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MEMOIR

ON THE

MEGATHERIUM,

OR

GIANT GROUND-SLOTH OF AMERICA

(Megatherium umericanum, Cuvier).

by
RICHARD OWEN, F.R.S., D.C.L.,

SUPERINTENDENT OF THE NATURAL HISTORY DEPARTMENTS IN THE BRITISH MUSEUM ;
. FULLERIAN PBOFB8SOB OF PHYSIOLOGY IN THE ROTAL INSTITUTION OF GREAT BRITAIN ;
FOREIGN ASSOCIATE OF THE INSTITUTE OF FRANCE, ETC.

WILLIAMS AND NORGATE,

14. HENEIETTA STEEET, COVENT GAEDEN, LONDON;

AND

20, SOUTH FEEDEEICK STEEET, EDINBUEGH.

1861.

TO

THE ROYAL SOCIETY OF LONDON,

This Work, which owes its origin to the Society's favourable reception of the
Authors commwiications on the Megatherium, and allied fossils, in the years
1851 and 1855, and the drawings for its illustration chiefly to the liberal
application thereto of part of the 'Government Grant,' is, by permission of the
President and Council, most gratefully and respectfully dedicated, by

THE AUTHOR.

CONTENTS.

Pafte

§ 1. Historical Introduction 3

§ 2. Of the Spinal Column 12

§ 3. Comparison of the Spinal Column 25

§ 4. Of the Skull 28

§ 5. Of the Teeth 36

§ 6. Comparison of the Skull and Dentition 41

§ 7. Of the Bones of the Anterior Extremity 45

§ 8. Of the Bones of the Posterior Extremity 62

§ 9. Comparison of the Bones of the Hind-foot 74

§10. Physiological Summary 77

§11. Geological Summary ^2

DESCRIPTION

OF

THE SKELETON OF THE MEGATHERIUM.

^ 1. Historical Introduction.

JjEFORE commencing the description of the skeleton of the Megatherium, now in
London, Plate I., which is the most complete that has yet reached Europe, a brief
statement may be premised of the chief steps which have led to the restoration of the
species (Megatherium Americanum, Cuvier and Blumenbach) to which it belongs.

Cuvier, in communicating to the ' Annales du Museum' (t. v. 1804) a translation
of the first memoir on this subject — that, viz. by Garriga and Bru, published at
Madrid in 1796, — gives all the requisite details respecting the discovery of the skele-
ton therein described, and adds his own more important deductions as to its affinities
from an examination and comparison of the plates of the Spanish work.

It appears that proofs of these plates were transmitted in 1795 to the Institute
of France, and that Cuvier, having been called upon by the ' Class of Sciences ' at
that period to give a report upon them, developed his views of the affinity of the
animal to the Sloths and other Edentates*, and proposed for it the name ' Mega-
therium-f-.'

M. Roume, the correspondent of the French Institute to whom that distinguished
scientific body were indebted for the proof impressions of Garriga's work, and who
had an opportunity of examining the skeleton itself at Madrid (which Cuvier never
enjoyed), inserted a brief notice of it in the 'Bulletin de la Soci6t6 Philomathique'
of the Republican year IV. (1795); in which, after particularly noticing that the pelvis

* " Je developpai des-lors l'affinitc de cet animal avec les paresseux et les autres Mentis." — Annales du
Museum, t. v. p. 377.

t fiiyat great, fli/piof beast.

a2

was open towards the abdomen, the pubis being absent, without any indication of its
having ever existed*, concludes that the animal was intermediate, as to form, between
the Cape Anteater (Orycteropus) and the Great Anteater of America (Myrmecophaga
jubata). But he adds, that M. Cuvier, from an examination of the engravings of the
skeleton, had recognized that the species was much more nearly allied to the Sloths
than to the Anteaters.

M. Abildgaard, a professor at Copenhagen, having had the opportunity of study-
ing the skeleton of the Megatherium at Madrid in 1793, published a short notice of
it in Danish, illustrated by a rude figure of the skull and of the hind limbs, and
referred the species to the Bruta of Linnaeus, an order afterwards modified to form
the Edentds of Cuvier: this notice, though published the year after Cuvier's Report,
appears to have been independent of it, and the conclusions to be the result of the
author's own observations and reflections. It is, therefore, to be regarded as an
additional testimony to the true affinities of the species.

Cuvier's comments on the figures in the engravings for Garriga's memoir are
accompanied by reduced copies of them, given in the above-cited volume of the
' Annales du Museum,' and afterwards in the fourth volume of the first edition of the
' Recherches sur les Ossemens Fossiles,' 4to, 1812. In both works Cuvier sums up
his conclusions as to the habits and food of the Megatherium, as follows: — "Its
teeth prove that it lived on vegetables, and its robust fore-feet armed with sharp
claws, make us believe that it was principally their roots which it attacked. Its
magnitude and its talons must have given it sufficient means of defence. It was not
of swift course, nor was this requisite, the animal needing neither to pursue nor to
escape-f-."

In the year 1821 Drs. Pander and D'Alton published their beautiful Monograph
on the Megatherium, the result of personal examination and depiction of the then
unique skeleton at Madrid ; they represent the bones more artistically and in more
natural juxtaposition than in the plates of Bru's memoir, but the subject being the
same, the same deficiencies, to be presently specified, were unavoidable.

As the accomplished and learned authors of the German work reasoned, like
Abildgaard, from actual inspection of the fossil skeleton, their conclusions as to the
nature, affinities and habits of the animal to which it belonged merit a respectful
consideration. In it they recognize, with Cuvier, all the important points of resem-

* " Son bassin est compost; des os sacrum, ileum et ischium, mais il n'y a point de pubis ni d'indication qu'il
ait existe". Ce bassin est ouvert du cot6 de l'abdomen." — p. 97. It will be shown in this memoir that the sup-
posed want of pubic bones was due to accidental mutilation of the pelvis of the skeleton at Madrid : the true
profile of the pelvis is given at ei, 63, 04, Plate I.

+ •* Ses dents prouvent qu'il vivoit de vegtitaux, et ses pieds de devant, robustes et armees d'ongles tranchans,
nous fait croire que c'Gtoit principalement leurs racines qu'il attaquoit. Sa grandeur et ses griffes devoient lui
fournir assez de moyens de defense. II n'e"toit pas prompt a la course, mais cela ne lui etoit pas necessaire,
n'ayant besoin ni de poursuivre ni de fuir." Tom. cit. ' Sur le Megatherium,' p. 29. See also the posthumous
edition of the ' Ossemens Fossiles,' 8vo, 1836, torn. viii. p. 363.

blance to the skeletons of the existing species of Sloth, of which they append excel-
lent figures. But, imbued with the principles of the transcendental and transmuta-
tive hypotheses, then prevalent in the schools of Germany, they regard the great Mega-
therium and Megalonyx as being not merely predecessors but progenitors of those
still lingering remnants of the tardigrade race, into which such ancestral giants are
supposed to have dwindled down by gradual degeneration and alteration of characters.
But they deem the living habits of the Megatherium to have been far different from
those of its puny scansorial progeny: it was, in their opinion, a fossorial animal;
and not merely an occasional digger of the soil, as Cuvier concluded, but altogether
a creature of subterranean habits ; some earth-whale, as it were, or colossal mole.
Pander and D'Alton nevertheless give to this animal, which they truly characterized
as one of the most extraordinary of its class, the name of * Riesen-faulthier,' Bra-
dy pus giganteus, or Gigantic Sloth*.

Cuvier, in preparing his new and enlarged edition of the famous ' Recherches sill-
ies Ossemens Fossiles,' availed himself, in the fifth volume, published in 1823, of the
labours of the German anatomists and draughtsmen, just cited, and substituted
copies of their figures for those which he had previously borrowed from Bru.

The teeth of the Megatherium are still described as being implanted by two roots,
and as being sixteen in number, formulized as m, l^: there is the same deficiency of
the sternal ribs, pubic bones and tail ; the manubrium sterni continues to be repre-
sented in a reversed position : but, with regard to the bones of the fore-foot, the
organization of which was involved in obscurity, owing to the faulty manner in which
Cuvier believed them to have been articulated, he endeavours to throw some new
light on their arrangement. After a comparison of the figures given by Pander and
D'Alton with the bones of the fore-foot in existing Edentata, Cuvier concludes that
the fore-feet in the Madrid skeleton are transposed, the right being on the left, and the
left on the right side ; that the index, medius and annular digits were the only ones
provided with claws; that the thumb was clawless, and the little finger rudimental
and concealed, in the living Megatherium, under the skin ; the hand being thereby
specially formed for cleaving the soil and digging, like that of the Dasypus gigas. On
this hypothesis, names are applied by Cuvier to certain bones of the carpus, none of
which had before been determined -f\ Cuvier, however, adds, that " in order to verify
his conjectures it must be necessary to have access to the skeleton itself, and to com-
pare separately each bone of the fore-foot with their homologues in that species of
Armadillo J." His ideas of the affinities of the Megatherium have now undergone some

* Das Riesen-Faulthier, &c. fol. 1821, p. 16.

f Recherches sur les Ossemens Fossiles, 4to. t. v. pt. 1. 1823, p. 185. pi. 16. fig. 13. The letters indica-
tive of the carpal bones have been omitted, by oversight, in the plates, but there is no difficulty in adding them
according to the description given by Cuvier in the text.

J " Mais on sent que, pour verifier ces conjectures, il faudroitetre aupres du squelette, et en comparer sepa-
rement tous les os avec leurs analogues dans ce tatou, ce que j'espere que quelque anatomiste espagnol ne
tardera pas a faire." — lb. p. 185.

6

modification : the following paragraph is added to the summary on this head given in
flu- earlier edition of the great work: — "Its analogies approximate it to different
genera of the Edentate family. It has the head and the shoulder of a Sloth, whilst
the legs and the feet offer a singular mixture of characters peculiar to the Anteaters
and Armadillos*." Cuvier concludes his account of the Megatherium in the second
edition of the 'Ossemens Fossiles,' by appending a note communicated to him by
M. Augusts St. Hilaike, " which announces," he says, " that the Megatherium had
pushed its affinity to the Armadillos so far as to be covered like them with a scaly
cuirass."

This opinion derived apparent confirmation from the description by Professor Weiss
of portions of an osseous tessellated dermal armour of some gigantic quadruped, sent
to Berlin by the traveller Sellow, which armour he figures, and attributes to the
Megatherium, in a " Geological memoir on the Provinces of S. Pedro do Sal and the
Banda Oriental," published in 1827 -J--

In the year 1832, a highly valuable and important collection of the bones of the
Megatherium, discovered in the Rio Salado, with a portion of a bony tessellated
dermal covering of an animal, found in Lake Averias, province of Buenos Ayres,
indicative of a frame as great as that of the skeleton from the Rio Salado, was
transmitted from Buenos Ayres by Sir Woodbine Parish, K.H., and presented by
him to the Royal College of Surgeons. These specimens formed the subject of a
memoir communicated by William Clift, Esq., F.R.S., to the Geological Society,
June 13, 1832J, in which, although, with the characteristic caution of the author, the
armour is not directly affirmed to belong to the Megatherium, nothing is stated to
prevent the inference that it formed part of the ' Remains' of that animal which it is
the object of the memoir to describe : and, in the description of the map engraved in
pi. 43, the specimen figured in pi. 46, with other portions of the bony armour, are
comprehended amongst " those Remains of the Megatherium which have hitherto
been sent to Europe §." Further countenance to the later opinion of Cuvier as to
the affinities of the Megatherium to the Armadillo, was afforded by a few remarks in
the text of Mr. Clift's memoir : thus, in noticing the " bony or pseudo-cartilaginous
pieces which unite the true ribs to the sternum," Mr. Clift adds, " as is also
the case in the Armadillo ||." And in the description of the caudal vertebrae, he
remarks, " they have the inferior spines (/'. e. the chevron or V-shaped bones), mani-
festing in this their relation to other Edentata, as the Myrmecophagce and Dasy-

* " Ses analogies le rapprochent des divers genres de la famille des ententes. II a la tete et l'cpaule d'un
paretseux, et ses jambes et ses pieds offrent un singulier melange de earacteres propres aux fourmilliers et aux
tatous."— lb. p. 189.

t Abhandlungen der Kon. Akad. der Wissenschaften zu Berlin, 1827.

I Geological Transactions, 2nd series, vol. ill. p. 437.

i Ibid. Description of the Plates. || Ibid. p. 439. f Ibid. p. 444.

The additional parts of the Megatherium, supplied by Sir Woodbine Parish, and
deficient in the skeleton at Madrid, were two of the ossified cartilages of the ribs,
two of the smaller bones of the sternum, twelve caudal vertebrae, and ten of the
separate ' chevron bones,' partly belonging to them and partly indicating other
caudal vertebrae : they also included a part of the os hyoides. Mr. Clift, with his
accustomed ingenuity and artistic ability, gives at one view an idea of "all the parts
hitherto known, or supposed to be known," of the Megatherium, by taking as his
basis the outline of the view of the skeleton given by Pander and D' Alton in the
first plate of their work ; leaving in simple outline those parts which are present in
the Madrid skeleton, but not in Sir Woodbine Parish's collection ; expressing by a
pale tint the parts in that collection which also exist in the Madrid skeleton; and
indicating by a dark tint the additional parts which are deficient in the Madrid
skeleton, and had not before been figured.

Besides the important elements thus added towards the completion of our know-
ledge of the skeleton, Mr. Clift was enabled to correct an error into which Cuvier
bad been led by a figure of a mutilated tooth in tab. 4. fig. 5, f, of Garriga's memoir,
which seemed to show that it had been implanted, as Cuvier describes it to have
been, by two fangs. Pander and D'Alton give a similar figure of one of the teeth
in their tab. 2. fig. 15. The figure of the natural size of one of the teeth of the Me-
gatherium transmitted by Sir Woodbine Parish, given in the third plate (pi. 45. fig. 2.)
of Mr. Clift's memoir, is the first accurate representation of these characteristic parts,
and shows that the implanted base is widely excavated for a persistent matrix, as in
the Sloths and Armadillos.

The prevalent belief among Comparative Anatomists and Naturalists at this
period, founded upon the additional observations by Weiss and Clift to those con-
tained in the second edition of the ' Ossemens Fossiles ' of Cuvier, may be gathered
from such notices as were then published of the opinions expressed by the eminent
professors of those sciences on the subject. Thus Dr. Robert Grant, treating of the
Armadillos, in his Lectures on Comparative Anatomy, says, "The Megatherium itself
appears to have been such a digging loricated animal, and in many of its bones
resembles the Armadillos *."

The Very Rev. Dr. Buckland, in his Bridgewater Treatise published in 1836, ad-
mitting the probability, from the evidence at that time adduced, that the Megathe-
rium had been defended by a bony tessellated armour, argues that — "A covering of

* Lecture, reported in the ' Lancet,' March 29, 1834 : and again, in 1839, the Megatherium is described by
Dr. Grant as being " allied in structure to the Bradypus, and shielded with cutaneous plates like the Dasypus."
— Thomson's 'British Annual' for 1839, p. 274. M. Desmarest, in the art. MegatMre of the ' Dictionnaire
des Sciences Naturelles,' 1823, writes asfollows : — " Leurs membres etaient robustes et terminer par cinq gros
doigts. Des observations recentes paroissent prouver que sa peau, epaissee et comme ossifiee, etait partagee
en une foule d'ecussons polygones et rapproch^s les uns des autres comme les pieces qui entrent dans la com-
position d'une mosaique." — " La forme des molaires et la taille de ces animaux semblent indiquer qu'ils se
nourrissoient de vegetaux et sans doute de racines."

such enormous weight would have been consistent with the general structure of the
Megatherium : its columnar hind legs and colossal tail were calculated to give it due
support ; and the strength of the loins and ribs, being very much greater than in the
Elephant, seems to have been necessary for carrying so ponderous a cuirass as that
which we suppose to have covered the body." He next calls attention to the broad
and rough flattened surface of a part of the crest of the ileum, to the broad summits
of the spines of many vertebrae, and also to the superior convex portion of certain
ribs, on which the armour could rest, as affording " evidence of pressure, similar to
that we find on the analogous parts of the skeleton of the Armadillo, from which,"
he remarks, "we might have inferred that the Megatherium also was covered with
heavy armour, even had no such armour been discovered near bones of this animal in
other parts of the same level district of Paraguay*."

The estimable and justly celebrated author of the * Bridgewater Treatise,' notwith-
standing his bias for the hypothesis of the affinities of the Megatherium to the Arma-
dillos, enunciates his conclusion with philosophic caution, and affirms that the other
" remarkable character of the Megatherium, in which it approaches most nearly to
the Armadillo and Chlamyphorus, consists in its hide having probably been covered
with a bony coat of armour, varying from three-fourths of an inch to an inch and a
half in thickness -f-." In the same work is given an original figure of the pelvis and
hind limb of the Megatherium, from a front view of those specimens in the Museum
of the College of Surgeons.

M. Laurillard, in the posthumous edition of the ' Ossemens Fossiles' of Cuvier,
published in 1836, whilst admitting it to be very possible for the Megatherium to
have been covered by a cuirass, appends a note of warning against too hastily attri-
buting to that animal the fragments of the gigantic osseous armour that had been
found in the same formations of South America ; because, in the casts of some of the
bones which were transmitted with that armour by Sir Woodbine Parish, M. Lau-
rillard had recognized a calcaneum, an astragalus and a scaphoid, differing from
those of the living Armadillos only by their size and by some merely specific modifi-
cations X-

But that which Baron Cuvier and M. Laurillard had ventured to regard as
very possible, and Dr. Buckland as probable, M. de Blainville a few years later
announced to be a positive fact. He communicated, in 1839, to the Academy of
Sciences of the French Institute, a statement that bones of the Megatherium had
recently been discovered, accompanied with fragments of a carapace belonging in-
dubitably to the same animal; and he adds that the association of a bony armour
with the internal skeleton of the Megatherium can be demonstrated as surely by
d. priori reasoning as by the h. posteriori fact ; but he adduces no observations or

* Geology and Mineralogy considered with reference to Natural Theology, vol. i. pp. 160 and 161.

t Ibid. p. 159.

J Recherches sur lea Ossemens Fossiles, 8vo, 1836, torn. viii. p. 354.

arguments from the skeleton in addition to those of which Dr. Buckland had pre-
viously availed himself, simply affirming that " the Megatherium is proved to have
been certainly covered by an osteo-dermal carapace, by the disposition of the spinous
processes of the vertebrae, by the angles of the ribs, by the articulation of the pelvis
with the vertebral column," &c. ; and he concludes by announcing "that the Mega-
therium was a gigantic species of Armadillo, most nearly allied to the diminutive
Chlamyphorus *."

With regard to the fossils from South America, unequivocally referable to the
Armadillo family, I had myself pointed out the generic distinction of that large
quadruped, some bones of which had been transmitted along with the gigantic der-
mal armour by Sir Woodbine Parish, and proposed for it the name of ' Glyptodon '
in Sir Woodbine Parish's work on Buenos Ayres -f-; and afterwards, stimulated by
the general tendency of anatomists and palaeontologists to regard the Megatherium
as being, likewise, a gigantic Armadillo, I entered upon a critical review of all the
facts of the case which at that time had been obtained, and communicated the result
in a memoir to the Geological Society, read March 23, 1839 J. The general con-
clusions from this memoir were: —

1. The opinions of Cuvier and Weiss, in favour of the Megatherium being so
armed, rest on no better ground than the mere fact of bony armour of some gigantic
quadruped and the skeleton of the Megatherium having been discovered in the same
continent.

2. The skeleton, or its parts, which have been actually associated with the bony
armour above mentioned, belongs to a quadruped distinct from and less than the
Megatherium.

3. No part of the skeleton of the Megatherium presents those modifications which
are related to the support of a dermal covering.

4. The proportions of the component tesserae of the bony armour in question to
the skeleton of the Glyptodon are the same as those between the dermal tesserae and
skeleton of existing Armadillos, but are much smaller as compared with the bones of
the Megatherium.

5. No bony armour composed of tesserae having the same relative size to the bones
of the Megatherium as in the Glyptodon and existing Armadillos, has yet been dis-
covered.

In 1837 I had been put in possession of an additional test of the affinities of the
Megatherium, by portions of teeth, obtained by Mr. Charles Darwin at Punta Alta
in Northern Patagonia, from which specimens I was kindly permitted to take the
requisite sections for microscopical examination. Previous researches by Professor

* " Recherches sur l'anciennete des Edentes terrestres a la surface de la terre," Comptes Rendus de l'Acad.
dea Sciences, 1839, p. 65.

t P. 178 A, frontispiece, 8vo, 1838.

{ " On the Glyptodon clavipes," Transactions of the Geological Society, second series, vol. vi. p. 98.

