the Megatherium prove, however, that the dense layer separating the
internal substance from the cæmentum is not enamel, but presents the
same structure as the hard ‘dentine’ or ivory of the generality of
Mammalian teeth; and corresponds with the thin cylinder of hard
‘dentine’ in the tooth of the Sloth. No species of the Order _Bruta_ has
true enamel entering into the composition of its teeth; but the
modifications of structure which the teeth present in the different
genera of this order are considerable, and their complexity is not less
than that of the enamelled teeth of the Herbivorous Pachyderms and
Ruminantia, in consequence of the introduction of a dental substance
into their composition corresponding in structure with that of the teeth
of the _Myliobates_, _Psammodus_, and other cartilaginous fishes.
The microscopic investigation of the structure of the teeth of the
Megatherium was undertaken chiefly with the view of comparing this
structure with that of the teeth of the Sloth and Armadillo, and of thus
obtaining an insight into the food, and an additional test of the real
nature of the disputed affinities of the Megatherium. The central part
of the tooth (_c._ Pl. XXXI.) consists of a coarse ivory, like the
corresponding part of the tooth of the Sloth. It is traversed throughout
by medullary canals ¹⁄₁₅₀₀ of an inch in diameter, which are continued
from the pulp-cavity, and proceed, at an angle of 50°, to the plane of
the dense ivory, parallel to each other, with a slightly undulating
course, having regular interspaces, equal to one and a half diameters of
their own areæ, and generally anastomosing in pairs by a loop of which
the convexity is turned towards the origin of the tubes of the fine
dentine, as if each pair so joined consisted of a continuous reflected
canal, (_c._ fig. 1, Pl. XXXII.) The loops are generally formed close to
the fine dentine. In a few situations I have observed one of the
medullary canals continued across the fine dentine, and anastomosing
with the corresponding canals of the cæmentum. The interspaces of the
medullary canals of the coarse dentine are principally occupied by
calcigerous tubes which have an irregular course, anastomose
reticularly, and terminate in very fine cells. The more regular and
parallel calcigerous tubes, which constitute the thin layer of hard
dentine, are given off from the convexity of the terminal loops of the
medullary canals. The course of these tubes (_b._ fig. 1, Pl. XXXII.) is
rather more transversely to the axis of the tooth than the medullary
canals from which they are continued. They run parallel to each other,
but with minute undulations throughout their course, in which they are
separated by interspaces equal to one and a half their own diameter. As
they approach the cæmentum they divide and sub-divide, and grow more
wavy and irregular: their terminal branches take on a bent direction,
and form anastomoses, dilate into small cells, and many are seen to
become continuous with the radiating fibres or tubes of the cells or
corpuscles of the contiguous cæmentum. This substance enters largely
into the constitution of the compound tooth of the Megatherium: it is
characterized, like the cæmentum of the Elephant’s grinder, by the
presence of numerous radiated cells, or purkingian corpuscles, scattered
throughout its substance, but may be distinguished by wide medullary
canals which traverse it in a direction parallel with each other, and
forming a slight angle with the transverse axis of the tooth. These
canals are wider than those of the central coarse dentine, their
diameter being ¹⁄₁₂₀₀th of an inch; they are separated by interspaces
equal to from four to six of their own diameters, divide a few times
dichotomously in their course, and finally anastomose in loops, the
convexity of which is directed towards, and in most cases is in close
contiguity with, the layer of dense dentine.
Fine calcigerous tubes are every where given off at right angles from
the medullary canals of the cæmentum, which form a rich reticulation in
their interspaces, and a direct continuation between the loops of the
medullary canals and the calcigerous tubes of the dense dentine. The
cæmentum differs from the coarse dentine in the larger size and wider
interspaces of its medullary canals, and by the presence of the
bone-corpuscles in their interspaces; but they are brought into organic
communication with each other, not only by means of the tubes of the
dense dentine, but by occasional continuity of the medullary canals
across that substance. The tooth of the Megatherium thus offers an
unequivocal example of a course of nutriment from the dentine to the
cæmentum, and reciprocally. Retzius observes with respect to the human
tooth, that “the fine tubes of the cæmentum enter into immediate
communications with the cells and tubes of the dentine (zahnknochen), so
that this part can obtain from without the requisite humours after the
central pulp has almost ceased to exist.” In the Megatherium, however,
those anastomoses have not to perform a vicarious office, since the pulp
maintains its full size and functional activity during the whole period
of the animal’s existence. It relates to the higher organized condition,
and greater degree of vitality of the entire grinder in that extinct
species.
