of the tooth on which that genus is
mainly founded, with the descriptions and figures of the tooth of the
_Megalonyx Jeffersonii_, in the “Ossemens Fossiles,” they seem to differ
in so slight a degree as to warrant only a specific distinction, and
this difference even, viewing the various proportions of the teeth in
the same jaw of the _Megatherium_, is more satisfactorily established by
the characters pointed out by Dr. Harlan in the form and proportions of
the radius, than by those in the tooth itself.
The next notice of the _Megalonyx_ which I have consulted, in the hope
of meeting with additional and more precise information as to its real
generic characters, is an account given by the learned Professor
Doellinger,[38] of some fossil bones, collected by the accomplished
travellers Spix and Martius in the cave of Lassa Grande, near the
Arrayal de Torracigos, in Brazil. In this collection, however, it
unfortunately happens that there are no teeth, but only a few bones of
the extremities, including some ungueal phalanges, which Professor
Doellinger concludes, from their shape, the presence of an osseous
sheath for the claw, and the form of their articulation, to belong,
without doubt, to an animal of the Megatherioid kind, about the size of
an Ox. He particularly states that they are not bones of an immature
individual; but that they agree sufficiently with Cuvier’s descriptions
and figures of the _Megalonyx_ to be referred to that species of animal
(zu dieses thierart;) and he adds, what is certainly an interesting
fact, that the fossils in question form the first of the kind that had
been discovered out of North America.
Subsequently to the discovery of these bones, and of those of the
_Megalonyx laqueatus_ above alluded to, the remains of another great
Edentate animal were found in North America, and were deposited in the
Lyceum at New York; among these is a portion of the lower jaw with the
whole dental series of one side. It is thus described by Dr. Harlan.
“The fragment I am now about to describe is a portion of the dexter
lower jaw of the Megalonyx, containing four molar teeth; three of the
crowns of these teeth are perfect, that of the anterior one is
imperfect. These teeth differ considerably from each other in shape, and
increase in size from the front, the fourth and posterior tooth being
double the size of the first, and more compressed laterally; it is also
vertically concave on its external aspect, and vertically convex on its
internal aspect; the interior or mesial surface is strongly fluted, and
it has a deep longitudinal furrow on the dermal aspect, in which respect
it differs from the tooth of the _M. laqueatus_ previously described by
me, of which the dermal aspect is uniform, but to which, in all other
respects, it has a close resemblance. I suppose it therefore probable,
that this last may have belonged to the upper jaw. The three anterior
molars differ in shape and markings: they are vertically grooved, or
fluted, on their interior and posterior aspects, a transverse section
presenting an irregular cube. The length of the crown of the posterior
molar is two inches: the breadth about five-tenths of an inch: the
length of the tooth is three inches and six-tenths. The diameter of the
penultimate molar is eight-tenths by seven-tenths of an inch. The length
of this fragment of the jaw-bone is eight inches and four-tenths; the
height three inches and six-tenths: the length of the space occupied by
the alveolar sockets five inches and eight-tenths. The crown of the
tooth presents no protuberances, but resembles that of the Sloth; the
roots are hollow.”[39]
This fossil is referred by Dr. Harlan to his _Megalonyx laqueatus_; but,
pending the absence of other proof of the identity of species, in which,
as may be seen by comparing fig. 2, with fig. 4, in Pl. XVII., the teeth
differ widely in form, it would be obviously hazardous to adopt such an
approximation on hypothetical grounds.[40] In order, however, to obtain
more satisfactory evidence of the nature and amount of the difference
between the _Megalonyx laqueatus_, and the allied animal represented by
the above-described fragment of lower jaw, I wrote to my much respected
friend M. LAURILLARD, requesting him to send me a sketch of the teeth in
the cast of that lower jaw, which had been transmitted from New York to
the Garden of Plants. With full confidence in the characteristic
precision and accuracy of the drawing with which I have been obligingly
favoured by M. Laurillard, I am disposed to regard the amount of
difference recognizable in every tooth in the lower jaw in question
(fig, 3 and 4,) as compared with the molar tooth either of _Megalonyx
Jeffersonii_ (fig. 1,) or _Meg^x. laqueatus_ (fig. 2) to be such as to
justify its generic separation from _Megalonyx_ on the same grounds as
_Megalonyx_ is distinguished from _Megatherium_, and for the subgenus of
Megatherioid Edentata, thus indicated, I would propose the name of
MYLODON.[41] The species of which the fossil remains are described by
Dr. Harlan may be dedicated to that indefatigable Naturalist who has
contributed to natural science so much valuable information respecting
the Zoology, both recent and fossil, of the North American continent.
The fossil about to be described represents a second and smaller species
of the same genus, and I propose to call it _Mylodon Darwinii_, in
honour of its discoverer, of whose researches in the Southern division
of the New World it forms one of many new and interesting fruits.
This fossil was discovered in a bed of partly consolidated gravel at the
base of the cliff called Punta Alta, at Bahia Blanca in Northern
Patagonia: it consists of the lower jaw with the series of teeth entire
on both sides: but the extremity of the symphysis, the coronoid and
condyloid processes, and the angular process of the left ramus, are
wanting. The teeth are composed, as in _Bradypus_, _Megatherium_ and
_Megalonyx_, of a central pillar of coarse ivory, immediately invested
with a thin layer of fine and dense ivory, and the whole surrounded by a
thick coating of cement.