B

10

Rbtzius and myself into the structure of the teeth of the Mammalia generally, had
made me acquainted with the marked difference between the teeth of the Armadillos
and those of the Sloths in internal structure, and I now found that the Mega-
therium presented the same remarkable compound structure of the teeth as in the
Sloths, but with additional complexity, by which they still further departed from
the comparatively simple structure of the teeth of the Armadillos: the exami-
nation at the same time proved that there was no true enamel in the teeth of the
Megatherium*.

Another important evidence of the affinity of the Megatherium to the Sloths was
brought to light by a fragment of the skull from Punta Alta, which demonstrated a
fifth small molar tooth on each side of the upper jaw, thus showing that in the
number as well as in the structure and the kind of teeth the Megatherium agreed
with the Bradypodidw, and especially with the Ai or Three-toed Sloth ; the anterior
pair of molars not "manifesting the excess of size and laniary form which characterize
them in the Unau or Two-toed species-f-.

These additional evidences of the concordance of structure between the Mega-
therium and the Sloths, manifested by the hard and enduring parts which are most
intimately related to the food of the animal, induced me to reconsider the conclu-
sions of Cuvier, Pander and D'Alton, and Dr. Buckland relative to the sources of
its nutriment and its habits of life ; and ultimately to arrive at a conviction of the
correspondence of the food of the Megatherium with that of the Sloths, and of the
relation of the modified form of the Megatherium to its peculiar mode of obtaining
such food, the grounds for which conviction are given in my memoir on the
Mylodon robustus, published in 1842.

By the analogy of this smaller species of the great extinct terrestrial Sloths of
South America, I endeavoured to dissipate some of the doubt and obscurity which
shrouded the true structure of the fore and hind feet of the Megatherium J; but the
light so obtained served rather to increase the desire to inspect the skeleton itself at
Madrid, and obtain, ex visu, a conviction of the accuracy of my views ; for I partici-
pated entirely in the doubts expressed by my experienced colleague Mr. Clift§
relative to that skeleton, then unique in Europe, viz. as to " whether it had been
properly or improperly mounted, i. e. whether all the parts were of one or more indi-
viduals, whether they belong to the situation or position in which they are placed,
whether all the parts are genuine or partly modelled, or whether parts are eked out
by bones that do not belong to the part or situation in which they are collected :"
concurring at the same time with Mr. Clift, that " no blame was attributable to the
articulator, who, probably, had little or no guide in such a difficult task."

* Zoology of the Voyage of Her Majesty's Ship ' Beagle,' Fossil Mammalia, 4to, 1838-40, p. 103,
pis. 31 and 32. fig. 1. f Ibib. p. 102.

X Description of the Skeleton of the Mylodon robustus, 4to, pp. 102, 131-136.
i "Notice on the Megatherium," Geol. Trans. Second Series, vol. iii., description of plate 44.

11

Year after year, however, passed away without bringing with it the requisite liberty
from official duties for a visit to Madrid. I availed myself of the rare opportunities
afforded by journeys of scientific friends to that city to endeavour to obtain informa-
tion on some of the discrepant or doubtful points, and more especially relative to
the exact number of teeth or sockets of teeth in the skull ; but usually without any
satisfactory result, owing chiefly to the difficulty which the mode of preserving the
famous skeleton presents to any close inspection. Dr. Daubeny, the accomplished
Professor of Botany at Oxford, in reply to one of my requests, wrote to me from
Madrid : — " I have examined the Megatherium and can discern only four teeth in
either jaw, which are all perfect and double, but whether or not there be the rudiment
of a tooth behind cannot be distinctly ascertained, unless the glass case which covers
the specimen be removed ; for there is no door or any way of getting close to the
skeleton. I should advise a memorial to be drawn up stating the reasons for wishing
the point to be determined, in which case, perhaps, the authorities might consent to
allow a pane of glass to be removed."

Fortunately the necessity of the endeavour to overcome these obstacles was in great
measure obviated by the arrival in 1845, in this country, of a very important and
remarkable accession of remains of the Megatherium, discovered in 1837, near
Luxan, Buenos Ayres, which, with other fossils of large extinct South American
animals, were purchased by the Trustees of the British Museum. This collection
included an entire cranium and lower jaw of the Megatherium ; all the vertebrae
of the tail ; complete series of the bones of both fore- and hind-feet : — in short, parts
which, in combination with those previously deposited in the Museum of the Royal
College of Surgeons, by Sir Woodbine Parish and Mr. Darwin, completed the skele-
ton of the animal, including the hyoid and sesamoid bones, which are too often want-
ing in the skeletons of our recent and common quadrupeds.

Accurate plaster casts of the huge pelvis and most of the other bones of the Mega-
therium in the College Museum had been prepared at the expense of the College and
presented to the British Museum : and, after an examination and comparison of the
whole series of the remains of the Megatherium in both collections, and a consulta-
tion with the ingenious articulator of the Mylodon, Mr. Flower, I suggested to
the able keeper of the Mineralogical Department in the British Museum, Charles
Konig, K.H., F.R.S., the advantage which would arise, if similar plaster casts should
be taken of all the other bones of the skeleton, and if such casts, coloured to match
the original bones, should be mounted ; the originals being preserved separate, for
the greater facility of their comparison and for the advantage of examining their
articular surfaces.

The Trustees of the British Museum having taken the proposition under mature
consideration, the Council of the Royal College of Surgeons having liberally
sanctioned the moulding of all the requisite bones in their Museum, and my own con-
sent to superintend the co-adjustment and attitude of the skeleton having been given,

b2

12

the models and plaster casts were ordered by the Trustees to be made, and their
articulation was confided to Mr. Flower.

The result is the exhibition in our National Museum of the entire skeleton of the
Megatherium, Plate I., in a much more complete state, and, I believe I may add,
more natural attitude, than that of the same extraordinary quadruped, which
previously had been unique and the glory of the Royal Museum of Natural History
at Madrid.

For the full fruition by comparative anatomists and palaeontologists of so rich an
accession to our evidences of one of the strangest animals of a former world, there still
remained one condition, — viz. the power to employ a competent artist to depict the
skeleton and its several parts. There could be no question that the opportunity of
supplying the omissions, correcting the errors, and clearing up the doubts, in the
descriptions founded on the Madrid skeleton, ought to be embraced without loss of
time. The enlightened and liberal grant of £1000, placed by Lord John Russell,
then Prime Minister, at the disposal of the Council of the Royal Society, in aid of the
labours of men of science, seemed to me to afford the means of removing the only
difficulty that stood in the way of completing the object of my wishes. I therefore
submitted the case to the "Committee of Recommendations for the application of
the Government Grant," and the Council of the Royal Society was pleased to
approve the recommendation of the Committee, viz. "that £100 be granted to Pro-
fessor Owen, to be applied to the procurement of drawings of the undescribed and
unfigured or inaccurately figured parts of the skeleton of the Megatherium."

The present memoir, and its illustrations from the accurate pencil of Mr. Joseph
Dinkel, are the result of that recommendation ; and I have only to add, that the
descriptions and figures of the several parts of the skeleton of the Megatherium, now
in London, have been taken from the actual bones ; and the views of the entire
skeleton from the articulated casts, which are so beautifully exact, as, for all the
essential purposes of science, to be of the same value and utility as the bones them-
selves would be if so articulated together.

^ 2. Of the Spinal Column.

The skeleton of the Megatherium, like that of all other vertebrate animals, being
composed of a series of segments, similar in their composition, and referable under
all their modifications to a common type, answering to that which is figured as the
' typical vertebra' in my work on the Vertebrate Skeleton*, I shall commence its
description by one of those segments which deviate least from the archetypal cha-
racter ; and such segments we find to constitute the major part of the trunk, where
they form what, in human anatomy, would be termed ' dorsal vertebrae,' ' ribs,' ' car-
tilages of ribs' and ' sternal bones.'

* On the Archetype and Homologies of the Vertebrate Skeleton, 8vo, 1848, p. 81, fig. 16, and p. 82,
fig. 15.

13

Plate II. fig. 1, gives a front view of the fifth segment of the dorsal or thoracic
region of the trunk : it deviates from the archetype inasmuch as the neurapophyses,
n, n, have coalesced, as in other mammals, with the centrum, c, below, and are connate
with the neural spine, ns, above : the haemal arch is also vastly expanded in relation
to the greatly developed vascular centres which it was destined to encompass ; the
pleurapophyses, pi, pi, being elongated and bent down, and the hsemapophyses, h.h,
removed from the centrum and articulated to the ends of the pleurapophyses, and,
by a double synovial joint, .yV, to the ha?mal spine or sternal bone h.s.

The coalesced centrum and neural arch constitute the so-called 'dorsal vertebra,'
and the one selected is the fifth of that series counting backwards.

The centrum, c, or body of the vertebra, is wedge-shaped, with its base upwards,
forming the floor of the capacious neural canal, and the sides — slightly concave
lengthwise, almost flattened vertically — converge to the inferior surface, which is
formed by an obtuse ridge : the centrum expands slightly at its articular ends, and
so that the contour of the anterior one is rather oval than trihedral ; this surface is
slightly depressed at its middle, slightly convex in the rest of its extent ; the posterior
articular surface is larger than the anterior one and flatter, but is also a little
depressed at the middle: the two upper angles of the hinder end, probably con-
tributed by the neurapophyses in the development of the vertebra, are slightly pro-
duced and truncate, ottering each a smooth, flat, small subcircular surface, c', for
the head of the rib of the preceding vertebra; the corresponding part of the rib of
the present segment is marked c". The neurapophyses, n, n, rise each by a slender
base which has coalesced with the anterior half of the upper and outer angle of the
centrum : they diverge from each other and expand as they rise ; then, developing
some articular surfaces and exogenous processes from their outer surface, arch
towards each other, increasing rapidly in antero-posterior extent, and coalesce above
the neural canal ; where they support the zygapophyses, z, z, and the thick and strong
neural spine IM. The roof of the neural arch, thus formed, projects some way beyond
the anterior surface of the centrum, and extends almost to the posterior surface.
The inner surface of the neural arch is as remarkable for its even smoothness, as the
outer surface is for its various prominences and depressions. The outer side of the
basal half of the neurapophysis supports a large elliptical articular surface (n!), con-
cave from above downwards and backwards : the overhanging fore-part of the arch
supports the two flat oval anterior zygapophyses, z, z, the articular surfaces of which
look almost directly upwards ; on the under surface of the back part of the arch are
the two posterior zygapophyses looking almost downwards, and between these is a
rough longitudinal prominent ridge. The neural spine is moderately long, subcom-
pressed and snbtrihedral, with a sharp anterior margin, smooth sides, and a rough
thick posterior surface, developing a median longitudinal ridge: the summit expands
into a rough triangular almost flattened surface.

For the convenience of describing and comparing the different exogenous processes

14

developed from the neural arch in the class Mammalia, I have indicated them by
single-worded names ; having found that, although they varied much in size and a
little in relative position, when traced through the series, they could be identified
from species to species.

The most common and constant of these processes is that which usually stands
out, or transversely, from the base of the neural arch, and affords a joint or a surface
of confluence for the rib; this I have proposed to call ' diapophysis*' : the second
process has a range of variety in its position from the upper part of the diapophysis
to that of the anterior zygapophysis, but, as it is commonly somewhere between these
two, I have called it ' metapopkysis^ '; the third process projects most commonly more
or less backwards, from the base of the diapophysis, and I have termed it ' anapo-
physis%\ As each of the above processes developes in some species an articular
surface, and as each is usually more or less oblique in position, I have called those
processes which more constantly support such surfaces ' zygapophyses §'. The com-
parative anatomy of these and other ' exogenous' processes has been the subject of a
previous memoir ||, because their extreme degree of variety in the Order Edentata,
and extraordinary development in the Armadillos and true Anteaters, render them
of unusual importance in the question of the affinities of the Megatherium.

The narrow compressed base of the neurapophysis of the fifth dorsal vertebra in
that animal having risen above the centrum, developes on its outer surface a large
vertically oval articular surface, n', concave in the direction of its long axis, almost
flat transversely ; to which surface a corresponding convex articular surface, «", on
the upper part of the neck of the rib near the head, is adapted. Above the surface,
n', the diapophysis, d, stands out, short, thick, subdepressed, expanded at its extremity,
which is slightly produced backwards and supports on its outer surface an elliptical
articular concavity, d', with the long axis directed from above downwards and back-
wards, and articulated to a corresponding convexity, d", upon the tubercle of the rib.
A rugged tuberosity, m, on the upper and fore-part of the diapophysis represents in
rudiment the metapophysis. A smaller tuberosity is interposed between the anterior
zygapophysis, z, and the neurapophysial surface, n', for the rib. Thus the neural
arch of the vertebra in question presents ten distinct articulations besides the two
sutural ones now obliterated by its anchylosis to the centrum, which part has its two
large terminal articulations distinct from those for the head of the rib, d, which
I reckon among the neurapophysial ones. .The rib, which term I confine to the

* Lectures on the Comparative Anatomy of the Vertebrate Animals, 8vo, Longmans, 1846, vol. i. p. 42 ;
from cui trans, and airwpvtru processus.

f On the Anatomy of the Male Aurochs, Proceedings of the Zoological Society, November 14, 1848, p. 131 ;
from ficni inter, and uwtyvtris.

I On the Anatomy of the Aurochs, ut supra, p. 131 ; from dyd retro, and dvtyvms.

$ Lectures on the Comparative Anatomy of the Vertebrate Animals, p. 43 ; from Zvyov junctura, and
dxofvait ; these are called the " oblique or articular processes" in Human Anatomy.

II Philosophical Transactions, 1851.

15

pleurapophysis, or ' vertebral rib' of comparative anatomists, ' pars ossea costee' of
anthropotomy, presents a small flat subcircular surface, c", for articulation with that
on the base of the neurapophysis forming the upper angle of the body of the vertebra
in advance of the segment to which the rib belongs. The neck of the rib rapidly
expands as it quits the head, developes the convex oval surface, n", on its upper part
for the concavity on the neurapophysis of its own vertebra ; and is indented behind
where it joins the tubercle. The oval convex articular surface, d", for that on the
diapophysis, is situated on the upper and towards the back part of the tubercle. The
body of the rib is moderately convex on its outer side, more convex transversely on
its inner side, where the convexity is bounded by a groove on each side, extending
half-way down the rib, near its rather sharp margins: in its lower half the rib be-
comes a little broader and less thick. The outer surface of the rib is well marked
by grooves and ridges for muscular attachment ; the best-developed eminence being
at a short distance from the tubercle ; and the largest and deepest groove being
behind the tubercle.

The hsemapophysis, or 'sternal rib,' h, is a straight subcompressed bone, with a
very irregular surface, which is somewhat convex on the inner side, but is traversed
by strong oblique ridges with intervening deep and wide channels on part of the outer
side. The surface of junction with the pleurapophysis is a very rough and irregular
one for ligamentous union : the opposite end of the hsemapophysis divides into two
convex condyles, s1 s1, separated by an oblique, deep and rather narrow groove; the
outer condyle projects further than the other; on the shorter one the articular sur-
face passes continuously from one side to the other, describing a semicircle; on the
longer condyle the articulation is divided by a median constriction into two oval con-
vex surfaces. One half of each of the condyles articulates with a corresponding con-
cavity on the 'sternal bone' (hs) of its own segment, the other half of each condyle
with the contiguous sternal bone.

The sterneber or sternal bone, completing, as ' hsemal spine,' hs, the typical seg-
ment in question, is a cuboid piece, divided into an outer or peripheral, and an inner
or central portion. The outer portion is subpentagonal, having its four corners exca-
vated by as many concavities for the hsemapophysis ; the two upper concavities, s" s",
being divided by a flat rough tract, the two lower concavities by a rough tuberosity.
The outer surface is flat and rough. The inner portion, or that next the cavity of
the chest, is larger than the other and has a hexagonal contour ; the four angular
concave articular surfaces, s", *", for the hsemapophyses being separated, at the sides
of the bone, by rough tracts ; and, above and below, by a flat articular surface, hs, by
which the bone articulates with contiguous sternal bones. The inner surface of this
portion is flat and rough, having been apparently covered by a strong aponeurosis in
the living animal. Thus the whole bone presents not fewer than ten articular sur-
faces, viz. a flat semicircular one above or in front of, and a similar one behind, the
posterior division ; and two concave articulations, s",s', on each side of both divisions
for the double condyles of two pairs of haemapophyses.

16

The chief modification in the sixth segment of the chest is the development of a
third articular surface at the back part of the base of the spine, between the two
posterior zygapophyses ; and the somewhat greater production of the ridge which
stands out from the fore-part of the base of the spine between the anterior zygapo-
physes. The inferior or haemal arch is also augmented by increased length of the
ribs. The key-bone of that arch, sterneber or haemal spine, Plate XI. figs. 4-7,
repeats the characters of that of the previous segment, save that the sternal articula-
tion, s, and the haemapophysial ones, hp, hp, of the central division of the bone are
more continuous, as shown in fig. 6. Fig. 5 shows the surface which was presented
towards the integument ; fig. 4 that which was turned towards the cavity of the chest ;
fig. 7 is a side view showing the four articular cavities for the double condyles of the
sixth and seventh haemapophyses ; fig. 6 shows the under surface of this remarkable
type of sternal bone.

In the seventh dorsal vertebra (Plate III. figs. 1, 2, 3), a third articular surface,
mz, fig. 1, is developed between the two anterior zygapophyses, z, z, to join that upon
the back part of the sixth vertebra; so that there are three zygapophyses, a median
and two lateral, on both the fore and the back part (mz1, fig. 2) of the arch of this
vertebra, making, with the three articular surfaces on each side (fig. 3, c', n', d') for
the ribs, and with the anterior and posterior surfaces of the centrum, not fewer than
fourteen joints. In this and the two following segments of the back (.Dsandg,
Plate I.), the ribs attain their greatest length. In the tenth segment the haem-
apophyses cease to articulate below directly with a haemal spine. The median zyg-
apophyses continue to be developed both before and behind to the twelfth dorsal
vertebra inclusive. In the thirteenth (Plate X. fig. 4) this supplementary articula-
tion is suppressed behind ; and the costal articulations have disappeared from the
diapophyses d. Those on the neurapophyses are almost circular, «', and those on the
upper and posterior angles of the centrum, c', have increased in size. The metapo-
physis (m), which was indicated by a protuberance above the diapophysis in the pre-
ceding dorsals, begins to assume the form of a rugged thick vertical ridge. The
fourteenth dorsal vertebra shows the progressive increase of size of the centrum, and
the absence of the median zygapophysis before as well as behind; and in it the
costal articulation on the centrum for the penultimate rib is lost, as well as that on
the diapophysis for the antepenultimate one, and only the subcircular concave neural
surface for the rib remains. In the fifteenth dorsal vertebra the posterior zygapo-
physes are convex transversely at their inner border, slightly concave in the rest of
their extent; the back part of the neural spine between these processes is deeply
grooved ; the metapophysial ridge increases in height and length ; a short and thick
anapophysis is developed from the back part of the base of the diapophysis, and on
the under part of the anapophysis there is a distinct, nearly flat, articular surface.
The sixteenth dorsal vertebra (Plate III. figs. 4 and 5, Plate X. fig. 5) offers a
corresponding modification at the fore-part of each neurapophysis, in the develop-
ment of a short, strong, wedge-shaped process, p, fig. 5, answering to the parapo-

17

physis in Myr-mecophaga* and Dasypus, with an articular surface, pa (Plate III.
fig. 4), on its upper part for junction with the anapophysis of the preceding vertebra.
The metapophysis, m, projects forward above the zygapophysis, the articular surface
of which (z, fig. 4) is continued upward upon the metapophysis (mz, fig. 4). The
anapophysis, a a, fig. 5, Plate X., forms a strong thick square plate of bone project-
ing upward, outward and backward from the diapophysis. The anapophysial arti-
cular surface, a', a', fig. 5, Plate III., is on the under and back part of this plate,
nearly parallel with the posterior zygapophysis, z', the convex inner border of which
has increased in thickness. The body of the vertebra assumes, with its larger size,
a more decided trihedral or wedge-shaped figure at this part of the spine.

The rib of the thirteenth segment loses the convex articular surface on the tubercle,
which is attached by ligaments to the diapophysis. The rib of the fourteenth seg-
ment loses the small flat surface at the extremity of the head ; and only the large
convex surface on the upper part of that end of the neck remains, which surface,
extending to the free end of the neck, reduces that part to an edge: the tubercle
exists in this rib, and is rough for ligamentous insertions as in the preceding. In the
fifteenth (Plate III. figs. 1 and 3) and sixteenth (ib. fig. 2) ribs the tubercle subsides ;
the neck of the rib is defined by the rugosity of its whole upper surface, save that
part where the articular convexity, n", remains for articulating with the neural arch,
as shown at «', fig. 5, Plate III.