The conical cavities (_d._ Pl. XXXI.) attest the size and form of the
persistent pulp; the diameter of its base is equal to the part of the
crown of the tooth which is formed by the coarse and fine dentine. From
the gradual thinning off and final disappearance of these substances as
they reach the base of the tooth, I conclude that they were both formed
at the expense of the pulp. The fine tubes and cells must have been
excavated in its peripheral layer for the reception of the hardening
salts of the dense dentine, and the rest converted into the parallel
series of medullary canals with their respective systems of calcigerous
tubes, in a manner closely analogous to the development of the entire
tooth of the Orycteropus. The coarser dentine of the tooth of the
Megatherium differs, in fact, from the entire tooth of the
_Orycteropus_, only in that the parallel medullary canals and their
radiating calcigerous tubes are not separated from the contiguous canals
by a distinct layer of cæmentum, and that the medullary canals
anastomose at their peripheral extremities. The wide spaces, (_e._ Pl.
XXXI.) indicate the thickness of the dental capsule by the ossification
of which the exterior stratum of cement was formed. It was not until I
knew the true structure of the tooth of the Megatherium, that I could
comprehend the mode of its formation. The parallel layers of enamel in
the Elephant’s grinder are formed, as is well known, by membranous
plates passing from the coronal end of the closed capsule towards the
base of the tooth; but a certain extent of enamel can only thus be
formed, and when the crown of the grinder has once protruded, and come
into use, the enamel cannot be added to. The modification of the
structure of the tooth of the Megatherium readily permits the
uninterrupted and continuous formation of the dense substance which is
analogous to the enamel of the Elephant’s grinder.
With respect to the question of the respective affinities of the
Megatherium to the Bradypodoid or Dasypodoid families, the result of
this examination of the teeth speaks strongly for its closer
relationship with the former group: the Megalonyx, Mylodon, and
Scelidotherium, in like manner correspond in the structure of their
teeth with the Sloth, and differ from the Armadillo.
If from a similarity of dental structure we may predicate a similarity
of food, it may reasonably be conjectured that the leaves and soft
succulent sprouts of trees may have been the staple diet of the
Megatherioid quadrupeds, as of the existing Sloths. Their enormous
claws, I conclude, from the fossorial character of the powerful
mechanism by which they were worked, to have been employed, not, as in
the Sloths, to carry the animal to the food, but to bring the food
within the reach of the animal, by uprooting the trees on which it grew.
In the remains of the Megatherium we have evidence of the frame-work of
a quadruped equal to the task of undermining and hauling down the
largest members of a tropical forest. In the latter operation it is
obvious that the immediate application of the anterior extremities to
the trunk of the tree would demand a corresponding fulcrum, to be
effectual, and it is the necessity for an adequate basis of support and
resistance to such an application of the fore extremities which gives
the explanation to the anomalous development of the pelvis, tail, and
hinder extremities in the Megatherioid quadrupeds. No wonder, therefore,
that their type of structure is so peculiar; for where shall we now find
quadrupeds equal, like them, to the habitual task of uprooting trees for
food?
DESCRIPTION OF FRAGMENTS OF BONES, AND OF OSSEOUS TESSELATED DERMAL
COVERING OF LARGE EDENTATA.
It is now determined that there once existed in South America, besides
the Megatherium, the Megalonyx, and the allied genera described in the
preceding pages of the present work, gigantic species of the order
_Bruta_ belonging to the Armadillo family, and defended, like the small
existing representatives of that family, by a tesselated bony dermal
covering. The largest known species of these extinct _Dasypodidæ_ is the
_Glyptodon clavipes_, of which the armour and parts of the skeleton have
been described by MM. Weiss and D’Alton in the Berlin Transactions for
1827 and 1834: and the generic and specific characters and name, with an
account of the dental system, and bones of the extremities, were
recorded in the Geological Proceedings for March 1839. It would seem
that parts of the same, or a nearly allied gigantic species were
described in the same year by M. Lund; under the name of _Hoplophorus_.