The haemal spine (' sterneber') of the eighth segment, Plate XI. figs. 8-12, may be
the last of the so-called bones of the sternum. It is divided, like those in advance,
into a peripheral and a central portion. The peripheral portion (fig. 9) is of a sub-
quadrate form, the four corresponding articular surfaces (ha, ha') for the hseuiapo-
physes almost touching each other at their margins ; the outer roughened surface is
convex; the anterior haemapophysial articular surface is suppressed on the left side of
this division of the bone, fig. 10, to which the corresponding haemapophysis seems to
have been united by ligament. The central division of the bone (fig. 8) presents the
median flat surface (s, fig. 10) on its upper or fore-part for the antecedent sterneber,
and the concave haemapophysial surface, hp,hp, on each side of its anterior half; but
posteriorly the haemapophysial surface on each side is confluent with that of the
same side belonging to the peripheral division of the bone, which thus presents at its
lower or hinder part only two long oval concave articular surfaces (hp a, figs. 1 1 & 12)
for the pair of heemapophyses of the ninth segment of the chest, and the concavities
almost meet at the under surface of the sterneber, which there presents no articular
surface for a succeeding one. The number of articular surfaces, therefore, of this
bone is reduced to six ; one, s, for the antecedent sterneber, two on the anterior half
of the right side, hp, ha, fig. 12, for the bifid condyle of the eighth haemapophysis,
one on the anterior half of the left side, ha, fig. 10, for one of the condyles of the
opposite haemapophysis ; and a pair of surfaces on the posterior and lateral parts, hpa,
* Philosophical Transactions, 1851, Plate L. fig. 21,p,pa.

C

18

for the ninth pair of haemapophyses which terminate each by a single convex condyle.
It is possible that a more simplified sterneber may have intervened between the
haemapophyses of the tenth segment.

In the three segments of the trunk, Plate I. L i, a, ^ succeeding the last of the
dorsal series, both pleurapophyses and haemapophyses are wanting as distinct ossified
parts, and those segments are reduced to the coalesced elements, constituting the
' lumbar vertebrae' of Human Anatomy. The accessory articulations between the
parapophyses and anapophyses are continued in these vertebrae, which do not become
anchylosed together in the Megatherium as in the Mylodon.

I next proceed to trace the modifications of the segments as they recede from the
typical one in the opposite direction or towards the head.

The fourth dorsal segment much resembles the fifth, which has been taken as the
type; the pleurapophyses are shorter, especially at their cervix; but the complex
articulations of these and of the haemapophyses are repeated.

In the third segment the pleurapophysis, pi, fig. 3, Plate IX., and haemapophysis,
ib. h, are anchylosed together: both are shortened, but the pleurapophysis in a
greater degree ; this retains its three articular surfaces, c", n", d", on the head, neck
and tubercle ; and the haemapophysis, h, has its double condyle, s1, s", at the sternal
end, fig. 3 b, the inner one being single, the outer one divided by a narrow groove
into an anterior and a posterior convexity. The concave border of the rib is less
produced than in the fifth segment.

In the second dorsal segment (Plate I. D a) the neural spine is increased in
height, and the metapophysial tubercle is diminished in size. The vertebral, pi, and
sternal, h, parts of the rib (Plate IX. fig. 2) are anchylosed, and both are shortened :
the former retains its three articular surfaces on the head, c", neck, n", and tubercle,
d", that on the tubercle being the largest, and being partially divided into two con-
vexities (fig. 2a, d). The pleurapophysis {pi, fig. 2) is diminished in length, but
increases in breadth to its place of coalescence with the haemapophysis (h) ; the
convex articular surface (fig. 2b, V) on the outer condyle of the haemapophysis is not
divided : the inner condyle, d, is much reduced and has only a small articular
surface.

The first dorsal segment (Plate I. D i) is remarkable for the superior height and
antero-posterior extent of the neural spine, the summit of which expands into a
broad flat subtriangular rough surface, Plate IV. fig. 5. D i, ns. The anterior mar-
gin of the spine is sharp and produced. The anterior zygapophyses are not so near
each other as in the succeeding vertebrae; and they are continued outwardly upon
the base of the metapophysis, which is here more distinct from the diapophysis than
in the succeeding vertebrae ; and the articular surfaces of the zygapophyses are
slightly concave transversely. The costal concavity below the diapophysis is con-
tinuous with the smaller articular surface upon the side of the neurapophysis.

The pleurapophysis (Plate IX. pi, fig. 1) is much reduced in length, and is con-

19

fluent below with a short, thick, broad, subquadrate haemapophysis, h. The short neck
of the rib terminates in a small obtuse end without any distinct articular surface : this
end seems to have been imbedded in the ligamentous mass between the seventh cer-
vical and first dorsal vertebrae : the short neck quickly expands into the shaft of the
rib : a small elliptical surface, fig. 1 a, n", on the upper part of the neck, is continued
at its outer end into the larger convex surface upon the upper part of the tubercle,
d": from the middle of the anterior border of this surface a strong ridge is continued
down the outer surface of the rib to its hinder border. The sternal end of the haem-
apophysis, fig. 1 b, presents a large subtriangular surface, slightly concave in one
direction, slightly convex in the other, adapted to a similar single concavo-convex
surface on the side of the much developed and modified haemal spine called 'manu-
brium sterni.'

This bone (Plate XI. figs. 1, 2, 3) is of an oval or cordiform figure, with a pro-
minence on each side near its inferior truncated apex, below the lateral articulations,
hp, for the first pair of sternal ribs. The outer surface is principally concave length-
wise, and is sculptured by the impressions of the coarse aponeurotic structures which
have been adherent to it in the living animal : a short median longitudinal ridge pro-
jects from its lower part. The inner surface is chiefly convex, but is very irregular.
At its upper half a strong median prominence divides the shallow rough depressions,
cl, for the attachment of the clavicular ligaments : these depressions are deepest above
the costal articulations which are supported on well-marked triangular prominences.
Between these prominences the bone is rather concave. A strong rough tuberosity
projects below the lower angle of the costal surface. The contracted inferior end of
the manubrium terminates in an oval convex articular surface,*', for the second ster-
nal bone. There are no articular surfaces for the haemapophyses of the second dorsal
segment.

The skeletal segment (Plates I. C?\ IV. fig. 7) in advance of the first of the
dorsal series is reduced to its centrum and neural arch ; both pleurapophysis,
haemapophysis and haemal spine are absent ; and its remaining anchylosed elements
constitute the 'seventh cervical vertebra' of Descriptive Anatomy. This is most
remarkable for the great development of the neural spine (Plate IV. fig. 7, ns),
which exceeds that of the first dorsal vertebra: the summit is similarly expanded
and flattened above (Plate IV. fig. 5, ns 7) ; but the anterior margin is still more
produced, forming a low angle about half-way down the spine (fig. 7, ns). The
anterior zygapophyses are more remote from each other than in the first dorsal, and
their articular surfaces are chiefly supported by the inner side of the base of the
metapophyses, figs. 5, 7 w», which are here well developed and more distinct from the
diapophyses, d, fig. 7, than in the dorsal region. The diapophyses, figs. 5 &c7,d, are
strong, rugged, stand out from the sides of the neural arch, and terminate in rough
truncate ends. The posterior zygapophyses (figs. 6 & 7, *') are slightly convex.

In the sixth cervical vertebra (Plates I. Ce; V. figs. 5 & 6) the spine, ns, is much

c 2

20

shortened, though still long and large in proportion to the neural arch. The met-
apophyses, m, m, stand out from the upper part of the side of the neural arch
behind the anterior zygapophyses, z. The diapophyses, d, d, are developed from the
base of the neural arch, which has descended lower upon the sides of the centrum ;
and now we find another element — the pleurapophysis, pi — restored to the segment,
but reduced to rudimental proportions and anchylosed at two points. Its vertebral
end is bifid ; one portion, answering to the head of the rib, has coalesced with the
side of the centrum (at p, fig. 5) ; the other, answering to the tubercle, has united
with the under part of the diapophysis, d: what may be termed the body of the rib
is a short but broad rhomboidal plate (fig. 6, pi), projecting outward, downward and
a little backward. The space intercepted between the pleurapophysis and diapophy-
sis forms the canal, v, for the vertebral artery.

The fifth cervical (Plate I. Cs) differs from the sixth by its smaller dimensions,
especially by its shorter spine, and by the diminution in the breadth of the pleurapo-
physis, which terminates by a thick obtuse end : it sends out, however, a thin plate
forwards from its vertebral end. The metapophysis is a large obtuse tubercle,
Plate IV. fig. 5, m ».

In the fourth and third cervicals (Plate V. figs. 3 & 4) the neural spine is still
more reduced, and, contracting from its base, assumes a triangular shape, fig. 4, ns.
The anterior zygapophyses, fig. 3, z, are concave transversely, and look upward and
inward ; the posterior ones, fig. 4, z', are convex, with the reverse aspect : the met-
apophysis, m, continues to be developed as a distinct tuberosity, external and posterior
to the prozygapophyses ; and the pleurapophysis continues to send forward the
pointed plate from its fore-part, fig. 4, pi, its outer end, pi, being thick and tuberous,
like the diapophysis, d, above.

The dentata (Plate V. figs. 1 & 2) has its spine extended in the antero-posterior
direction, and of great strength, though low ; with a thick angular ridge projecting
from its fore-part and overhanging the neural canal; it is broad, flattened and
almost vertical behind, and has a subbifid summit (Plate IV. fig. 5, *, and Plate V.
fig. 1, ns). There are no metapophyses and no anterior zygapophyses, but the ana-
logous articular surfaces (figs. 41 & 42, zn) have descended upon the antero-lateral
parts of the coalesced centrum of the atlas or ' odontoid process,' and are adapted to
corresponding surfaces of the bases of the neural arch of the atlas. The posterior
zygapophyses, z', are wide apart, and are convex. The diapophysis, d, is short and
obtuse ; the pleurapophysis, pi, still developes its anterior angle, Plate V. fig. 2, pi1.
The fore-part of the odontoid process, o, is a rounded tuberosity, on the under sur-
face of which is the oval, slightly convex surface for articulating with the hypapo-
physis (Plate IV. fig. 3, o, hy), which has coalesced with the neural arch, ns, of the
atlas, and is commonly called the 'body of the atlas.' The under surface of the
centrum of the dentata developes a hypapophysial ridge.

The atlas, viewed from behind, as in Plate IV. fig. 3, is a large, transversely ob-

21

long, subdepressed, shuttle-shaped bone, perforated by a large aperture, quadrate
below for its detached centrum the ' odontoid process,' arched above for the spinal
cord.

The thinnest and smallest part of the ring of the atlas is formed by the hypapo-
physis, hy, which has coalesced with part of the bases of the neural arch, nn, and
has supplanted, as it were, the proper centrum, o, Plate V. figs. 1 & 2, which has
remained anchylosed to that of the axis. The upper surface of the hypapophysis
presents a shallow articular surface, o, Plate IV. fig. 3, for that centrum to rest and
turn upon. The hinder half of the base of the neurapophysis developes, on each side,
a slightly concave, subcircular, articular surface, z', for a moveable articulation with
that on the side of the odontoid, zn, fig. 1, Plate V. The atlas is perforated an-
terior and external to this by a foramen, Plate IV. fig. 1, s, answering to that called
' foramen alare posterius ' in the Horse, in which it gives passage to the posterior
branch of the occipital artery ; in the Megatherium the foramen or canal is bridged
over by a narrow oblique bar of bone, dividing its external outlet into two, r & s,
and through the hinder, s, of the divisions it is probable that a branch of the second
spinal nerve may have passed.

The diapophysis, d, is a broad, depressed, rounded aliform process, with a protu-
berance from its under and back part, like the rudiment of a pleurapophysis, pi.
Anterior to this process the under surface of the diapophysis is deeply and widely
excavated and perforated by the vertebral artery, the canal for which, opening upon
the upper surface of the diapophysis, is then continued obliquely inward, perforating
at q, fig. 4, Plate IV. the upper part of the neural arch, just within the upper part of
the condyloid concavities. A large part of the canal for the first spinal nerve, fig. 1, v,
opens into the outer commencement of the vertebral canal, and answers to that
called 'foramen alare anterius' in the Horse, which transmits the inferior branch
of the first spinal nerve as well as the anterior branch of the occipital artery. The
condyloid concavities, fig. 2, nz, are semioval, large and deep, and occupy nearly the
whole of the anterior surface of the neural arch, being separated above by a rough
tract of three inches' extent, upon which the vertebral canals open. There is a tri-
angular rough surface at the back and inner part of each condyloid concavity.

Such are the modifications of the different cervical vertebrae of the Megatherium.
With regard to the dorsal vertebrae, their chief characteristics may be briefly recapi-
tulated as follows : —

The first dorsal vertebra is distinguished by the confluence of the neuro-costal and
dia-costal surfaces, and by the superior height of the spine.

The second to the fifth dorsals inclusive, like the first, have only the ordinary pair
of zygapophyses before and behind, but have the neural and diapophysial surfaces
for the rib distinct.

The sixth dorsal is recognizable by having a third median zygapophysis behind,
but not in front. The seventh to the twelfth dorsals inclusive have the three zyg-

22

apophyses both before and behind. The thirteenth dorsal has the median zygapo-
physis in front but not behind : the costal surface has disappeared from the diapo-
physis. The fourteenth dorsal has only the ordinary pair of zygapophyses before and
behind, but may be distinguished from the second, third, fourth and fifth dorsals by
the absence of the costal articulation on the diapophysis, and of that on the upper
and hinder angle of the centrum. The fifteenth dorsal has an anapophysis on each
side with an articular surface, and has only the costal articulation on the neurapo-
physis. The sixteenth dorsal has on each side, at the fore part of the neural arch, a
parapophysis with a superior articular surface, and behind, an anapophysis with an
inferior articular surface. But it differs from the lumbar vertebrae by the costal sur-
face on the neurapophysis.

Having now described the principal characters of those segments of the skeleton,
the centrums and neural arches of which are comprehended in Anthropotomy under
the term of ' true vertebrae,' on account of their freedom of motion on each other, I
next proceed to the description of the ' false vertebrae ;' and first, of those that, being
anchylosed together, form the ' sacrum.'

This part of the skeleton includes five vertebrae (Plate VII. 1-5), which are not only
anchylosed to each other, but to both the iliac and ischial bones : the length of
the sacrum is 22 inches, its extreme breadth across the fifth vertebra, fig. 1, dt, is
20 inches. The centrum of the first vertebra (Plate VI. c) presents a transversely
oblong, subquadrate, flattened, articular surface for that of the last lumbar vertebra,
with its margin a little produced forwards, and developed below into a pair of rough
ridges, * *. The neural arch overhangs this surface, and developes a metapophysis
from the fore part of each side of its base, with a broad articular surface on its under
part, and a similar surface above (Plate VII. fig. 1, z), representing the anterior
zygapophysis ; the two surfaces meeting at a right angle at their inner borders.
The broad diapophysis of the first sacral, 1, is perforated by a small subvertical
canal, d', at its confluence with the ilium, and is separated from the corresponding
part of the next diapophysis by a larger orifice, o i, which is the first of the four
superior or posterior sacral outlets. The neural arch of the first sacral vertebra, n 1,
is separated from that of the second, «», by a narrow transversely elongated ellip-
tical vacuity. The neural arches of the three succeeding vertebrae are completely
confluent: a pair of triangular closely approximated apertures, n *, divides the base
of the neural spine of the fourth, ns *, from that, ns s, of the fifth sacral vertebra. The
neural spines of the first four sacral vertebrae have coalesced into a strong vertical
ridge, ns, ns «, increasing in thickness as it extends backwards, and being there from
two-thirds of an inch to one inch and a half thick across the broken summit. The
second posterior sacral canal, o 3, intervenes between the diapophysis of the second
and that of the third vertebra. The metapophysis, m «, of the fourth appears as a low
angular tubercle above and a little behind the diapophysis of the third sacral, 3. The
diapophysis of the fourth sacral, d 4, extends outwards beyond the ilium, as a sub-

23

depressed broad process with a rough free extremity ; the back part of the process
coalesces with a similar but stronger and longer diapophysis of the fifth sacral, d*,
from the fore-part of the base of which a tuberous metapophysis, m 5, projects upward
and forward. The third, o 3, and fourth, o 4, upper sacral outlets are wider apart than
the second and first; the sacrum expanding posteriorly. The back and under part
of the diapophysis of both the fourth and fifth vertebrae coalesce with the ischium
and with the thick and strong parapophysis extended from the side of the centrums.
The neural arch of the fifth sacral developes a pair of posterior zygapophyses, Plate
VII. z', z', with a flat surface looking outward and a little downward, and with the
lower angle continued upon a small rough subarticular surface. The posterior
surface of the last sacral vertebra, Plate VII. fig. 2, is on the same vertical parallel
as the posterior zygapophyses ; it is nearly flat and transversely elliptic. The neural
canal of the sacrum, the anterior aperture of which is 3 inches in vertical and 4 inches
in transverse diameter, expands in the sacrum, and opens below by three wide fora-
mina on each side: of these the first and second are of great size: into the second
foramen the third upper sacral canal leads: the third lower sacral foramen, which
is the smallest, corresponds with the fourth upper one: the fifth canal for the fifth
pair of sacral nerves broadly grooves the back part of the parapophysis and side of
the centrum. The posterior aperture of the neural canal is 2 inches in vertical and
4 inches 3 lines in transverse diameter. Both diapophyses and parapophyses of the
first three sacral vertebrae coalesce with the ilia. The sacrum is concave below both
transversely and lengthwise.

The tail of the Megatherium was of great strength : it is so long as to touch the
ground when the trunk is raised at an angle of forty-five degrees from the horizontal
position : it includes eighteen vertebrae, which progressively diminish in size from the
first to the last, Plate I. C d, 1-18.

The first vertebra, Plate II. fig. 2, is remarkable for the length and strength of its
diapophyses, d, which are expanded at both ends, and, like those of the sacral verte-
brae, are probably lengthened out by connate or coalesced pleurapophyses, pi. The
base of the process, dp, extends from the side of the centrum to the base of the neural
arch, is widely excavated behind for the passage of the first pair of caudal nerves,
and is subcompressed before it expands into its rugged free termination. These pro-
cesses are shorter than those of the last sacral vertebra. The neural canal, n, is tri-
angular, 3 inches in vertical and 3 inches 9 lines in transverse diameter. The neural
arch developes two posterior zygapophyses, z1, with their articular surfaces looking
downwards and outwards: two anterior zygapophyses, z, with their articular sur-
faces looking upwards and inwards, these being strengthened by a strong tuberous
metapophysis, m, on their outer side: the spine, ns, is of moderate length, carinate
behind, obtuse and slightly expanded above. From the under part, of the trans-
versely elliptical centrum are developed two hypapophyses, hy, each with an oblong
articular surface, Plate X. fig. 6, hy, to which is joined a haemapophysis, Plate II. //.

24

Each haemapophysis is a long slender conical bone, with an articular surface at each
angle of the base, hy1, hi/', and an obtuse slightly inflected apex : the inner side of
the bone is slightly concave, the outer one convex transversely : a rough tuberosity
dividing it from the inflected apex. The essential differences between the first caudal
segment and the dorsal one delineated on the same Plate are, that the pleurapophy-
sis, pi, is short and anchylosed to the diapophysis, the haemopophyses, h, articulate
with the centrum, and the haemal spine is absent, in fig. 2.

The second caudal vertebra (Plate VIII. figs. 1 and 2) differs from the first in
having an anterior, fig. 2, hy, as well as a posterior, ib. hy', pair of hypapophyses ;
and in the confluence of the haemapophyses, fig. 1 , h, at their apices forming the so-called
' chevron bone' (os en chevron, Cuvier). This vertebra is smaller than the first caudal
in all its parts except the haemapophyses, and in all its dimensions except the vertical
diameter, which is due to the development of the coalesced parts of those elements
into a long and strong haemal spine, hs. The anterior hypapophyses, fig. 2, hy, which
are the smallest, articulate with the surface on the back part of the base of the haem-
apophyses of the first caudal vertebra : the posterior hypapophyses, hy', which are
more oblong and closer together, articulate with the anterior and larger pair of sur-
faces, hy', of their own haemapophyses, fig. 1, h. There is a strong rough tuberosity
projecting backwards external to each of the anterior hypapophyses. The posterior
articular surface is, in the present instance, developed only on the right haemapo-
physis, fig. 1, hy"; on the left it is represented by a rough tubercle.

From the third to the fifth caudal vertebrae inclusive, the proximal end of each
haemapophysis has both the large anterior transversely oblong surface for its own
centrum, and the smaller subcircular posterior surface for the next centrum : the
spine, or coalesced portions, of the third pair is the longest in the caudal series ; be-
yond this it progressively diminishes. In the sixth caudal vertebra the haemapo-
physes have a rough protuberance instead of the posterior articular surface. After
the eighth the protuberance subsides to a rough ridge. In the eleventh caudal
(Plate VIII. figs. 3-6) the distal end of the coalesced and shortened haemapophyses,
hy, is truncate, and as broad as the divided bases. The under surface of the corre-
sponding centrums of the sixth to the eleventh caudal offers the articular surface on
the anterior pair of hypapophyses, fig. 6, hy' ; the posterior pair, ib. hy, are rough
tuberosities. The posterior zygapophyses (figs. 3, 4, 5, z' z') retain their articular
surfaces to the tenth caudal: in the eleventh they are mere angular projections. The
metapophyses, mm, are continued to the fourteenth caudal. The neural spine is
reduced to a low tuberosity on the thirteenth caudal (fig. 7, ns): the neural arch con-
tinues complete to the sixteenth, fig. 8, n: in the seventeenth, fig. 10, the neurapophyses,
ii, are mere exogenous ridges, bounding the sides of an open neural groove. The
haemapophyses are continued to the fourteenth vertebra: two pair of rough low
hypapophysial tubercles, hy, hy, fig. 9, continue to be developed to the sixteenth :
they subside on the penultimate caudal, fig. 11, in which the diapophyses are repre-

25

sented by an obtuse ridge on each side of the centrum. In the last centrum, figs. 12,
13, all the processes have disappeared, and it presents the form of a low rounded
cone, with a smooth concave pentangular base, fig. 13, and a rough tuberous
summit, fig. 12. This simplified modification of the central element terminates the
vertebral series.

fy 3. Comparison of the Spinal Column with that of other Bruta.

In the number of the true vertebra?, and of those in each kind, the Myrmecophaga
jubata, amongst the Bruta, agrees with the Megatherium. The Ai, or Three-toed
Sloth, Bradypus tridactylus, has the same number of dorsal and lumbar vertebrae,
but has two more in the cervical region; the Unau, or Two-toed Sloth, Cholaipus
didactylus, has the same number of cervical and lumbar, but has eight additional
dorsal vertebrae, being the greatest number known in any mammalian quadruped.
The Short-tailed Manis (Manis brevicaudata) has seven cervical and sixteen dorsal
vertebrae, but it differs from the Megatherium in having five lumbar vertebrae. The
Armadillo tribe (Dasypodida?) differ most from the Megatherium in the inferior num-
ber of the dorsal vertebrae, which do not exceed eleven in some species, nor twelve in
any. The Orycterope, Orycteropus capensis, shows its affinity to the Armadillos in
having but thirteen dorsal vertebrae : and, like them, it has five lumbar vertebrae.
With regard to the structure of the vertebrae, the Anteaters, both hairy (Myrmeco-
phaga) and scaled (Manis) most resemble the Megatherium in the length and the
uniform backward inclination of the spinous processes ; but these processes are not so
long in proportion to their antero-posterior extent. The spinous processes of the dorsal
vertebrae are short and, in the hinder ones, obsolete in the Sloths : the Unau shows
the nearest affinity to the Megatherium by having a few of the anterior dorsal spines
better developed than in the Ai. In the Orycterope the last dorsal spine is vertical,
indicating a centre of motion in the trunk, those behind and those before slightly
converging towards this centre : nothing like this appears in the Megatherium.

In the development of the accessory articular surfaces upon both anapophysis and
parapophysis of the last dorsal and lumbar vertebrae, the Megatherium manifests a
more direct departure from the Sloths and a proportionate affinity to the Anteaters.
The Armadillos, which likewise possess these accessory joints, have superadded pecu-
liarities of the posterior dorsal and lumbar vertebrae, in relation to the support of
their peculiar bony armour, of which the Megatherium offers as little trace as do the
Myrmeeophagae : I allude to the progressively and rapidly increasing length of the
metapophysis *. These, in the lumbar region, equal in length the spinous process
itself; to which the metapophyses bear the same relation in the support of the over-
arched carapace that the tie-bearers do to the king-post in the architecture of a roof.
From the fact of the metapophyses in the dorsal and lumbar vertebrae of the Mega-
therium, Plate I. and Plate III. figs. 4 and b,m, not being developed beyond the state

* See Plate XLIX. figs. 18 & 19, m, Philosophical Transactions, 1851.

D

26

of a tubercle, I long ago drew the inference that, like the Sloths and Auteaters, it was
not covered by a bony armour*.

With regard to the cervical vertebrae, the fact of the Megatherium having the
normal number in the Mammalian class, seven — if it were not sufficiently established
by the well-adjusted articulations of those in the skeleton here described, rendering
any supplemental vertebrae inadmissible, — would have been made most probable by
the same number being present in the skeleton of the Megatherium at Madrid, and
in the more complete skeleton of the Mylodon in the Museum of the Royal College
of Surgeons in London. Moreover, that one of the Megatherioids had seven cervical
vertebrae and no more is certain : the skeleton of the Scelidotherium, discovered and
deposited by Mr. Darwin in the Museum of the Royal College of Surgeons, having
been imbedded, without disturbance of the true vertebrae, and those of the neck being
exposed in the ordinary number, and in their natural juxtaposition, on the removal
of the stony matrix-f-.

The atlas in both the Ai andUnau presents but two perforations on each side upon
the upper surface; one in front, the other behind the base of the transverse process,
and this is less produced and is of a quadrate rather than a triangular form.

The dentata of the Unau resembles more that of the Megatherium in the size of
the spinous process than that of the Ai does ; but the spine in the Unau is pointed
behind, not bifurcate. In the forms and proportions of the spines of the succeeding
cervical vertebrae the Unau approaches nearer to the Megatherium than does any
other existing Edentate species; but the spine of the seventh cervical is by no means
proportionally so developed, and metapophyses are not present in any. The Arma-
dillos are distinguished from all other Bruta by the great breadth, the shortness and
the anchylosis of the middle cervical vertebrae. In the Anteaters (Myrmecophaga)
the spine of the dentata is low and is extended more forwards than backwards ; the
spines of the other cervical vertebrae are still less elevated. In the long-tailed
Manis very similar proportions of the cervical spines prevail.

The closest correspondence with the Megatherium in the form and structure of the
cervical vertebrae is presented as might be expected by its extinct congeners, the
Mylodon and Scelidotherium.

A resemblance of the Armadillos to the Megatherium has been pointed out in the
ossification of the sternal ribs, but this is a character common to the order Edentata,
and is consequently equally manifested by the Sloths. The Anteaters most resemble
the Megatherium in the double joints by which the sternal ribs articulate with the
sternum. There is, however, a character by which the Sloths peculiarly resemble the
Megatherium, viz. in the anchylosis of the sternal with the vertebral portion of the
rib in those of the first three dorsal segments. The Unau most resembles the Mega-
therium in the form of the manubrium sterni, having the same prolongation of that

* Geological Transactions, 2nd Series, vol. vi. p. 101 (1839).

t Description of the Fossil Mammalia collected during the Voyage of the Beagle, 4to. 1 838-40. pi. 20. p. 84.

27

bone in advance of the expanded part giving articulation to the first rib. This pro-
longation, which is not present in the Bradypus tridactylus, relates to the complete
development of the clavicles in the Cholcepus didactylus, and serves, as in the Mega-
therium, for their ligamentous attachment. In other existing Bruta the manubrium
sterni has a broader and shorter figure, and is generally emarginate anteriorly.
The succeeding sternal bones present the nearest resemblance to those of the Mega-
therium in the genus Myrmecophaga, in which they are divided into two parts, each
having articular surfaces for those on the bifurcate ends of the ossified sternal ribs ;
but here the broad depressed portion of the sternal bone is external, the stumpy cylin-
drical part internal or toward the thoracic cavity. The small subcubical sternebers
of the Sloths represent the peripheral divisions only of the more complex bones in
the Megatherium. Only the Sloths and Anteaters resemble the Megatherium in the
small number of the lumbar vertebrae, and the Megatherium most resembles the
Myrmecophaga in the number and complexity of the articulations between these.
The genera Manis, Dasypus and Orycteropus have five lumbar vertebrae.

Both the Mylodon and Scelidotherium have three lumbar vertebras ; but these had
coalesced with each other and with the sacrum in the skeleton of the Mylodon robustus
described by me*.

The Myrmecophaga; have five sacral vertebrae as in the Megatherium : the Oryc-
teropus has six : so likewise has the Bradypus tridactylus : the Bradypus didactylus
has seven sacral vertebrae; the Armadillos depart furthest from the Megatherium in
the unusual number of vertebrae which coalesce to form the sacrum, these ranging
from eight to ten in the different species.

It would be unsafe, however, to infer that the Megatherioids were more nearly
allied to the Anteaters than to the Sloths in respect of the structure of the sacrum
of the Megatherium, for the Mylodon robustus has seven sacral vertebrae, like the
Bradypus didactylus. The posterior expansion of the sacrum and its junction with
the ischia, so as to circumscribe the great ischiatic notch, is a character common to
the Order Bruta. The sacrum early anchyloses with the iliac bones in the Sloths ;
and the broad and expanded ilia of these animals, with the long and slender anterior
parts of the ischia and pubes, circumscribing a large obturator foramen, and bounding
a capacious pelvis in front by a narrow symphysis, are characters by which the Sloths,
of all existing Edentata, offer most resemblance in their pelvis to the Megatherium.

The part of the skeleton in which the Sloths differ most from the Megatherium is
the tail, which is so short as to be hardly visible in the entire animal ; whilst the Ant-
eaters and Armadillos present the extensive and complex development of caudal ver-
tebrae which characterizes the Megatherioid skeletons. In all the existing species of
the ground-dwelling Edentate families the tail is relatively longer and more slender than
in the Megatherium ; it is even prehensile in the Manis longicaudatus and in the Myr-
mecophaga didactyla ; but of the modifications of the terminal vertebrae on which that
power depends there is no trace in the Megatherium any more than in the Mylodon.
* Description of the Skeleton of the Mylodon robustus, 4to. 1842.

d2

28

The haemapophyses are distinct, but short and stumpy, in the first caudal vertebra
of the Dasypus longicaudutus, Pr. Max. : in the Myrmecophaga jubata they present
proportions much more nearly resembling those in the first caudal vertebra of the
Megatherium, and they are equally disjoined at their distal ends*.

The Mylodon in the number, as well as the proportions and structure of the caudal
vertebrae, makes the nearest approach to the Megatherium ; the haemapophyses are
equally distinct from each other in the first caudal*)*.

Upon the whole, our deductions from the characters of the parts of the skeleton
described in the present section of this memoir, would lead to an inference that Me-
gatherium was nearer akin to Myrmecophaga than to Bradypus ; nevertheless the cer-
vical vertebrae, the condition of the anterior ribs, the form of the manubrium sterni,
and the pelvis, illustrate the intermediate nature of the giant's affinities, and afford
an additional instance to many others which I have had occasion to point out, of a
closer adherence to a common type in extinct animals than in the existing species to
which they may be most nearly allied.

§ 4. Of the Skull.

The skull of the Megatherium is chiefly remarkable for its small relative size,
especially in respect of breadth, for the slight diminution of this diameter from the
occiput to the end of the nasal bones, for the length and strength of the ascending
and descending processes from the orbital end of the zygomatic arch, and for the
peculiar depth of the lower jaw, especially at its middle part, where it lodges the long
molar teeth.

In the skulls and portions of skulls which have come under my observation, all the
cranial and most of the facial sutures, save those that unite the tympanic to the rest
of the temporal bone, had been obliterated, and the originally complex assemblage of
bones forming the cranium and upper jaw had been reduced to a continuous whole.

Viewed from behind, the skull is remarkable for the degree to which the pterygoid,
palatine and maxillary portions descend below the true ' basis cranii ': in consequence
of which, the foramen magnum and occipital condyles appear to be situated in the
upper half of the direct back-view, Plate XIV. fig. 1.

The basioccipital (i) is a broad depressed plate with a thin smoothly rounded con-
cave posterior border : it slightly increases in thickness as it advances forwards to
blend with the basisphenoid (Plate XV. j), but developes on each side a rough protu-
berance for muscular attachments, anterior to the occipital condyles. In a side view,
Plate XII., these condyles form the most prominent parts of the occipital region,
which, as it rises above them, slopes forward, giving a very low character of intelli-
gence to the cranium.

* See Philosophical Transactions, 1851, Plate LIII. fig. 60.
f Memoir on the Mylodon, 4to. p. 69, pi. 8, fig. 5, b, b.

29

Each condyle (Plate XIV. fig. 1,», ») is a convexity of a subtriangular form, with
the base straight and the sides curving to an obtuse apex : the extent of their convex
curvature in the vertical or antero-posterior direction (Plate XII. fig. 1, 2) equals that
of a semicircle, and indicates that the Megatherium possessed considerable freedom
and extent of motion of the head. The parallel bases of the condyles are turned
toward each other and flank the sides of the foramen magnum, Plate XIV. fig. l,o.
This foramen is a wide or full ellipse with the longer axis transverse, and the plane,
through the greater prominence of the upper border, inclined forward and down-
ward at an angle of 135° with the basioccipital. The plane of the occiput forms,
with that of the basis cranii, an angle of 105°. The occipital region (ib. 1, 3, a, s)
is semicircular, its contour being formed by a thick rugged ridge continued from
the superoccipital (3) on each side to the mastoid, »: the osseous wall included be-
tween this arched ridge and the condyles (2), is divided into four shallow depres-
sions, by a sharp median vertical ridge terminating below at a venous foramen just
above the upper border of the foramen magnum, and by a pair of thick obtuse
ridges (r) extending from the sides of the arched ridge obliquely inwards to near the
apices of the occipital condyles, and thence to the paroccipitals (4). The spaces so
defined are roughened by muscular impressions.

The paroccipital (Plate XIV. fig. 1, 4) is a moderately developed triangular tube-
rosity abutting against the back part of the articular cavity for the stylohyal (Plate
XV. ie), and separated by the outer occipital depression from the mastoid (s). The
precondyloid canals (ib. p), which transmit the motory-lingual or ninth pair of nerves,
begin by a large oblique aperture at the middle of the inn'er side of the base of the
condyles and extend along a course of 1\ inches in extent forward and outward to
open upon the back part of the rough fossa between the paroccipital (4) and petrosal
(i«) externally, and the basioccipital and basisphenoid internally, which fossa answers
to the • foramen lacerum in basi cranii ' of ordinary mammals. The carotid fora-
mina (c) open upon the fore-part of this fossa. The external precondyloid foramina
(p, p) are situated on the inner side of the paroccipitals, are each an inch in diameter,
and thus indicate the considerable muscular development of the tongue, and its great
use in stripping off the leaves and smaller branches of the trees affording the nourish-
ment of the Megatherium.

The basisphenoid (Plate XV. 5) presents on each of its hinder angles a low
rugged subcircular protuberance (x) for muscular insertions ; in advance of which it
slightly expands, the lateral margins inclining downwards so as to form the begin-
ning of the smooth channel that contracts and deepens as it advances towards the
posterior aperture of the bony nostrils. The broad subvertical pterygoid plates (24) are
continued insensibly from the deflected margins of the basisphenoid ; they are smooth
on the inside, irregularly channelled for muscular attachments on the outside. The
posterior aperture of the bony nasal canal (Plate XIV. fig. 1, n) is a long and nar-
row vertical oval, bounded above by a low transverse ridge (Plate XV. r), which
extends from the hinder border of the bony palate about two inches backwards upon

B

30

the smooth surface of the pterygoid plates, where it gradually suhsides : this ridge
forms the angular termination of the bony palate (ib. ») behind. The substance of
the basisphenoid is excavated by air-sinuses, continued backward from the ordinary
position of the sphenoidal sinuses, and extending into the basioccipital as far back
as the jugular foramina,^. They are exposed by fracture of the walls in the pro-
tuberances marked .r, x, in Plate XV.

From the state of the sutures already alluded to, the limits of the superoccipital,
parietal, and frontal bones cannot be defined. Above the arched superoccipital ridge
(Plate XIII. fig. 2, >) there are two semielliptical rough depressions {a, a) for mus-
cular attachments; and, in advance of these, the upper surface of the cranium shows
two other similar but shallower muscular impressions, b, b. The smooth surface of the
parietal, gradually narrowed to an inch in width ( 7) between the temporal ridges (t),
again as gradually expands into the frontal region (n), and it is perforated, a little
anterior to the middle of the temporal fossa, by a submedian vascular (venous?)
foramen (v), about half an inch in diameter. The temporal fossae (Plates XII. &
XIII. v, t, is) are remarkable for their great antero-posterior extent, and for the
encroachment upon them by the peculiar process (c) sent upward and backward
from the malar bone, as. They are partially defined anteriorly by the extension of a
postorbital process (Plate XII. fig. 1, is, Plate XIII. fig. 2, 12) downward to the malar
bone(j«); but, beneath and within this slender process, they communicate freely with
the orbits, 0. The posterior boundary ridge, continued from that on the parietal bone,
curves forward below and is continued into the sharp upper border of the zygoma,
Plate XII. fig. 1,17. The surface of the temporal fossae is grooved and perforated
posteriorly by large vessels, and is everywhere strongly impressed by the attachments
of muscular fasciculi. The base of the zygomatic process of the temporal bone has
an extensive origin (Plate XIII. fig. 2, v), not less than 6 inches in antero-poste-
rior extent ;* its free portion, to where it joins the malar, being 3 inches in length.
It is a strong trihedral bar of bone, rather concave on the upper and outer sides, and
forming on the underside the glenoid articular cavity for the lower jaw. This cavity
(Plate XV. g), of an oval form with the long axis transverse, measures 3 inches by
2§ inches, and is half an inch in depth. Behind this cavity the base of the zygoma
(Plate XII. fig. I,*?) has coalesced with the mastoid (s) and petrosal (i«) elements of
the temporal, which combine to form the meatus auditorius externus, e. This canal
is subcircular, about 10 lines in diameter at the deeper part, where it is formed by
the above elements, but doubtless wider at its outer part, where it was completed by
the tympanic bone. This bone is wanting in the skulls of the Megatherium hitherto
transmitted to England : the absence of any fractured surface upon the contour of
the orifice of the auditory canal indicates, however, that the bone was a free element
of the temporal in the Megatherium as in the Mylodon* and Glossotherium-f*.

* Description of the Skeleton of an Extinct Gigantic Sloth (Mylodon robustus, Owkn), 4to, 1842, p. 28.
t Zoology of the Voyage of the Beagle, * Fossil Mammalia,' 4to, 1840, p. 59. pi. 16. fig. 4.

31

The mastoid (Plates XII. & XIII. fig. 1, s) forms a rugged process, in depth or length
not exceeding the paroccipital (4), but of greater breadth and thickness; above it,
externally, and probably in the line of the primitive suture with the squamosal, is a
venous foramen, Plate XII. fig. 1, v. The petromastoid — probably the petrosal part
in a greater degree — forms the hemispheric articular cavity (Plate XV. :a) for the
stylohyal (Plate I. ss), the anterior rugged wall of which cavity extends downwards
farther than any other part of the proper basis cranii, Plate XII. fig. 1, 16 : the petrosal,
anterior to this, sends down a shorter rough pyramidal process. The carotid
foramen (Plate XV. c), a full ellipse with diameters of 5 lines and 4 lines, is situated
between the petrosal and basisphenoid at the fore-part of that oblong depression
which is terminated behind by the large precondyloid foramen.

The stylohyal (Plate I. 38) has the form of a hammer, with a long, slightly bent
handle, terminated by an obliquely truncated rough surface for syndesmosis with the
ceratohyal. At the opposite end the handle is subcom pressed, and the head is formed
by a sudden expansion in the vertical direction, terminated posteriorly by a straight
but rugged margin, and with the upper end produced, thickened, and forming a
smooth convexity, or condyle, adapted to the cavity above described in the petro-
mastoid. The lower end of the head or expanded part of the hammer-shaped bone
is more produced, more rugged, and terminates obtusely. The outer surface has a
wide depression at the middle, which is rough, with several short and well-marked
ridges. The length of the specimen described is 8 inches, the breadth or depth of
the expanded end is 3 inches and a half.