Of the valuable and interesting discoveries of this able Naturalist I
regret that I was not aware until the appearance of a notice of them in
the Comptes Rendus for April, 1839.[64] Amongst the fragments of bony
tesselated armour in Mr. Darwin’s collection are a few pieces which were
found by him, associated with remains of Toxodon and Glossotherium near
the Rio Negro in Banda Oriental.[65] These fragments, if we may judge
from their thickness, must have belonged to an animal at least as large
as the _Glyptodon clavipes_; but the pattern differs in the greater
equality of size of the component tesseræ. The thickness of the largest
fragment is one inch and a half, the tesseræ vary in diameter from one
inch to half an inch, and are separated by grooves about two lines in
depth, and two in diameter. The pattern formed by the anastomosis of
these grooves is an irregular net-work; the contour of the tesseræ is
either unevenly subcircular, hexagonal, pentagonal, or even four-sided;
with the sides more or less unequal. In those portions of this armour,
where one of the tesseræ exceeds the contiguous ones in size, the
imagination may readily conceive it to be the centre of a rosette,
around which the smaller ones arrange themselves, but there is no
regular system of rosettes, as in the portions of the dermal armour of
the Glyptodon figured by Weiss, and those brought to England by Sir
Woodbine Parish, in which the central piece is double the size of the
marginal ones.
The portions of the tesselated bony dermal covering of a Dasypodoid
quadruped, figured in Pl. XXXII. figs. 5 and 4, of the natural size,
were discovered folded round the middle and ungueal phalanges, figs. 2
and 3, at Punta Alta, in Bahia Blanca, in an earthy bed interstratified
with the conglomerate containing the remains of the fossil Edentals.
In one of these fragments, measuring six inches long by five broad, the
tesseræ are arranged in rosettes, and so closely correspond in size and
pattern with the bony armour described by M. Lund, as characterizing his
species, _Hoplophorus euphractus_, that I feel no hesitation in
referring them to that animal. One of the pattern rosettes is figured at
fig. 4, together with the thickness of the armour at this part, and the
coarse tubulo-cellular structure of the bone. Another portion of dermal
armour from the same locality, gives the pattern shown in fig. 5, formed
by square or pentagonal tesseræ, arranged in transverse rows; it is
certain that this portion of armour belonged to the same animal as the
preceding piece; and probably that it constituted part of the transverse
dorsal bands of the _Hoplophorus_.
The middle and ungueal phalanx, as well as the portions of armour, are
given of the natural size, in Pl. XXXII. The upper and outer surface of
the phalanx, is shown in fig. 2. It is smooth and flat; joins the inner
surface by a sharp edge, which runs along the upper and inner side of
the bone; and passes by a gradual convexity to the under surface; the
ridge corresponding with the base of the claw, is feebly developed at
the under and lateral parts of the base of the claw. Below the double
trochlear joint for the middle phalanx, there are two articular surfaces
for two large sesamoid bones.
The middle phalanx corresponds in its small antero-posterior diameter
and wedge-shape, with that of the great Glyptodon: but the terminal
phalanx is longer and deeper, in proportion to its breadth.
Among the collection of fossils from Punta Alta, in Bahia Blanca, there
is an interesting fragment of the head of a gigantic animal of the
Edentate order, including the glenoid cavity, and part of the zygomatic
process of the left side. The articular surface for the lower jaw,
exhibits, in its flatness, extent, and the absence of a posterior ridge,
the well-marked characteristics of this part of the Edental structure.
It measures two inches four lines in the transverse, and two inches two
lines in the antero-posterior diameter. The commencement of the
zygomatic process presents a vertical diameter of two inches, and a
transverse diameter of eight lines at the thickest part. It is slightly
concave at its lower border, and convex above. The small portion of the
cranial parietes, which is preserved, exhibits the cellular structure
consequent upon the great extension and development of the nasal
air-sinuses: this condition of the cranial parietes, has already been
noticed in the description of the more perfect skulls of the large
extinct Edentata.
NOTICE OF FRAGMENTS OF MOLAR TEETH OF A
MASTODON.
Of the remains of this gigantic extinct Pachyderm, observed by Mr.