The upper part of the coalesced frontals (Plate XIII. fig. 2, 11) forms a smooth tri-
angular plate, rapidly expanding to the postorbital processes (it) and very slightly
convex. Some indistinct traces of the fronto-nasal suture seem to show that the
nasal bones (Plate XIII. fig. 2, u) extended backward beyond the transverse parallel
of the postorbital processes : more distinct traces of the naso-maxillary sutures (21),
show that the coalesced nasals were 2 inches 9 lines across at their narrow posterior
part, where they are flat above : at first slightly contracting, they then gradually ex-
pand, and become more and more convex transversely to their anterior extremity.
Here the nasal bones are also thickened, are rugged for the firmer attachment of the
cartilaginous parts of the nose, and their under surface, being excavated by two longi-
tudinal grooves, the thickened terminal surface is divided into a middle (Plate XIV.
fig. 2, m) and two lateral («, n) parts, the latter being convex and subangular, and
the middle expansion slightly excavated. As in the Two-toed Sloth (Choloepus didac-
tylus), the under surface of each nasal bone sends off a terminal plate or process for
the attachment of a turbinal cartilage or ossicle. A narrow median groove indi-
cates the original suture between the nasal bones along their anterior half.

The cranial cavity of the Megatherium is considerably smaller than the cranial
part of the skull, the outer wall or plate of bone being separated by large irregular
air-cells from the vitreous plate, or that case of bone which immediately invested the

E 2

32

brain and its membranes. The vertical diameter of the cranial cavity is 4 inches
8 lines ; its transverse diameter, which is greatest at the posterior third part of the
cavity, corresponding with the posterior part of the cerebrum, is 6 inches. The
brain of the Megatherium, to judge from its bony case, must have been less, by
nearly one half, than that of the Elephant; but with the cerebellum relatively larger
and situated more posteriorly to the cerebral hemispheres : whence it may be inferred
that the Megatherium was a beast of less intelligence, and with the command of fewer
resources, or less varied instincts, than the Elephant.

The ' maxilla superior,' or maxillary bone, may be divided into a palatal, alveolar,
and facial portion: the latter (Plate XII. fig. 1, 21) is remarkable for the excess
of its vertical over its antero-posterior extent : it forms, with the coalesced lacry-
mal (/), the anterior and part of the inferior boundary of the orbit by a strong sub-
vertical outstanding plate, curved with the convexity forward, perforated at the
middle part of its base by the antorbital canal (/•), which is double on the left side, and
near the upper part of its thick obtuse margin by the lacrymal canal (/) : it is smooth
behind, or next the orbit, rather rough and irregular in front : a rough, shallow de-
pression (Plate XIV. fig. 2, s) near the upper part of this surface indicates the origin
of a strong labial muscle. The outer surface of the facial plate of the maxillary is
smooth and slightly undulated ; it evidently extends as far as the postorbital process
upwards and backwards, in connexion with the nasal bone : its anterior border
(Plate XII. fig. 1, «), terminating the side of the nostril, is vertical, slightly concave
and sharp, and is smoothly excavated on the inner side or towards the nasal cavity.
The lower part of this nasal wall presents a deep and rough sutural notch for articu-
lation with the premaxillary bone.

The alveolar part of the maxillary (Plate XV. i, v) extends about an inch below
the suborbital process. The extent of the alveolar tract is 10 inches ; its greatest
breadth is 2 inches 4 lines, viz. between the second and third teeth. The number of
alveoli is five. The first (?) has a subtriangular transverse section, with the apex very
obtusely rounded off and turned forward ; the borders of this alveolus are sharp and
somewhat produced below the level of the surrounding bone. The second alveolus (ii)
is close to the first, and the corresponding teeth are nearly in contact; its transverse
section is quadrate, the hinder side being the broadest, the outer side the narrowest;
the fore-side is more curved than the back one. The partition between this and the
third alveolus is thicker than the preceding one, and the teeth stand further apart.
The third and fourth sockets are most nearly of a square form, but the transverse
diameter predominates ; the fifth socket (v) is suddenly reduced in size, and re-
sembles most the first in form, but with the rounded apex of the triangle turned
backwards.

No trace of the suture between the maxillary (21) and palatine (20) bones remains:
the alveolar border beyond the fifth socket (v) rapidly contracts to the thin vertical
pterygoid plate («).

33

The bony palate terminates behind in an angular notch, formed by the ridges (r, r)
before described. The bony palate forms a narrow tract, with parallel lateral borders
gently diverging at the fore and back part of the tract, which is very slightly concave
transversely: it is perforated by numerous foramina; two long ones, like fissures
(Plate XV. v, v), opposite the interspace between the third and fourth molars, seem
to represent the post-palatal foramina; there are, also, some large foramina (u, u)
between the first alveoli. The extent of the palatal part of the maxillary in advance
of these alveoli is about 1 inch to the hindmost part of the premaxillary (22), and 2J
inches to the apex of the process (21) articulating with that bone.

The premaxillaries (Plates XV., XII. fig. 1, and XIII. figs. 1 & 2, 22) have coalesced
along the major part of their extent, leaving only a median fissure on their upper
surface (Plate XIII. fig. 2, 22), of about 1^ inch in length, at about the same distance
from their slightly expanded anterior ends ; at their under surface (Plate XV.)
the same fissure is more advanced, and contracts to a few foramina. They form a
slender, elongated, subdepressed, four-sided portion of bone, and constitute a singular
anterior termination of the skull.

At the base or back part this portion of bone measures 4^ inches across; the fore-
end is 2 inches 9 lines across ; the narrowest part, near this end, is 2 inches 4 lines
across ; the vertical diameter is pretty nearly throughout 1 inch 6 lines, but decreases
anteriorly. The posterior third of the bone sends upward from its median line a
ridge, which enlarges as it approaches the corresponding ridge from the maxillaries,
and there presents a smooth and gradually expanding groove at its upper part, for
the support of the vomer or its cartilaginous septal prolongation (Plate XIV. fig. 2).
Anterior to the median ridge begins the groove which sinks into the fissure, and is
then again continued forward as a groove to within an inch of the fore-end of the
bone: this part (Plate XV. 22) seems crossed by a rough plate or cap of bone, flat,
and about an inch in breadth at its upper part, and there terminating behind, as it
does below, in a free margin.

The under surface of the premaxillary mass (Plate XV. 22) is rather convex antero-
posteriorly, as also transversely along its middle third : the groove indicating the
primitive suture runs along the whole of this surface, and sinking into its fore-part,
opens by two or three foramina into the fissure which is seen on the upper surface.
The back part of the under surface of each premaxillary is notched to receive a
triangular process of the palatine part of the maxillary (21): the more slender median
parts of the notches partly divide the prepalatal or incisive fissure (*), which thus
presents the form of a chevron.

The malar (Plate XII. fig. 1,*) is a singularly developed mass of bone, and has
always attracted attention as one of the most remarkable features of the skull,
from the period of the earliest notices of the Megatherium. Its bulk and complex
shape appear to relate to the unusual share which a modified and largely developed
masseter muscle must have taken in the act of mastication.

34

Firmly articulated by extensive reciprocally indented sutures, at one end with the
maxillary (si), at the other end with the zygomatic bones (a?), and giving an extensive
surface of attachment, by a peculiar upward prolongation, to fasciculi of the temporal
muscle, it afforded the requisite fixity for the origins of the large and complex mas-
seter.

The suture with the maxillary is in great part obliterated in the skull under descrip-
tion ; but a portion remaining on both sides shows that the malar ascended to the
level of the antorbital foramen, Plate XIV. fig. 2, r : it forms the lower and a great
part of the hinder boundary of the orbit ; the latter by a triangular, slightly bent
postorbital process (Plate XII. fig. 1 , a), which almost touches the corresponding
more slender process of the frontal (ib. u). The ascending process (ib. c) is a long,
narrow, unequal-sided triangle with an obtuse apex ; the descending process (d) is a
longer and stronger one, extending, when the mouth is shut, outside and for three
inches below the alveolar border of the lower jaw : its extremity is obtuse and re-
curved. The fourth process (ib. b), which may be called the 'zygomatic' one, ex-
tends beneath the end of the corresponding process of the temporal bone, but the
obliteration of the suture in the present skull prevents a precise definition of its
limits. The whole outer surface of the malar is slightly convex, moderately smooth,
with a defined surface for muscular attachment near the back part of the base of
the descending process. The inner surface shows, by its well-marked ridges and
depressions, the vigorous action of the muscular fasciculi which derived their origin
from that part.

The orbit (Plate XII. fig. 1, o), of proportionally small size, as in all large mam-
malian quadrupeds, presents a long vertically oval form ; or rather, by the convex
border of the malar (a), is reniforra. Its peripheral contour is almost completed by
the descending postorbital process of the frontal (ib. u) in the present skull ; anterior
to which the prominent boundary is effaced by a broad smooth channel, where the
orbital surface is more directly continued upon the facial surface of the maxillary:
this part answers to the supraciliary notch in quadrupeds. The lacrymal bone being
completely coalescent, if not connate, with the maxillary, is recognisable only by the
lacrymal foramen (ib. /), which is just within or behind the obtuse anterior border of
the orbit. Admitting the essential presence of the lacrymal by this character, it then
combines with the frontal, maxillary and malar bones, to form the contour of the
orbit. Within this frame, the orbit, as already remarked, communicates extensively
with the temporal fossa.

The anterior aperture of the bony nasal canal (Plate XIV. fig. 2, m, n, w) is sub-
circular, and is formed by the nasals, maxillaries and premaxillaries ; the deep ver-
tical sides being contributed wholly by the maxillaries.

The formation of the external bony aperture of the organ of hearing has already
been described.

Mandible. — The chief characteristic of the mandible or lower jaw is the near

35

equality of its vertical to its horizontal or longitudinal extent, due to the height of
the coronoid process, and more especially to the depth of the dentigerous part of the
bone. The latter dimension relates to the interesting modification of the principle
of maintenance of the efficiency of the masticating machinery, as contrasted with that
in the great proboscidian quadrupeds with a similar diet to the Megatherium.

The condyle of the jaw (Plate XVI. fig. 1, a) is transversely elliptical, 3 inches in
the long, 1 inch 10 lines in the short, diameter: it is moderately convex, and least so
from behind forwards : it seems but a small surface for the articulation of so massive
a bone, laden with large teeth, to the cranium ; but the adequate firmness of suspen-
sion was afforded by the enormous muscles which seem to have embraced every other
part of the ascending ramus of the mandible. The coronoid process (Plates XII.
and XVI. fig. 2, b) was lofty compared with its antero-posterior diameter: it is
mutilated in the present skull, but seems to be entire in that of the skeleton at
Madrid ; and its form and extent may be appreciated in the figures published
by Bru * and Pander-J~. It is much compressed, begins to curve upward imme-
diately anterior to the neck of the condyle, being continued from the middle of that
part. The angular process (ib. <•) of the lower jaw curves backward 4^ inches below
the condyle : it is a broad triangular plate, moderately convex externally, concave
internally and chiefly by a slight inward bending of the lower margin, Plate XVI.
fig. 2, c. A few ridges on the comparatively smooth outer surface indicate the inser-
tions of muscles ; but the inner surface is strongly sculptured by pits and grooves
with strong intervening bony crests. The oblique beginning of the dentary canal
(e) is situated 6 inches below the condyle, and the foramen is 2 inches from the last
alveolus, but above its level. The anterior border of the base of the coronoid pro-
cess is below the interspace between the fourth and fifth alveolus ; on its inner side
is a large elliptical outlet of a division of the dentary canal, Plate XVI. figs. 1 & 2,/.
The outer and inner surfaces of the coronoid process present characters antilogous
to those on the same surfaces of the angular process, in regard to muscular traces,
but the concavity is on the outer side of the coronoid.

The lower contour line of the mandible, which is usually continued forward,
straight, or with a gentle curve or undulation, in ordinary quadrupeds, is interrupted
in the Megatherium about one foot from the apex of the angular process by a notch,
from which the contour line describes an abrupt deep convex curve below the molar
teeth, d, and then as suddenly rises and passes by a concave curve to the under
side of the long and slender symphysis, Plate XII. fig. 2, d, d. The depth of the
dentigerous part of the horizontal ramus is 9 inches 6 lines ; it is slightly convex
externally, and forms a flat deep vertical wall internally, ib. d, Hi.

* Gabkioa, J. Description del esqueletto de un quadrupedo muy corpulento y raro, &c. fol. Madrid,
1796, Lam. i. and ii. fig. 1.

t Das Riesen-Faulthier (Bradypus giganteus), fol. trans., Bonn, 1821, tab. iii. See also Cuviee, ' Ossemens
Fossiles,' 4to, torn. v. part 1. pi. 16. figs. 1 and 2.

36

The antero-posterior extent of the alveolar border is 9 inches (Plate XVI. fig. 1).
The first socket is irregularly four-sided, the front side being the shortest, slightly
convex, and with the angles rounded off between it and the outer and inner sides :
the outer side forms rather an acute angle with the hinder side. The area of the
second socket is more regularly quadrate, with the transverse diameter the longest;
that of the third socket is nearly a true square ; the fourth and last is similar to, but
smaller than, the first, with the shortest and most curved side at the back part, and
with the antero-posterior diameter a little exceeding the transverse one. The inter-
vals between the alveoli are narrow and subequal.

The rami of the jaw are blended together at the symphysis, which is of great extent,
Plate XII. fig. 2, Plate XVI. fig. \,d,d: it begins posteriorly at the fore-part of
the mandibular convexity, opposite the second alveolus, whence the symphysis
rapidly contracts to the shape of a scoop or spout, which is prolonged 8^ inches
from the alveolar part, and terminates in a thick, rough, rounded and emarginate
extremity : the canal at the upper part of this spout-like symphysis is semicylindrical,
slightly bent down at the end, and 3 inches in diameter ; it becomes roughened by
numerous small vascular impressions near the end, but elsewhere is smooth, and has
obviously served for the support, during acts of protrusion and retraction, of a long
cylindrical tongue. The margins of the canal are thick and rounded. The 'mental
foramina,' or anterior outlets of the dental canal (Plate XVI. fig. 2,g), are two on the
right side and three on the left, from 4 lines to 8 lines in diameter.

§ 5. Of the Teeth.

The teeth are of one kind, molars, five on each side of the upper jaw (Plate XV.
and Plate XVII. fig. 2, i,ii,iii,iv,v), four on each side of the lower jaw (Plate XVI.
fig. 1, Plate XII. fig. 2, i, ii, Hi, iv), eighteen in total number.

In the upper jaw, the first or anterior molar (/) is the second in point of size, the
last (v) being the least. The first molar (Plate XVII. fig. 2, i) is 8^ inches in length';
the pulp-cavity extends six inches from the base ; it presents two slight curvatures,
one having the convexity turned forward, and the other inward. The transverse
section (Plate XV. i) gives an irregular semicircle, with the convexity turned for-
ward, and the flat side next the second tooth ; the angles at which this side joins the
curve are rounded ; the outer angle is somewhat produced, and the outer side of the
curve is flattened. The central axis of the tooth, formed by the vaso-dentine *, is
irregularly tetragonal ; the cement is thick on the anterior and posterior surfaces,
thin on the sides of the tooth.

The second molar (Plate XV., Plate XVII. fig. 1 & fig. 2, ii) is the largest of the
upper series ; it exceeds 9 inches in length, is of a tetragonal form, with two slight
curvatures, as in the first molar. The posterior and broadest side is nearly flat, the

* See my ' Odontography,' and Art. Tbbth in ' Cyclopaedia of Anatomy,' vol. iv. for the definition of the
different dental tissues.

37

anterior side somewhat convex, the outer and narrowest side is concave, the inner side
is sinuous, having a median longitudinal eminence between two longitudinal concavi-
ties. The central axis of vaso-dentine (Plate XVII. fig. 2, v) is more compressed from
before backwards than in the preceding tooth, and its posterior surface is concave ;
the two transverse ridges of the grinding surface of the tooth formed by the dentine
(ib. d, d) are nearly equal ; but the sloping side formed by the vaso-dentine is larger
than that formed by the cement (ib. c).

The third tooth (Plate XV. its, Plate XVII. fig. 2, Hi) is of nearly the same size
and form as the second, but is somewhat narrower ; the anterior and outer angle is
less rounded off, and the external longitudinal depression is deeper: it is further
removed from the second tooth than this is from the first.

The fourth molar (Plate XV. iv, Plate XVII. fig. 2, iv) is smaller than the two
preceding, but of nearly equal length, viz. 8^ inches, and is distinguished from the
other teeth by being curved in only one direction, and that in a very slight degree,
the concavity looking, as in the other teeth, outward : the central axis of the tooth,
in reference to the anterior and posterior planes of the skull, is straight : the anterior
and posterior layers of cement decrease in thickness as they approach the base of the
tooth, so as to describe a slight curve, the convexity of which is turned, on both sides,
towards the adjoining tooth. The fourth molar is tetragonal, and with more equal
sides than the two preceding teeth ; the outer and inner sides are concave, the ante-
rior and posterior ones convex ; the angles are rounded, but the anterior and inner
angle is more produced than the rest. The grinding surface presents two equal trans-
verse ridges, the contiguous sides of which are the longest.

The fifth molar (Plate XIV. fig. 1, Plate XV. v, Plate XVII. fig. 2, *) is 5 inches
in length, 1 inch 2 lines in transverse, and 10^ lines in antero-posterior diameter:
its principal curvature presents its concavity forward, or toward that of the anterior
tooth ; the curve in the transverse axis of the skull is scarcely appreciable. The
transverse section of this tooth is rhomboidal, with the angles rounded, and with the
longest diameter intersecting the antero-internal and the postero-external angles.
The dentinal axis is transversely quadrilateral, with the posterior angles entire, and
t lie posterior surface concave: the layer of cement which covers this surface is the
thickest, and its posterior surface is convex: the layers which cover the outer and
inner sides of the tooth are, as in the rest, the thinnest ; the anterior layer is less than
one-third the thickness of the posterior layer. The anterior ridge of dentine is
slightly prominent, and the posterior alone forms the summit of a transverse emi-
nence with sloping sides, but these diverge at a more open angle than in the pre-
ceding teeth.

At the date of the publication of my ' Odontography,' no specimen of the lower
jaw of the Megatherium had reached England, and certain detached teeth with slight
differences from those known to belong to the upper jaw were conjecturally referred
to the lower one*. The entire bone, with the dental series complete (Plate XVI.,

* Odontography, p. 341.
F

38

Plate XII. fig. 2, i,ii,iii,iv), shows that three of those teeth were rightly so referred;
but that the small molar alluded to at p. 342, op. cit., does not belong to the lower
jaw, which has only four teeth in each ramus.

The first molar (Plate XVI. and XII. fig. 2, /) is 8 inches in length, with a
pulp-cavity of 5 inches in depth; it presents a curve, with the convexity forwards,
which is more marked than in any of the upper molars. The anterior surface is so
much less convex transversely than in the first upper molar, that the transverse sec-
tion presents a tetragonal rather than a semicylindrical figure ; the anterior side,
however, being only three-fourths the breadth of the posterior one, by which the first
lower molar may be distinguished from all the tetragonal teeth of the upper jaw.
Both the inner and outer sides are slightly concave transversely, the posterior side is
moderately convex. The posterior ridge has a base twice as thick as the shorter
anterior ridge. The greatest transverse breadth of the crown is 2 inches, the great-
est fore and aft breadth is 1 inch 7 lines.

The second molar (Plates XVI. and XII. fig. 2, ii) is the largest, at least the
broadest transversely, of those of the lower jaw. ft is 9 inches in length, with a
minor curvature, convex forward. The anterior side is the broadest, being more
extended inward than the posterior side: its transverse diameter is 2 inches 3 lines,
the fore and aft diameter of the crown is 1 inch JO lines: the base of the hinder
eminence in the latter diameter exceeds that of the front eminence chiefly by the
greater extent of dentine exposed.

The third molar (Plates XVI. and XII. fig. 2, Hi) is of the same length as the
second, but has its two diameters more nearly equal, the transverse section being
nearly square, the anterior division being rather the broadest transversely, and of
equal thickness from before backwards. Both this and the preceding tooth are con-
vex transversely before and behind, concave at the sides.

The last lower molar (Plates XVI. and XII. fig. 2, iv), with an equal antero-
posterior diameter to the preceding, is shorter and narrower transversely, especially
in regard to its posterior division, which is more rounded, or convex transversely,
behind, than in any of the antecedent teeth. The hinder slope of the hinder ridge
is more nearly horizontal, and those towards the middle of the tooth are less deep:
the modification of the grinding surface of this tooth relating to the flatter surface
of the fifth molar above, and its greater antero-posterior extent as compared with its
breadth compensating for the absence of a fifth molar in the lower jaw. The grind-
ing surface of the four lower molars equals that included between the anterior ridge
of the first molar and the posterior ridge of the last molar in the upper jaw.

Each molar has its base undivided, but excavated by a deep conical pulp-cavity
(Plate XVII. fig. 2,p,p), from the apex of which cavity a fissure is continued to the
middle of the grinding surface of the tooth, where it is more conspicuous in the upper
(Plate XV.) than in the lower molars. Plate XVII. fig. 2, exhibits a longitudinal
section of the five molars of the upper jaw, in situ. The central axis of vaso-dentine
(v) is surrounded by a thin layer of true or hard dentine (d), and this is coated by

39

cement (c, c), which is of great thickness on the fore and hind surfaces, but is thin
where it covers the outer and inner sides of the tooth.