Darwin at Santa Fé, in Entre Rios, and on the banks of the Tercero, the
fragments of the teeth and portions of the skeleton which reached
England, are not sufficient to lead to a determination of the species;
but sufficiently prove it to have been nearly allied, if not identical,
with the _Mastodon angustidens_ of Cuvier, and unquestionably distinct
from the _Mastodon giganteum_ of the United States.
NOTICE OF THE REMAINS OF A SPECIES OF
EQUUS,
_Found associated with the extinct Edentals and Toxodon at Punta Alta,
in Bahia Blanca, and with the Mastodon and Toxodon at Santa Fé, in Entre
Rios._
The first of these remains is a superior molar tooth of the right side;
it was embedded in the quartz shingle, formed of pebbles strongly
cemented together with calcareous matter, which adhered as closely to
the tooth in question, as the corresponding matrix did to the associated
fossil remains. The tooth was as completely fossilized as the remains of
the Mylodon, Megatherium, and Scelidothere; and was so far decomposed,
that in the attempt to detach the adherent matrix, it became partially
resolved into its component curved lamellæ. Every point of comparison
that could be established proved it to differ from the tooth of the
common _Equus Caballus_ only in a slight inferiority of size.
The second evidence of the co-existence of the horse with the extinct
Mammals of the tertiary epoch of South America reposes on a more perfect
tooth, likewise of the upper jaw, from the red argillaceous earth of the
Pampas at Bajada de Santa Fé, in the Province of Entre Rios.[66]
This tooth is figured at Pl. XXXII. fig. 13 and 14, from which the
anatomist can judge of its close correspondence with a middle molar of
the left side of the upper jaw.
This tooth agreed so closely in colour and condition with the remains of
the Mastodon and Toxodon, from the same locality, that I have no doubt
respecting the contemporaneous existence of the individual horse, of
which it once formed part.
This evidence of the former existence of a genus, which, as regards
South America, had become extinct, and has a second time been introduced
into that Continent, is not one of the least interesting fruits of Mr.
Darwin’s palæontological discoveries.
DESCRIPTION OF REMAINS OF RODENTIA, INCLUDING THE JAWS AND TEETH OF AN
EXTINCT SPECIES OF
CTENOMYS.
The fragment of the upper jaw, figured in Pl. XXXII. fig. 6, exhibits
the first and second molar _in situ_, and the socket of the third and
fourth molar, of a Rodent, which by the form and number of the upper
maxillary teeth is referable to the genus Ctenomys. The molars are a
little larger, the longitudinal groove on their external surface is
somewhat deeper, and the last molar is relatively wider than in the
existing subterraneous species,—the Tucutucu (_Ctenomys Brasiliensis_,
Bl.), of whose habits so interesting an account is given in the
description of the Mammalia of the present Collection (No. IV. p. 79).
The form of the grinding surface of the first and second upper molar is
shown below the fig. 6, and three views of the second grinder are given
at figs. 7, 8, and 9. The fragment of the lower jaw of the same fossil
Rodent is figured at fig. 10 and 11. The long anterior incisor is
relatively narrower than in the _Ctenomys Brasiliensis_. I have not had
the means of comparing this fossil with the _Ctenomys Magellanicus_; but
since it is probable that the _Ct. Magellanicus_ may not be specifically
different from the _Ct. Brasiliensis_, it may be concluded that the
present fossil is equally distinct from both.
The portion of the right hind-foot of the Rodent figured at fig. 12,
includes the calcaneum, astragalus, cuboides, external and middle
cuneiform bones, and the metatarsals and proximal phalanges of the toes
corresponding with the three middle toes of five-toed quadrupeds. The
metatarsals are chiefly remarkable for the well-developed
double-trochlear articular surface, and intermediate ridge. These
remains, as well as the jaws and teeth of the Ctenomys, were discovered
at Monte Hermoso in Bahia Blanca.
In the same reddish earthy stratum of that locality, Mr. Darwin
discovered the decomposed molar of a Rodent, equalling in size, and
closely resembling in the disposition of its oblique component laminæ,
the hinder molar of the Capybara (_Hydrochærus_). The fossil differs,
however, in the greater relative breadth of the component laminæ.
I have, lastly, to notice the head of a femur, and some fragments of
pelvic bones from the same formation which bear the same proportion to
the tooth above alluded to, as subsists between the teeth and bones of
the Capybara, and which are sufficient to prove that there once has
existed in South America a species of the family _Caviidæ_, as large as
the present _Capybara_, but now apparently extinct.