As the outer layer of the vaso-dentine is first formed by the centripetal calcification
of the pulp, the thin crust of that substance at the open base of the tooth includes a
space equal to the vaso-dentine at the crown of the tooth : the contraction of the
base of the tooth is due to the progressively-diminishing thickness of the cement as
it approaches that part ; the intervening vacancy (m, m) in the socket indicating the
primitive thickness of the vascular capsule, by the ossification of which the cement
is formed.

The vaso-dentine (Plate XVII. fig. 3, v) is traversed throughout by medullary
canals, TcToth of an inch in diameter, which are continued from the pulp-cavity and
proceed, at an angle of 50° to the plane of the hard dentine, parallel to each other,
with a slightly undulating course, having regular interspaces, equal to one diameter
and a half of their own area, generally anastomosing in pairs by a loop (ib. /, I), the
convexity of which is turned toward the origin of the tubes of the hard dentine,
forming a continuous reflected canal.

The loops are situated near, and for the most part close, to the hard dentine. In
a few places one of the medullary canals may be observed to extend across the hard
dentine, and to anastomose with a corresponding canal in the cement. The inter-
spaces of the medullary canals of the vaso-dentine are principally occupied by den-
tinal tubes, which have an irregular course, form reticulate anastomoses, and termi-
nate in very minute cells, at least one hundred times smaller than the calcigerous
radiated cells of the cement.

The more regular and parallel tubes, which traverse the thin layer of unvascular
dentine (ib. d), are given off from the convexity of the terminal loops of the medullary
canals. The course of these tubes is more directly transverse to the axis of the tooth
than is that of the medullary canals from which they are continued. They run par-
allel with each other, but with fine undulations throughout their course. They have
a diameter of 10 oootn °^ an mcn> and have interspaces of about twice that diameter.
As the dentinal tubes approach the cement they divide and subdivide, and become
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 tubes of the cells of the contiguous cement.

The cement (ib. c), which enters so largely into the composition of the grinders of
the Megatherium, is characterized in that extinct animal by the size, number, and
regularity of the vascular or medullary canals (ib. m, m) which traverse it. They pre-
sent the diameter of ia10oth of an inch, and are separated by intervals equal to from
four to six of their own diameters. Commencing at the outer surface of the cement,
they traverse it in a direction slightly inclined from the transverse axis towards the
crown of the tooth, running parallel with each other; they divide a few times dicho-
tomously in their course, and finally anastomose in loops, the convexity of which is

f2

40

directed towards, and in most cases is in close contiguity with, the layer of hard
dentine. Fine tiihules are sent off, generally at right angles, from the medullary
canals, which quickly divide and subdivide, form anastomosing reticulations, and
communicate freely with the similar tubules that radiate from the lacuna; or calci-
gerous cells, ib. r, r. These cells are dispersed throughout the dentine, and present
an oblong form, with the long axis transverse to that of the tooth, measuring smooth
of an inch in diameter. The cavity of the cell, which is not quite occupied by their
opake contents, is often very clearly demonstrated. The tubes, which radiate from
the cells nearest the hard dentine, and from the terminal loops of the vascular canals,
intercommunicate freely with the tubes of the hard dentine. The tooth of the Mega-
therium thus offers an unequivocal example of a course of nutriment from the den-
tine to the cement, and reciprocally.

In the structure which the fossil teeth of the Megatherium and its extinct con-
geners clearly demonstrate, we have striking evidence of the rich organization of
those once-deemed extravascular parts, and that they were pervaded by vital activity.
All the constituents of the blood freely circulated through the vascular dentine and
the cement, and the vessels of each substance intercommunicated by a few canals
continued across the hard or unvascular dentine.

With respect to those minuter tubes, the more important as being more imme-
diately engaged in nutrition, which pervade every part of the tooth, characterizing
by their difference of length and course the three constituent substances, they form
one continuous and freely intercommunicating system of strengthening and repara-
tive vessels, by which the plasma of the blood was distributed throughout the entire
tooth for its nutrition and maintenance in a healthy state*.

The grinding surface of the molars of the Megatherium differs, on account of the
greater thickness of the cement on their anterior and posterior surfaces, from those
of all the smaller Megatherioids, in presenting two transverse ridges; one of the
sloping sides of each ridge being formed by the cement, the other by the vascular
dentine; whilst the unvascular dentine, as the hardest constituent, forms the sum-
mit of the ridge, like the plate of enamel between the dentine and cement in the
Elephant's grinder. The great length of the teeth and concomitant depth of the
jaws, the close set series of the teeth, and the narrow palate, are also strong features
of resemblance between the Megatherium and Elephant in their dental and maxillary

* The first statement of the continuation of filamentary processes of the pulp into the tissue of the growing
tooth was published in the ' Comptes Rendus de l'Acadcmie des Sciences,' Paris, 1839, p. 787, and the earliest

observation of their continuation into the dentinal tubuli was, I believe, recorded in the following passage:

" I had the tusk and pulp of the great Elephant at the Zoological Gardens longitudinally divided, soon after
the death of that animal in the summer of 1847. Although the pulp could be easily detached from the inner
surface of the pulp-cavity, it was not without a certain resistance ; and when the edges of the co-adapted pulp
and tooth were examined by a strong lens, the filamentary processes from the outer surface of the pulp could
be seen stretching as they were withdrawn from the dentinal tubes before they broke." — Art. Teeth, Cyclo-
paedia of Anatomy, vol. iv. p. 929.

41

organization. In both these gigantic phyllophagous quadrupeds provision has like-
wise been made for the maintenance of the grinding machinery in an effective state ;
but the fertility of the Creative resources is well displayed by the different modes in
which this provision has been effected : in the Elephant, it is by the formation of
new teeth to supply the place of the old when worn out; in the Megatherium, by the
constant repair of the teeth in use, to the base of which new matter is added in pro-
portion as the old is worn away from the crown. Thus the extinct Megatherium had
both the same structure and mode of growth and renovation of the molar teeth,
as are manifested in the present day by the diminutive Sloths.

§ 6. Comparison of the Skull and Dentition.

The important affinity indicated by the dentition is confirmed by the characters
of the skull. In no other edentate family, save the Bradypodidae, is the cheek-
bone so nearly developed to the megatherioid proportions of that bone ; in no other
does it ascend above the zygoma into the temporal fossa or descend below the level
of the molar teeth. The large and complex malar bone is also associated, in the
Sloths, with a terminal position of the great anterior and posterior orifices of the
cranium, with terminal occipital condyles, and in the Ai {Bradypus tridactylus) with
a sloping occipital region. The cranial division of the skull is relatively as great in
the Sloths as in the Megatherium ; and the actual capacity of the cerebral cavity is
masked by a similar expansion of the air-cells, which almost everywhere surround
that cavity, and raise the outer plate of its bony parietes above the inner one. The
occiput presents the same expanded proportions, the same broad depressed basilar
plate, and the anterior condyloid foramina are of large relative size. The tympanic
is small ; it nearly completes a circular frame for the ear-drum, to which function it
is limited, and it long remains a separate bone. The detached and, in the skull de-
scribed, lost tympanies of the Megatherium have been evidently restricted to the
same office. The temporal fossa, in the Sloths, is long and large, and communicates
freely with the orbit, the outer boundary of which, however, is not completed in any
living species of Sloth. The nasals become confluent in old Sloths, and develope
turbinal laminae from their under surface. The premaxillaries are edentulous and
without any ascending process. The rami of the lower jaw expand and branch out
behind into a coronoid, a condyloid, and a long and deep angular process, and they
are anchylosed anteriorly at a broad sloping symphysis. Only in the genus Bradypus,
amongst known existing quadrupeds, do the alveoli of both jaws correspond in num-
ber, simplicity, relative depth and position with those of the Megatherium. The
still more important agreement between these existing and extinct Bruta, in the
peculiar structure of the teeth, yields the crowning proof that it is to the diminutive
arboreal Sloths that the gigantic Megatherium and its less bulky though larger extinct
congeners have the closest natural affinity.

The chief differences observable in the cranial anatomy of the Sloths, as compared

42

with that of the Megatherium, are the greater relative depth and breadth, and the
more convex outline of the coronal aspect of the skull ; but this difference would be,
doubtless, much less marked in the immature than in the adult Megatherium. The
zygomatic process, in the Sloths, is relatively shorter, and does not attain the malar
bone ; this, therefore, has not the middle process for supporting the zygoma, and is two-
pronged, instead of being, as in (he Megatherium, four-pronged. The chief characters
by which the Megatherium deviates, in its cranial structure, from the bradypodal and
approaches to the myrmecophagal type, are the elongation of the slender edentulous
fore-part of the upper jaw, and of the corresponding grooved slender edentulous part
of the lower one: but the prolongation of the upper jaw is due to relatively longer
premaxillaries than are developed in any of the true Edentata. The zygomatic
arches, moreover, are more defective in the Anteaters and Pangolins than in the
Sloths ; the malar part especially being minute or obsolete. Only in the Orycterope
and Armadillos, amongst the existing Bruta, is the zygomatic arch complete, but it is
simple, without ascending or descending processes. The great Glyptodon, indeed,
exemplifies that, tendency to community of characters so often presented by extinct
species, in an inferior prolongation of the malar bone analogous to that of the Sloths
and Megatherium. With other existing mammals than those of the Edentate order,
it would be lost time to pursue the present comparison with a view to the elucidation
of the affinities of the Megathere. It needs only to place the skull of this animal by
the side of those of the Elephant, Rhinoceros, Sivatherium, Ox, Elk, Horse, Dugong,
or other vegetable-feeding mammal of corresponding or approximate size, to be struck
with the peculiarities of the fossil, and to be convinced that the habits and mode of
feeding of the Megatherium had been such as are no longer manifested by the larger
Herbivora of the present day.

It remains then to inquire whether, among the extinct forms of the mammalian
class to which was assigned the office of restraining the too luxuriant vegetation of a
former world, there be any that, fiom their cranial or dental characters, may be con-
cluded to have resembled the Megatherium in the mode of performing that task.

The skull of the Mylodon, while it presents all the essential resemblances to that of
the Megatherium which have been pointed out in the skull of the Sloth, as, e.g. the
long cranium, the terminal position of the occipital condyles, and of the occipital
and nasal apertures, and the large and complicated malar bones, approximates still
more closely to the Megatherium in the junction of the malar with the zygomatic
process of the temporal, and in the relative depression and flatness of the elongated
cranium. But in thus receding from the existing Sloths, the Mylodon does not
approach any other existing genus, but only another member of its own peculiar
extinct family.

The most marked differences in the skulls of the Mylodon and Megatherium de-
pend on the minor length of the teeth and consequent depth of their sockets in the
smaller species, which require a less vertical extent of the maxillary bone in the

43

molar region and of the corresponding part of the lower jaw, the lower border of
which is consequently nearly straight in the Mylodon, as it is in the existing Sloths.
So great a proportional extent of the descending process of the malar bone is con-
sequently not required, and this process is more oblique in direction, and is relatively
broader and thinner than in the Megatherium. In these differences also the Mylo-
don shows its closer resemblance to the Sloths. The basioccipital is relatively
broader, and the occipital condyles are wider apart in the Mylodon; the occipital
plane is more inclined and the zygomatic process proportionally weaker than in the
Megatherium. The minor depth of the lower jaw and the lighter grinding instru-
ments call for a less extensive origin of the temporal muscles, and accordingly the
superior boundaries of their fossa: are separated in the adult Mylodon by a wide and
smooth parietal tract*, as in the Sloths. The postfrontal process is rudimentary in
the Mylodon, and the postorbital process of the malar bone is obsolete ; the orbit is
consequently without any bony boundary behind ; the malar, accordingly, has but
three processes, and is thus less complicated than in the Megatherium. The inter-
orbital part of the skull is relatively narrower, the maxillary part relatively broader,
than in the Megathere. No trace of premaxillaries was present in the skull of the
Mylodon robustus described by me, and the broad truncated symphysis of the lower
jaw indicates that they must have been very small if they existed : the peculiar length
of the premaxillaries in the Megatherium, and the corresponding prolongation of the
long and narrow symphysis mandibular, offer the most conspicuous differences in the
conformation of the skull, and proportionally remove that genus from the existing
Sloths. The bony palate is absolutely broader in the much smaller skull of the
Mylodon than in the Megatherium, and it gradually contracts from the first to the
fifth molar: it is, e.g. 5 inches in breadth between the first molars in the Mylodon
robustus, and only 2 inches in breadth between the same teeth in the Megatherium.

The opportunity of a comparison of the skull of the Megalonyx with that of the
Megatherium is yet a desideratum : it would probably demonstrate some inter-
mediate modifications between the latter and the Mylodon.

The extinct megatherioid animal of which, after the Mylodon and the Megathe-
rium, the most complete cranium has hitherto been obtained, is the Scelidotherium-f~.

In many respects the skull of this animal resembles that of the Megatherium more
than does that of the Mylodon. The plane of the occiput is rather less inclined
from below forward than in the Megatherium, but more resembles that part than in
the Mylodon : the upper boundaries of the temporal fossae more nearly approximate
than in the Mylodon : the bony palate is narrower, and its sides more parallel than in
the Mylodon ; but instead of being concave transversely, as in the Megatherium, it
is convex : the alveoli are nearer together than in the Mylodon, and the first is not
separated by a wider diastema than the rest. The symphysis of the lower jaw is much

* Memoir on the Mylodon, 4to, 1842, pi. 3.

t Fossil Mammalia of the ' Voyage of the Beagle,' p. 73. pis. 20, 21, & 22.

44

prolonged, but is less deeply channelled above than in the Megatherium, and is not
so distinctly denned by the abrupt increase of depth of the ramus behind it which
characterizes the Megatherium ; the molar part of the mandible makes, however, a
greater convexity below than in the Mylodon.

With these marks of approximation to the Megatherium there are, however, the
same differences as in the Mylodon in regard to the widely open orbit, the more simple,
trifurcate, malar bone, the minor depth of the alveolar portions of the jaws, and the
straighter outline of the lower border of the mandible. In both the Mylodon and
Scelidothere the coronoid process is relatively shorter than in the Megathere, and
the foramen near the fore-part of the base of that process is outside and below that
base, not on the inside of it as in the Megatherium. The mastoid process is relatively
shorter and the stylohyal pit is shallower; the lacrymal bone is more distinct, and
the foramen is larger in the Scelidothere than in the Mylodon or Megatherium. In
all the essential characters of the lower jaw, as in the number, structure and kind of
teeth, the extinct megatherioid quadrupeds more closely resemble each other, and the
existing Sloths, than any other known existing or extinct animals.

The number of the teeth, their length, equable breadth and thickness, and absence
of fangs, their deeply excavated base, and the unlimited growth resulting from the per-
sistent matrix, together with their composition of cement, dentine and vaso-dentine,
without any true enamel, are characters common to the Megatherioids and Sloths.
The form of the teeth differs in, and characterizes, each genus. It would seem that
the Megalonyx, in the elliptical or subcylindrical shape of such of its teeth as are
known, more closely resembled the existing Sloths than do the other Megatherioids.
The Unau (Cholaepus didactylus) resembles the Mylodon in the distance between the
first and the second molars of the upper jaw; but the advanced molar assumes, in
that existing Sloth, the form and proportions as well as the position of a canine, and
the corresponding tooth of the lower jaw is similarly developed and separated from
the other three teeth by a nearly equal interval. In the Ai (Bradypus tridactylus)
the first molar in both jaws presents nearly the same proportionate size to the rest, as
in the Megatherium, and is not separated from them by a wider interval than the
rest, as it is in the Unau and Mylodon. In both species of existing Sloth, the last
molar of the lower jaw is as simple in form as in the Megatherium : in the Mylodon
and Scelidotherium it is larger and more complex in shape, the grinding surface
being divided into two lobes by two oblique channels, which traverse longitudinally
one the outer the other the inner side of the tooth : these grooves are more shallow
in the Scelidothere than in the Mylodon, and the lobes of the tooth are more equal
and more compressed. The grinding surface itself, in all the molars of both Mylo-
don and Scelidothere, resembles that in the Sloths; the two transverse ridges are
developed only in the teeth of the Megatherium, which are longer in proportion to
their thickness than in the Mylodon, Megalonyx, or Scelidothere. These modifica-
tions, with the narrow palate, the close-set series of teeth, their great length, and the

45

concomitant depth of the jaws, are features of resemblance to the maxillary and dental
characters of the Elephant; but the fundamental structure of the teeth, not only of
the Megatherium, but of all its extinct congeners, is manifested in the present day
exclusively by the restricted and diminutive family of the Bradypodidce. I conclude,
therefore, the present section of this memoir by repeating the remark which I was
led to make in a former memoir*, relative to the existing Sloths : — " These Mammals
present to the zoologist, conversant only with living species, a singular exception in
their dental characters to the rest of their class ; but there has been a time when
this peculiar dentition was manifested under as various modifications as may now be
traced in some of the more common dental types in existing orders of Mammalia."

Comparative Table of Dimensions of the Skull of the Megathere, Mylodon, and

SCEU DOTH ERE.

Megatherium
Americanum.

Mylodon
robustus.

Scelidotherium
leptocephalum.

Cranium.
Length from the occipital condyles to the fore-end of the upper

jaw

Length from the occipital condyles to the fore-part of the

malar bone

Length from the fore-part of the malar bone to the fore-end

of the upper jaw

Breadth across the widest part of the zygomatic arches

Least breadth at the interspace of those arches

Breadth of the fore-part of the nasal bones

Mandible.

Length

Breadth between the hinder ends (angles) of the rami

Breadth between the condyles

Breadth between the posterior sockets of the teeth

Breadth between the anterior sockets of the teeth

Breadth across the fore-part of the symphysis

Depth of ascending ramus from the upper part of the condyle
Depth of ascending ramus at the fore-part of the base of the

coronoid process

Depth of horizontal ramus at the fore-part of the first socket

Length of the symphysis following the outer curve

Fore and aft extent of base of coronoid process

From the back part of the condyle to the end of the angular

process

From the end of the angular process to the last socket

From the first socket to the anterior margin of the jaw ,

Extent of the alveolar series

Breadth of the condyle

ft. in. tin.

2 7 0

1 8 0

0 10 6

0 6 0

0 4 6

0 9 0

0 8 0

1 1 0
0 9 0

ft. in. tin.

1 6 6
1 2 0

0 5
0 10
0 5
0 3

1 3 6

0 6 3

0 4 2

0 2 10

0 4 0

0 5 4

0 5 7

0 3 7

0 3 0

0 4 3

0 3 8

3 2

6 6

3 6

5 4

2 6

ft. in. tin.

1 8 4

0 11 6

8 8

7 0

3 7

2 8

1 6

0 4

0 1

0 1

0 1

0 2

0 5

o

0

0 7 10

0 4 4

0 1 10

§ 7- Of the Bones of the interior Extremity.

The bones of the limbs of the Megatherium are not less fraught with interest to
the comparative anatomist and physiologist than are those of the trunk and head,
by reason of their peculiar proportions and configurations, and, more especially,
as the unguiculate type on which they are constructed is exemplified in a quadruped
of such enormous bulk. The anterior extremities exceed the posterior ones in

* On the Mylodon robustus, 4to, p. 45.
G

46

length : in their bony structure they include a complete clavicle with the scapula, a
humerus, an antibrachium consisting of fully developed and reciprocally rotating
radius and ulna, the carpus, metacarpus and four digits ; and manifest, in short, all
the main perfections of brachial structure, save the opposable thumb, observable in
the Mammalian class. These perfections, moreover, are associated with proportions
and processes indicative of enormous strength, and bespeak a limb fitted not only to
take its full share in the support of the body, but to be employed in operations in
which unusual resistance has manifestly to be overcome. In no respect, perhaps,
does the Megatherium more strikingly differ in its osseous structure from the existing
quadrupeds of corresponding bulk, than in the bony fulcra of the anterior extremity.

The scapula (Plate I. n, and Plate XVIII. figs. 1 & 2) presents a vast expanse of
bone, with a double spinous process ; the normal one expanding into a large acromion
which is continued into and is confluent with the coracoid process. The scapula
presents an inequilateral triangular form, of which the acromion is the apex ; the
main body or plate of the bone approaches to the form of the geometrical trapezium,
except that the extent of the posterior border or base is such as to destroy the
parallelism of the upper and lower borders. The upper border (b, c) is the shortest ;
but, in one specimen, owing to the greater development of the basal border it appears
to begin, as in Plate I., at the part of the base marked a in Plate XVIII., and to
form a low angle, as if continued about one-fourth of the distance from the base
parallel with the lower border, whilst the rest of the costa inclines downwards or
converges towards the neck of the scapula, with a slight concave outline. The upper
border increases in thickness as it passes into the origin of the coracoid, c. The
base of the scapula, from the point a, is straight as far as the origin of the spine ;
it then bends, with a convex curve, and increases in thickness to the inferior angle of
the scapula (d) close to which commences the second or lower spine. The inferior
costa of the scapula (d) extends forwards straight and parallel with the lower spine,
for some way, and is so continued into that spine that the fore-part of this might be
regarded as the inferior and anterior angle of the scapula bent outwards; but it
curves down to what may be more correctly considered as that angle at e, fig. 1,
Plate XVIII.

The normal spine of the scapula, notwithstanding its being the superior of the two,
commences at only one-fourth of the entire base of the bone from the inferior angle ;
it is thence continued forwards, gradually rising or gaining depth, parallel with the
inferior costa, and thus marks out a very large proportional extent of the outer
surface of the bone for the supra-spinal fossa, Plate I. si. As the spine advances it
increases in breadth as well as depth, until it springs clear of the main body of the
bone as the acromial process, k. This large, thick and rough process arches forward
and upward over the glenoid articular cavity (g), and meets and coalesces with the
coracoid (c), spanning the anterior outlet (/) of the supra-spinal fossa by a strong
and broad bridge of bone. Through the reciprocal modification of the coracoid, the
passage for certain vessels and nerves, which is usually in Mammals a mere notch of
the upper border of the scapula, is converted into a complete foramen, ft.

47

The second spine, which answers to the ridge denning the upper part of the fossa for the
' teres major ' muscle in Man, runs parallel with the upper spine, and then bends down to
terminate in the low angle (Plate XVIII. fig. 1, e), projecting from the inferior costa of the
scapula, about one-third of the length of that border from the glenoid cavity (g). The
shape of that cavityis almost an ellipse (Plate XVIII. fig. 2, g), the lower end being scarcely
more contracted than the upper one : both these parts of the periphery are rather more
produced than the lateral borders. The entire margin of this moderately deep and well-
defined articular cavity is thin, but convex. The inner surface of the scapula (Plate XVIII.
fig. 1) is divided into many shallow depressions by intermuscular ridges (i, i), having a
general tendency to converge from the basal border, where most begin, towards the
centre of the bone, though none extend so far. The inferior costa (d) is continued
more directly towards the inner side of the glenoid cavity by a smooth, convex rising of
the inner surface, which grows broader as it gradually subsides : the fore-part of the
lower costa (fig. 1, e) is rather a continuation of the inferior spine; between which and
the portion of the lower costa (d), is the wide channel, attesting the size and force of the
homologue of the teres major. The vigour of the enormous subscapularis muscle is
manifested by the intermuscular ridges (i, i).

Compared with the scapula of the Mylodon, that of the Megatherium differs in the
lower position of the spine, and the consequent greater expanse of the supraspinal fossa :
the base is straighter and relatively longer, the inferior costa relatively shorter. The
ridge extending from the inferior and posterior angle forwards upon the under surface
of the scapula, is relatively stronger in the Megatherium, and gives to the homologue
of the inferior costa in the Sloth's blade-bone the character of a second spine. As such
' second spine,' with a prolongation of the scapula below, characterizes that bone in the
Anteaters, this indication of affinity to Myrmecophaga, or rather manifestation of the
more general characters of the order Bruta, by the Megatherium, is not without interest.
The confluence of the acromion with the coracoid is peculiar to the Sloths amongst
existing Mammals.

Clavicle. — Before the discovery of the Megatherium, Man was usually cited in Manuals
of Comparative Anatomy as the largest animal possessing a ' collar-bone ' : he is, in fact,
the largest existing animal so endowed. But whilst the length of the human clavicle
averages but 6 inches, that of the Megatherium is 15 inches. In its general shape and
sigmoid curve, this bone (Plate XIX. fig. 1) singularly resembles the human clavicle,
but is thicker in proportion to its length. No single bone would have better excused
. the common conclusion of the mediaeval anatomists as to the nature of large fossil bones,
viz. that they were those of human giants, than the collar-bone of the Megatherium.

The sternal end is expanded, obliquely truncate, with a rough, irregular, undulating,
but mainly convex articular surface, much fitted for strong ligamentous attachment to
the manubrium sterni, and with a narrow, rather flattened facet above this surface,
where it abuts against the same part in its fellow, as shown at Plate XXVII. as.

The shaft of the bone is most contracted at its sternal third part ; thence it gradually

H

48

expands to the acromial end ; the anterior surface is moderately smootli and convex :
the posterior surface (Plate XIX. fig. 1) is rough and more flattened, and is traversed
obliquely by a broad ridge, which terminates outwardly, or developes the rugged upper
border of the acromial half of the bone. This expanded end bends downward, as the
sternal end bends upward. There is a large tuberosity on the outer or fore-part of the
acromial expansion, which terminates in an oblong convexity, adapted to the concavity
beneath the expanded end of the acromion. The strong aponeurotic character of the
periosteum of the clavicle is well shown by the linear decussating ridges on most parts
of the surface of the shaft. The bone is solid.

The closer affinity of the Bradypus didactylus, as compared with the Brad, tridactylus,
to the Megatherium, is illustrated by the complete clavicles which attach the scapula?
to the sternum ; but they are straighter, relatively more slender, and more suddenly
expanded at the sternal end than in the Megatherium. One species or variety of
Three-toed Sloth has only a small styliform clavicular bone, appended to the coracoid
process. The clavicle in Orycteropus and Mynnecophaga is complete, but has a single
curvature. The clavicle of the Mylodon is intermediate, in its form and proportions,
between that of the Megatherium and that of the Two-toed Sloth.

The supposed peculiarity in the articulation of the clavicle of the Megatherium with
the first rib instead of the sternum, which Cuvier inferred from the figures and descrip-
tions of the fossil animal which had been published in his time *, is shown, by the more
perfect specimens since received, not to exist in nature ; and the suspicion expressed by the
great anatomist, viz. that it might be due to some misarticulation in the Madrid skele-
ton, is thus proved to be well-founded. The true connexions of the sternal ends of the
clavicles with each other and with the manubrium sterni, are well shown in the view of
the skeleton in the British Museum, given in Plate XXVII.

Humerus. — The humerus (Plate XIX. figs. 2 — 5) is remarkable for the vast expanse
of the lower fourth part of the bone ; but this is limited to the transverse direction ; so
that, viewed sideways (as in Plate I.), the humerus of the Megatherium appears to be a
comparatively weak and slender bone: the whole shaft, however, gives indications of
the force of the thick muscular masses which surrounded and operated on it.

The head of the bone (Plate XIX. fig. 4) presents a smooth convexity of an ellip-
tical form, corresponding with that of the glenoid cavity of the scapula ; the long axis
of the ellipse is from before backward. The head rises clear above the outer and inner
tuberosities, neither of which are so developed as to interfere, as in ungulate quadru-
peds, with the free rotation of the bone.

The rugged surface of the inner tuberosity (ib. figs. 2 and 3, a) slopes downward and
inward (ulnad) from the peripheral groove of the head, marking the attachment of the

* " D'apres lea figures et lea descriptions, il paraitrait que cette clavicule s'articulerait, non pas avec le
sternum comme a l'ordinaire, mais avec le bas de la premiere cote qui est recourbee, et presente une concavitc"
pour la recevoir. Ce serai t une singularity dont je ne connais pas d'exemple." — Ossemens Fossiles, Ed. 1835,
8vo, torn. viii. p. 349.

49

joint-capsule. The pectoral ridge is continued from the lower, slightly outstanding,
part of this tuberosity. The outer tuberosity (ib. b) projects from a lower level, is larger
and more prominent than the inner one ; and is divided into two subequal rough facets.
The outer deltoid ridge begins from the outer side of the tuberosity ; the inner deltoid
ridge (ib. d) some inches lower down, and nearer the inner side of the shaft: these
ridges converge, strengthen as they descend, and coalesce at the beginning of the lower
third of the shaft, defining a long and narrow angular tract for the implantation of the
powerful deltoid muscle. A tuberosity (ib. c) is developed from the outer (radial) side
of the humerus, one-third down the bone, from which a strong ridge descends along the
same side of the middle third : this ridge is defined below, and divided from the supi-
nator ridge, by a deep and smooth oblique channel (ib. e) for the passage of vessels and
nerves from the back to the fore-part of the bone. The supinator ridge (ib. f) is the
most prominent feature of the lower third of the humerus ; it presents a long, triangular,
rough outer facet, widening as it descends, and with a secondary ridge from its middle
part. The longitudinal contour of this facet forms an obtuse angle with the lower half
of the ridge, extending to the outer condyle of the humerus : the outer facet of this half
is rough, triangular, with the base upward. The pectoral ridge (ib. fig. 3, i) terminates
in a low tuberosity (ib. h) on the inner side of the middle of the shaft ; whence a second
ridge is continued upward upon the back of the shaft. This surface is flatter than the
fore-part, especially at its lower expanded third ; at the bottom of which, midway between
the outer and inner supracondyloid productions, and just above the lower articular sur-
face, is a small but well-defined olecranal depression. The inner supracondyloid angular
production (ib. k) has the flat rough facet only upon its lower half. Owing to the pro-
duction of these ridges, the articular condyles themselves (ib. g, I) appear to occupy but
a small part of the distal end of the bone ; for the extreme breadth of this end being
1 3 inches, that of the articular surface is but 7^ inches. It consists of two convexities,
side by side, divided by a narrow and deep channel, continued from the front non-arti-
cular surface half-way towards the back part of the bone. Both convexities have a full
elliptic periphery ; the outer one (ib. figs. 2, 3, 5, g) with the long axis from before
backward, the inner one (ib. I) with the long axis from side to side : the outer condyle is
the larger and more prominent of the two ; it forms more than a hemisphere, the antero-
posterior contour describing full three-fourths of a circle. The extent of flexion and
extension of the fore-arm on the arm is thus shown to be considerable. The articular
surface continued from one condyle to the other is concave transversely.

The centre of the shaft of the humerus is occupied throughout by a coarse cancellous
structure. A very small medullary artery penetrates the back part of the bone, below
the pectoral tuberosity, the canal extending obliquely downward and outward.

The Myrniecophaga didactyla, amongst existing Bruta, most resembles the Megathe-
rium in the development of the supinator or outer supracondyloid ridge.

In its general proportions, the humerus of the Megatherium resembles that of the
Megalonyx; and is more slender, in proportion to its length, than in the Mylodon or

H 2

50

Scelidothere. The articular head forms a larger proportion of a sphere, and projects
more freely beyond the tuberosities : these are relatively smaller, and are more equal
than in the Mylodon or Scelidothere: the external tuberosity, in particular, is more
developed in these smaller Megathcrioids. In them also the external ridge is continued
from above the ' musculo-spiral ' groove inwards, along the front of the humerus to the
apex of the deltoidal tract, forming its outer boundary : in the Megatherium, a smooth
concave surface divides the outer ridge from the deltoidal elevation, which is absolutely
narrower. The vertical outline of the back part of the shaft of the humerus in the
Megatherium is almost straight, being but a little bent forwards at its lower third, as it
is likewise in the Megalonyx ; and, in both, the olecranal depression is well defined : in
the Mylodon and Scelidotherium, the same outline of the humerus is slightly concave ;
the lower third of the bone being, as it were, a little bent back below the deltoidal
platform; and the olecranal fossa is not defined. The inner supracondyloid plate is
produced at its upper part into a strong tuberosity, in both the Mylodon and Scelido-
therium, but not in the Megatherium.

In the existing Sloths, the humerus at this part, viz. above the inner condyle, is per-
forated in one genus (Cholcepus) and not in another (Ackeus, F. Ccjvier) : and the same
difference occurs in the great extinct Sloths. In the Megatherium the humerus is
imperforate, as it is in the Mylodon: in the Megalonyx and Scelidotherium it is per-
forated above the inner condyle. Yet the Megalonyx most resembles the Megathe-
rium, not only in the general proportions of the humerus, but in the configuration of
its two articular ends. The inner or ulnar condyle, e. g., is convex in every direction in
the Megalonyx as in the Megatherium : in the Mylodon and Scelidotherium it is convex
only from before backwards, but is concave from side to side. In the more robust pro-
portions, in the shape of the articulations and the development of the processes of the
humerus, the Mylodon and Scelidotherium as closely resemble each other, as the Mega-
therium resembles the Megalonyx in the same characters ; yet the humerus of the Sceli-
dotherium has the inner perforation, and that of the Mylodon a groove merely, for the
brachial artery and nerve. This variety, and the corresponding one above noticed
between the Megatherium and Megalonyx, show the inapplicability of the final cause
commonly assigned to the ento-condyloid hole, the existence or otherwise of which
depends merely on the ossification or non-ossification of the aponeurosis extending from
the shaft of the humerus to the ento-condyloid process.

Ulna. — The bones of the fore-arm in the Megatherium, as in the Megalonyx, ex-
emplify by their greater length, as compared with those of the same segment in the
hind limb, the Bradypodal affinities of these huge extinct quadrupeds.

The ulna in the Megatherium (Plate XX. figs. 1, 2, 3) is, however, peculiar for the
vast expanse of its proximal end, in connexion with its long and slender shaft. The
olecranon (ib. a) is twice as broad as it is long ; its inner border, springing from the notch
which penetrates the inner and back part of the humeral articular fossa, extends obliquely
upward and inward for 3£ inches, the ridge or edge of the plate being about an inch

51

in thickness : the plate from this ridge sweeps round the back part of the bone, subsiding
and increasing in breadth as it approaches the radial side, where it terminates in a
tuberosity, divided by a groove from the radial articular cavity (ib. b), and prolonged
downward into the ridge bounding the radial side of the upper half of the ulna. The
olecranon projects rather backward than upward, and is strengthened and supported at
its highest part by a strong angular buttress, which gradually subsides upon the back of
the ulna.

The great ' sigmoid' articular surface is divided into two facets by a median portion,
which is produced forward in the longitudinal or vertical direction, and is convex from
side to side ; the divisions so denned are concave. The inner one (ib. c), for the inner
humeral condyle, describes a semicircle from behind forwards ; it has little more than
half that extent from side to side, and is encroached upon by a narrow, rather deep,
rough channel, continued from the inner origin of the olecranon, expanding, to near
the middle of the cavity. The outer division of the sigmoid fossa (ib. b) has reverse
proportions, the transverse being nearly double the extent of the longitudinal diameter,
and it is less concave than the inner division ; about half an inch of its lower border
bends a little back for the articular margin of the head of the radius, just above the
rough fossa, for the reception of the non-articular part of the same head : the rest of the
outer division of the sigmoid cavity receives the back part of the outer condyle of the
humerus. Below the outer division the radial fossa (ib. d) presents a triangular form,
bounded by a rough ridge externally and by a tuberosity internally. The ridge is con-
tinued upon the radial side of the shaft of the ulna to within a fourth part of its distal
end ; the surface of this side of the bone is irregularly sculptured, indicating the strong
ligamentous connexion between the ulna and radius at this part. The back and inner
sides of the shaft of the ulna are comparatively smooth.

A vascular canal, somewhat larger than the rest, is seen near the fore-part of the outer
surface, entering the bone obliquely upward. A rising of the surface, with a linear
series of three or four rough tubercles, marks the lower fourth of the inner side of the
bone ; a short wide longitudinal channel marks the back surface of the distal end
(fig. 3), which is slightly expanded and convex, and so impressed as to indicate its
ligamentous junction with the carpus.

The ulna of the Megatherium differs from that of the Mylodon, not only in its longer
and more slender proportions, but also in the absolutely as well as relatively minor
height or length of the olecranon ; in the much less relative vertical or longitudinal
extent of the outer division of the ' sigmoid' cavity ; and in the ' haversian' fossa on the
inner division. It differs in the much narrower channel dividing the articular cavity
from that part of the base of the olecranon which is continued into the posterior ridge
or border of the shaft ; it differs, also, in the convexity of the distal end and the absence
of the articular facet, which is distinctly present in that part of the ulna in the Mylo-
don and Scelidotherium.

Radius. — The radius (Plate XX. figs. 4, 5, 6), like the ulna, of the Megatherium

62

resembles in its longer and more slender proportions that bone in the Megalonyx, and
differs from the proportionally thicker and shorter radius of both the Mylodon and
Scelidotherium. The proximal end is circular, and is occupied by a smooth, moderately
shallow, articular cavity (ib. fig. 5), with a well-defined border, over which the articular
surface extends, on the ulnar side of the head, for about half an inch down ; which tract
is adapted to the lower portion of the outer division of the sigmoid cavity of the ulna.
The articular modification of the head of the radius is as completely adapted for the
superadded rotatory movements of the antibrachial bones, as in the human subject, to
the head of the radius of which the resemblance of that of the Megatherium is strikingly
close.

The shaft of the radius gradually narrows, in the antero-posterior diameter, along the
upper fourth part, but maintains the same diameter as the head, transversely. Three
inches below the head, on the inner and fore-part of the shaft, is the tuberosity (ib. a)
for the tendon of the biceps, which measures 3 inches in long diameter and If inch
across. Here the bone bends a little outward (radiad) ; and the ridge bounding that
side is developed into what may be termed a process (ib. b), with a low angle, whence the
ridge is continued straight down the lower half of the shaft to near the tuberosity above
the styloid process (ib. c), where it curves outwardly to terminate in that tuberosity.
The fore-part of the shaft is moderately smooth and convex across ; it describes, length-
wise, a slight concavity ; on the inner side of the bone, a broad and very rugged tract
begins, about an inch and a half below the bicipital tuberosity, and extends along the
middle third of the shaft ; a less rough tract is continued thence, gradually expanding
to the cavity for the lower end of the ulna. The outer side of the shaft of the radius is
smooth, convex across, and with a slight convexity in its longitudinal contour : from the
external process downward the radius maintains an equal breadth near the lower end, and
there it expands in all directions to form the large articular cavity (ib. d and fig. 6)
for the major part of the carpus. Above this cavity, on the ulnar side, is the rough
and shallow depression for the ligamentous junction of the corresponding end of the
ulna; on the front side a broad low ridge extends obliquely from the suprastyloid
tuberosity to the border of the cavity ; on the back part three oblong, short, thick ridges
or tubercles divide the surface into four channels, for tendons ; three of these are longi-
tudinal and parallel, progressively increasing in width from the innermost (or one next
the ulna) to the third ; the outermost passes obliquely between the suprastyloid tubercle
and the styloid process. This process is short and thick, rounded at the end ; flattened
in front, with the smooth articular surface continued for a few lines upon the lower
border of this surface, from the general articular cavity which is extended over the lower
end of the radius including the styloid process. This cavity presents a triangular form
with the angles rounded off; the base next the ulna is short and oblique ; the anterior
border or side is the longest, the opposite border is the thickest, and is notched near the
styloid process : the cavity is moderately concave, and articulates with the scaphoid and
lunar bones of the carpus.

• 53

In the Mylodon, the radius is not only thicker in proportion to its length, but is more
extensively and deeply impressed by the muscles of the fore-arm, especially on the back
part of the bone. The tuberosity for the insertion of the biceps is further from the
proximal joint, and augments the power of the muscle in the same degree : the proximal
articular cavity is of an oval form.

Carpus. — The carpus (Plate XXI. s — u) consists of seven bones, four in the proximal
and three in the distal row.

Scapho-trapezium. — The first of the proximal row (ib. s) includes the bone (ib. t\
answering to the first of the distal row in Man and most Mammals, and is consequently
a 'scapho-trapezium' (s, t)*, as it is also in the Sloths, the Mylodon and Scelido-
therium. It is of an irregular triangular shape, with its base applied to the 'lunare'
(ib. I), and with the apex somewhat twisted. It presents a broad convex articular
surface (ib. s) for the outer half of the concavity of the radius ; and this surface is con-
tinuous with a crescentic one of about one inch in breadth, which covers the proximal
part of the side of the bone next the os lunare. The palmar or anterior horn of the
crescent is continuous with an oval flat articular surface joining the os magnum (ib. g) :
the opposite or dorsal horn is separated by a rough tract from a convex subquadrate
surface which also articulates with the os magnum. On the outer side of this surface,
and, like it, on the fore-part of the bone, is a surface concave in one direction and convex
in the opposite direction, for articulation with the small trapezoides (ib. z). External
to this, the fore-part of the produced and twisted apex of the bone, which represents the
trapezium, articulates with the stunted metacarpal of the pollex (ib. m i), chiefly by
ligament, but also by a small elliptical flat surface which seems to have been covered
with articular cartilage. Between the two facets for the os magnum the bone is deeply
excavated and has been perforated by blood-vessels.