This fact, together with the greater part of those which have been
recorded in the foregoing pages of the present work, establishes the
correspondence, in regard to the characteristic type, which exists
between the present and extinct animals of the South American Continent:
we have abundant evidence likewise of the greater number of generic and
specific modifications of these fundamental types which the animals of a
former epoch exhibited, and also of the vastly superior size which some
of the species attained.
At the same time it has been shewn that some of the present laws of the
geographical distribution of animals would not have been applicable to
South America, at the period when the Megatherioids, Toxodon, and
Macrauchenia existed: since the Horse, and according to M. Lund, the
Antelope and the Hyæna, were then associated with those more strictly
South American forms. The Horse, which, as regards the American
continent, had once become extinct, has again been introduced, and now
ranges in countless troops over the pampas and savannahs of the new
world. If the small Opossums of South America had been in like manner
imported into Europe, and were now established like the Squirrels and
Dormice in the forests of France, an analogous case would exist to that
of the Horse in South America, as the fossil Didelphys of Montmartre
proves.
With respect to the geological contemporaneity of the fossils collected
by him, Mr. Darwin subjoins the following observations:—
“The remains of the following animals were embedded together at Punta
Alta in Bahia Blanca:—The _Megatherium Cuvierii_, _Megalonyx
Jeffersonii_, _Mylodon Darwinii_, _Scelidotherium leptocephalum_,
_Toxodon Platensis (?)_ a Horse and a small Dasypodoid quadruped,
mentioned p. 107; at St. Fé in Entre Rios, a Horse, a Mastodon,
_Toxodon Platensis_, and some large animal with a tesselated osseous
dermal covering; on the banks of the Tercero the Mastodon, Toxodon,
and, according to the Jesuit Falkner, some animal with the same kind
of covering; near the Rio Negro in Banda Oriental, the _Toxodon
Platensis_, Glossotherium, and some animal with the same kind of
covering. To these two latter animals the _Glyptodon clavipes_,
described by Mr. Owen in the Geological Transactions, may, from the
locality where it was discovered, and from the similarity of the
deposit which covers the greater part of Banda Oriental, almost
certainly be added, as having been contemporaneous. From nearly the
same reasons, it is probable that the Rodents found at Monte Hermoso
in Bahia Blanca, co-existed with the several gigantic mammifers from
Punta Alta. I have, also, shown in the Introduction, that the
_Macrauchenia Patachonica_, must have been coeval, or nearly so, with
the last mentioned animals. Although we have no evidence of the
geological age of the deposits in some of the localities just
specified, yet from the presence of the same fossil mammifers in
others, of the age of which we have fair means of judging, (in
relation to the usual standard of comparison, of the amount of change
in the specific forms of the invertebrate inhabitants of the sea,) we
may safely infer that _most_ of the animals described in this volume,
and likewise the Glyptodon, were strictly contemporaneous, and that
_all_ lived at about the same very recent period in the earth’s
history. Moreover, as some of the fossil animals, discovered in such
extraordinary numbers by M. Lund in the caves of Brazil, are identical
or closely related with some of those, which lately lived together in
La Plata and Patagonia, a certain degree of light is thus thrown on
the antiquity of the ancient Fauna of Brazil, which otherwise would
have been left involved in complete darkness.”
LONDON:
PRINTED BY STEWART AND MURRAY,
OLD BAILEY.
[Illustration:
Pl. I.
_G. Scharf del et lithog._ _Printed by C. Hullmandel._
_Base of the Skull of Taxodon Platensis._
_Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. II.
_G. Scharf del et lithog._ _Printed by C. Hullmandel._
_Side View of the Skull of Taxodon._
_One-third the Natural Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. III.
_G. Scharf del et lithog._ _Printed by C. Hullmandel._
_Top View of the Skull of the Taxodon._
_One-third the Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. IV.
_G. Scharf del et lithog._ _Printed by C. Hullmandel._
_Taxodon Platensis._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. V.
_G. Scharf del et lithog._ _Printed by C. Hullmandel._
_Fragments of the lower Jaw and Teeth of a Taxodon._
_Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. VI.
_G. Scharf del et lithog._ _Printed by C. Hullmandel._