Lunare. — The 'os lunare' (Plate XXI. I) offers, as in Man, some rude resemblance
to a crescent ; its proximal surface, very convex from before backward and rather convex
from side to side, is wholly covered by the smooth articular surface which plays upon
the ulnar half of the terminal cavity of the radius ; and this surface is continued upon
the radial side of the bone to form there the crescentic tract, adapted to the similarly-
shaped tract on the scaphoid. The inner horn of this tract is continuous with the surface,
convex at the fore-part, then deeply concave from before backwards, for the os magnum
(ib. g) : this articular surface is continuous with a similarly deeply excavated and irre-
gular one on the ulnar half of the fore-part of the bone, which is subdivided into three
facets, the middle one for the os cuneiforme, the two smaller ones for two parts of the
os unciforme.

Cuneiforme. — The ' os cuneiforme ' (ib. c), which is the smallest of the three proximal
carpals, presents at its radial side a triangular convex surface for articulation with the

• This is the bone called 'cuneiforme' in Cuvieb's chapter on the Megatherium, and which is marked r
in the copy of Dr. Pander's figure of the Madrid skeleton introduced into plate 217, fig. 3, of the edition of
the ' Ossemens Fossiles,' 8vo, 1835, here cited.

54

lunare; and at its fore-part an irregular concavo-convex oblong surface for the unci-
forme (ib. «). Its proximal surface is tuberous and rough for ligamentous attachment
to the ulna, except where the smooth articular surface for the os pisiforme is situated :
this surface is in great part convex.

In the Mylodon the os cuneiforme is the largest of the carpal bones.

Pisiforme. — The os pisiforme (Plate XXI. p) is conical with an obtuse apex, having
on its base the articular surface for the os cuneiforme, and with the rest of its exterior
surface more or less irregular, for implantation of a tendon and ligaments.

Trapezoides. — The homologue of the trapezium being connate with the scaphoid, and
noticed in the description of that compound bone, the trapezoides (ib. z) is the first
independent carpal bone of the distal row. It is the least of the carpal series, and is a
relatively smaller and flatter bone than in the Mylodon: the proximal or scaphoidal
surface is convex transversely, concave from behind forward, and plays in a correspond-
ing concavo-convex surface in the scaphoid. The distal surface is almost wholly convex:
both surfaces are joined by a small articular facet on the radial side of the bone, which
is adapted to a corresponding facet in the small metacarpal bone of the thumb ; and by
a more extended articular surface on the ulnar side of the bone for junction with the os
magnum.

Magnum. — This bone, arbitrarily so termed, comes next after the trapezoides and
pisiforme, in the order of size, being much inferior to the other carpals. It is almost
wholly covered by smooth articular surfaces. The small non-articular rough surface (ib. g)
exposed upon the back of the wrist is of a transversely extended hexagonal figure, with
the outer and inner sides the shortest. The surface for the lunare is concave anteriorly,
but very convex in the greater part of its extent. It is continuous at its radial border,
with the two surfaces, one concave, the other flat, for the scaphoides ; and at its ulnar
border with the flat surface for the unciforme. The concave scaphoidal surface is con-
tinuous with the surface for the trapezoides, which is much narrower than is the same
surface in the Mylodon, the scaphoidal surface being broader than in the Mylodon.
The largest articular surface is that for the base of the great middle metacarpal, which
is slightly convex except at its fore-part, which is produced into a cone.

Unciforme. — Of all the carpals the os unciforme (ib. u) differs most in form from that
of the Mylodon ; it is a thick transversely extended bone, the free tuberous surface on
the back of the wrist being hexagonal, with the two inner and the two outer sides very
short : one of the outer sides is formed by a protuberance separating the articular sur-
faces for the os cuneiforme and fifth metacarpal, which meet at an acute angle in the
Mylodon.

The proximal oblong concavo-convex surface for the cuneiforme is continuous, at the
radial side of the bone, with the surfaces for the lunare and os magnum ; and the latter
with the broad surface along the distal part of the base which is obscurely divided into
three facets, the middle and largest for the base of the fourth metacarpal, the next in
size for the outer facet or the base of the third metacarpal, and the outermost and

55

smallest facet for the small part of the base of the fifth metacarpal which articulates
with the carpus. The inner or palmar rough surface of the unciforme has an oblong
tuberosity, which is narrower than that upon the dorsal surface.

Metacarpus. — The innermost or first metacarpal (Plate XXI., I, m), answering to
that of the pollex or thumb, resolves, by its rough obtuse distal termination, as well as
by its diminutive size, one of the points considered doubtful by Cuviek, in the structure
of the fore-foot of the Megatherium, by proving that the pollex was absent, or repre-
sented solely by the rudimental metacarpal, which must have been concealed beneath
the integument. The bone is of an irregular subcubical shape, broader than it is long.
At its base are two separate subcircular flat surfaces articulating with corresponding
facets on the trapezial part of the scapho-trapezium : on the side next the second meta-
carpal is a convex articular surface, having the lower part obscurely defined for articu-
lation with the trapezoides, and the rest lodged in the concavity, partly articular, partly
rough, upon the outside of the base of the second metacarpal. The outer side of the
rudimental first metacarpal is obliquely flattened, the surface here being apparently for
the insertion of a strong tendon.

The metacarpal of the second digit (ib. m n *) has a very irregular exterior ; its
comparatively small base is excavated obliquely to receive the fore-part of the trape-
zoides ; on the outer or radial side it shows a triangular excavation, the lower half of
which has a smooth articular surface for the rudimental metacarpal, m I. On the
ulnar side of the base there is a convex articular surface divided into a proximal narrow
tract for the os magnum, and a distal broader tract for the contiguous side of the base of
the middle metacarpal. Above or beyond this the middle third of the bone supports a
rugged protuberance, which has been attached by ligament to a similar rough surface on
the middle metacarpal ; so that little more than one-third of the bone projects freely
from the metacarpus. This free part is subcompressed and expands in the direction
from the back to the palm of the hand, so as to form the surface for a trochlear articu-
lation of great extent in that sense. A ridge from the back surface of the metacarpal
expands into a tuberosity near the dorsal end of that articulation ; and a similar
but smaller tuberosity projects from the palmar end of the same articulation, along each
side of which there runs a thickish edge. The distal surface itself presents a median
vertical or longitudinal prominence, beyond the lower half of which the articular surface
is produced laterally so as to make the surface concave transversely on each side the
median ridge ; whilst at the upper half the whole surface is convex transversely, as it is,
in a minor degree, longitudinally.

The middle metacarpal (ib. m in) is a little longer than the second, but is twice as
thick, with a quadrate transverse section, the four sides of the shaft being flat and sharply
defined. The base of the bone is produced ' ulnad' and ' proximad,' so as to be wedged
between the fourth metacarpal, the magnum and the unciforme. The articular surface
on the ulnar side of this production for the fourth metacarpal is extensive, and for the
* Kel'erred to the 'annulaire' or fourth digit in the ' Ossemens Fossiles,' ed. cit. pi. 216, fig. 8.

I

56

most part slightly convex : the surface on what may be termed the truncate end of the
production, for articulation with the unciforme, is slightly convex on the dorsal half,
slightly concave on the palmar half. The surface for the magnum on the proper base of
the metacarpal sinks into a conical cavity near its dorsal end ; the rest being nearly fiat.
On the radial side of the base is an oblong articular tract for the second metacarpal,
and beyond this the extensive rough surface for the ligamentous connexion with the
same bone. The radial surface is grooved above the rough facet, obliquely by a wide and
moderately deep canal ; apparently for the passage of a tendon to the digit. The
dorsal surface shows an oblique broad tuberosity, extending from above the tendinal
groove to the upper or dorsal end of the distal articulation. The smooth and almost
flat dorsal surface gradually deepens into a broad and shallow oblique channel on the
ulnar side of the oblique tuberosity. The ulnar side of the bone, beyond the articular
surface for the fourth metacarpal, is occupied by a rugged flat tract for ligamentous
connexion with the same bone. The palmar surface is pretty smooth, flat transversely,
slightly concave lengthwise ; produced into a tubercle below the middle prominence
of the distal joint. The articular surface of this joint does not cover the whole distal
end of the bone ; it is long and rather narrow, extending obliquely from the palmar
forward to the dorsal surface ; the dorsal side of the bone being longer than the palmar
one. It is traversed lengthwise by a median prominence, convex transversely, almost
straight lengthwise ; and the surface is continued upon a flat tract on each side of the
prominence, that on the radial side of the prominence being the broadest.

The general form of this bone is more like that of a brick or of an ashlar stone for a
strong wall, than like that of the usual support of a flexible digit of a fore-paw or hand.

The chief difference between the middle metacarpals of the Megatherium and Mylo-
don is in the form of the distal articulation. This surface, in the smaller Megatherioid,
is convex from above downward, whilst in the Megatherium it is straight, or rather
concave, and it joins the dorsal surface at an acute angle. The lateral depressions of
the pulley are narrower in the Megatherium, and the vertical inflexions of the phalanx
must have been more limited than in the Mylodon.

The fourth metacarpal (ib. miv), as compared with the third, is longer and more
slender in the Megatherium than in the Mylodori ; but its articulation by an obliquely
extended base with the third and fifth metacarpals and the unciform bone, closely
corresponds with that in the Mylodon *.

The two oblique metacarpal surfaces are nearly parallel, the radial one is concave, the
ulnar one slightly convex ; both are separated by a sharp angle from the intermediate or
carpal surface, which is nearly square and is slightly concave. The proximal half of the
bone is bounded by four flat equal sides, with intervening angles, sharp on the radial
side. The upper and under sides are nearly smooth, with a broad low tuberosity near
the proximal end ; the outer and inner sides are rugged for close syndesmosis with the
adjoining metacarpals. Only the distal half of the bone stands freely out ; it expands
• Description of the Mylodon robustut, 4to, p. 92. pi. xv. m 4.

57

in vertical breadth to the articular surface for the fourth finger. The angle between
the upper and radial sides is rounded off; that between the upper and ulnar sides is, in
one specimen, developed into a sharp ridge.

The distal articular surface resembles that of the second metacarpal, but occupies a
smaller relative proportion of the whole distal surface ; the vertical prominence, convex
transversely, is broader, more concave vertically, and extends obliquely from the upper
and radial angle to the lower and ulnar one : the flat lateral extension of the articular
surface is confined to the radial side of the prominence. A single flat surface for a sesa-
moid bone is situated below, but distinct from, this part of the articulation. A large
and strong tuberosity projects below the prominent part of the joint ; an oblique channel
divides this tuberosity from a longer one on the ulnar side of the distal end of the meta-
carpal.

The distal articular surface of the fourth metacarpal in the Mylodon is reduced to a
small, vertical, oblong, nearly flat surface ; this difference relating to the stunted deve-
lopment and limited function of the digit it has to support. In the Megatherium such
simplification of the distal joint of the metacarpal is limited to the fifth of that series of
bones (ib. m v) : this metacarpal *, of the same length as the fourth, is more slender ; its
proximal end is wedge-shaped, the radial articular surface and the flattened outer facet
converging to the narrow rough tract which is joined by ligament to the carpus. The
articular surface on the radial side has a small terminal part obscurely marked off for a
facet on the unciforme ; the rest receives the convexity on the ulnar side of the fourth
metacarpal : beyond this is a rough surface for the syndesmotic union of the contiguous
bones. Rather more than half the fifth metacarpal stands freely out; it is traversed
above by a longitudinal ridge expanding into a broad tuberosity at the distal end ; the
bone is smooth and rounded on the palmar side. The small terminal articular surface
is oval, slightly concave vertically, concave transversely, upon the radial half of the distal
expansion. There is no articular surface for a sesamoid.

The metacarpals are so united and wedged together and with the carpus, as to trans-
mit from the oblique carpal surface which sustains the radius the weight of the fore
part of the Megatherium to the fifth digit, the stunted extremity of which was imbedded
in the marginal hoof-like callosity on which the ponderous quadruped trod, with the
claw-bearing toes bent inwards.

From the scapho-trapezium and lunare the weight was transmitted to the second row
of carpal bones ; and by the oblique production of the base of the second metacarpal,
and especially of the third metacarpal, it was concentrated through the medium of the
fourth metacarpal upon the fifth. The lateral pressure thus occasioned explains the
extent of syndesmotic and sutural union along the basal part of the metacarpals, and also
the squared angular shape of the constituents of this masonry of the huge fore-paw.

On comparing the structure of the carpus in the Megatherium with that in existing
Mammals, it is found to be repeated in the Unau or Two-toed Sloth (Bradypus (Choice-
• Beferred to the index or second digit in the ' Ossemens Fossiles,' ed. eit. pi. 16. fig. 11.

i2

58

pus) didactylus), and in that species or subgenus exclusively ; the carpal bones being
seven, and the reduction to that number resulting, also, from the connation of the
scaphoid and trapezium. A scapho-trapezial bone exists in the Ai or Three-toed Sloth
(Bradypus (Acheus) tridactylus), but the carpus is further reduced in this species or
subgenus to six bones by the confluence of the trapezoides with the os magnum. The
trapezius is a distinct ossicle in the Chlamyphorus, as in most other Armadillos ; in the
Basypus sex-cinctus it coalesces with the trapezoides. In the Pangolins (Manis) the
scaphoid coalesces with the lunare, not with the, trapezium. In the true Anteaters
(Myrmecophaga), and in Orycteropus, the ordinary eight carpal bones retain their indi-
viduality.

Digital Phalanges. — The stunted metacarpal of the pollex, in the Megatherium, bore
no rudiment of a digit. They were powerfully developed and unguiculate in the three
following digits.

The index digit (Plate XXI., n) has three phalanges: the proximal one (1) is almost
twice as broad, and more than twice as deep, as it is long ; the metacarpal surface pre-
sents a deep and wide, vertically elongated, subangular concavity, fitting the vertical pro-
minence and lateral facets of the distal joint of the metacarpal ; the distal surface of the
proximal phalanx presents a vertical angular fissure dividing two oblong convexities.
The mid-phalanx (ib. H, ») has a proximal trochlea playing on the preceding, the median
vertical ridge being overhung by the produced upper rough surface ; the distal articula-
tion repeats that of the proximal phalanx, but the median fissure is less deep, and the
lateral convexities are more regularly rounded and prominent. The ungual phalanx
(ib. n, 3) exceeds the length of both preceding phalanges ; the upper or dorsal side of
its base is produced backwards into an obtuse point; the rough sheath of the claw
extends along three-fourths of the phalanx ; it is convex radiad, vertical and flat ulnad ;
the core presents a rough edge radiad and a vertical rough surface ulnad, and is smooth
and convex transversely at its base, both above and below.

The proximal and middle phalanges of the digitus medius (ib. in, 1...2) are confluent,
the line of anchylosis being indicated by a vertical ridge along both the inner and the
outer sides, and by a curved ridge convex backwards on the upper or dorsal side. The
proximal articular surface presents a deep vertical channel, with a narrow elongated
subconcave surface continued from its radial side, and a still narrower flat surface from
the opposite side. The distal articular surface of the composite bone has a deep median
vertical groove dividing two convexities. The compound phalanx presents two rough
tuberosities below, one terminating each bank of the vertical articular proximal channel ;
there is a smooth depression above, and another below, close to the distal trochlea. The
enormous distal phalanx (ib. in, a), of twice the vertical breadth of the adjoining claw
phalanges, is very little longer than that of the second digit ; it is flattest on the ulnar
side, to which the upper convexity slightly inclines. The base or palmar part of the
phalanx is the broadest, is convex both lengthwise and transversely, and is perforated
by two canals, for the large vessels and nerves supplying the claw-core. The surface,

59

in which was implanted the flexor tendon, shows many linear impressions radiating from
the median tuberosity, forward and laterally. The articular surface excavates obliquely
the base of this phalanx which overhangs the joint ; the concave border of the median
angular prominence describes a semicircle from above forward and downward ; the ulnar
excavation is longer in that direction than the radial one. The well-marked trochlear
joint restricts the movements of the great claw to flexion and extension on a vertical
plane, whilst its position effectually prevents retraction of the claw, the preservation of
the effective condition of which is due to an opposite bend to that in the Cat-tribe,
as was well explained by Cuvier in his description of the Megalonyx*. The major
part of this enormous phalanx goes to form the sheath of bone protecting the base
of the claw. The bony core or peg on which the claw was fixed and moulded is
compressed, with a sharp edge above and flat below, where it projects beyond the
sheath.

The finger of the fourth digit has three phalanges and is unguiculate, with a claw
resembling in shape and size that of the second digit. The proximal phalanx (ib. iv, i)
is so compressed in the direction of its axis, that the proximal and distal articular surfaces
almost meet above, only about 1^ line's breadth of rough surface there intervening : the
proximal articular surface is a wide and moderately deep vertically-elongated channel,
with a semielliptic flat surface continued from the lower half of the radial border ; the
outer and inner sides of the phalanx are narrow, elliptic rough convexities ; the distal
articular surface is gently convex vertically, sinuous laterally. The second phalanx
(ib. iv, i) is deeper than long ; its upper surface is produced backward over the proximal
phalanx into a rough obtuse process ; the under surface is much shorter, but is broader
than the upper surface ; the convexity of the distal articular surface describes a semi-
circle from above downward, and is divided by a vertical trochlear groove into two parts,
of which the ulnar one is the larger. The ungual phalanx (ib. iv, s) is long, triedral,
with the radial side of the sheath vertical, the under surface rough and flattened, and
the ulnar surface sloping from above downward and outward to join the under surface.
The upper posterior tuberosity, overhanging the joint, inclines radiad, and is somewhat
flattened ; the proximal surface presents a median vertical rising for the channel in the
preceding joint.

The small elliptical surface on the radial half of the distal end of the fifth metacarpal
supports a very short lamelliform phalanx, produced outward some way beyond the
joint ; and to this is articulated a stunted second phalanx with a rough, obtuse, non-
articular end. In some instances the above two phalanges are blended together.

Amongst existing Mammals the Sloths alone present the connation of the scaphoid
with the trapezium, the Two-toed Anteater and the Armadillo that of the first with the
second phalanx. This latter character, peculiar to the middle digit in the Megathe-
rium, is limited also to the same digit in the Myrmecophaga didactyla ; in Basypus it
affects the third, fourth and fifth digits.

• Ossemens Fossilea, ed. cit. t. viii. p. 510.

60

The Bradypus tridactylus has but two flexible phalanges in each of the three ungui-
culate toes, but this reduction is due to the early anchylosis of the proximal phalanx
with the metacarpal, not, as in Megatherium, with the second phalanx. In the Bra-
dypus didactylus the unguiculate digits preserve the normal number of free pha-
langes.

The pollex is atrophied in the Megatherium, as it is in both existing species of Sloth ;
and, as in the Bradypus tridactylus, only the second, third and fourth digits support
claws ; but the fifth digit, instead of being wanting, as in the Ai, is developed, so far as
was needed for the purposes of terrestrial progression, in the Megatherium. The small
existing arboreal Sloths are seldom obliged to walk on the ground, and there can only
crawl along with difficulty. In the Mylodon the pollex was developed and unguiculate,
but both the fourth and fifth digits were terminated by a stunted second phalanx. In the
Unau, not only the fourth and fifth digits, but also the first are suppressed in the fore-
foot. Yet this is the Sloth, as already remarked, which so peculiarly illustrates the bra-
dypodal affinities of the Megatherium in the structure of the carpus, notwithstanding
the degree to which the adaptive modifications of the Megatherioid type of fore-foot
are carried in relation to the exclusively arboreal life of this small existing tardigrade.
The coalescence of the scaphoid and trapezium, which Cuvier was the first to recog-
nize in the existing Sloths, he continued to affirm in the latest edition of the ' Ossemens
Fossiles' to be peculiar to them. The bony structure of the fore-foot of the Megathe-
rium he regarded as most resembling that of the Dasypus gigas. M. Laurillard, after
the subsequent reception of casts of the carpal bones of the Megatherium, which had
been transmitted to England, with other bones of the Megatherium, by Sir Woodbine
Parish, K.H., inferred that the fore-foot of the Megatherium had a greater analogy with
that of the Myrmecophaga jubata, but he did not detect the connation of the scaphoid
with the trapezium. The form of the scapho-trapezial bone in both existing Sloths
bears an unmistakeable resemblance to that in the Megatherium, but in the Unau it
describes a deeper curve towards the palmar aspect, and the trapezial portion (described
by DeBlainville as the sesamoid of the pollex*) is relatively longer than in the Mega-
therium. The base of the stunted metacarpal of the pollex is expanded, and abuts by
one part against the trapezium and by another against the base of the second meta-
carpal. The trapezoides is a small bone articulated, as in the Megatherium, with the
scapho-trapezial, the os magnum, and the second metacarpal ; the os magnum presents
almost the same pentagonal contour, dorsally, as in the Megatherium, the anterior facet
being also partly convex for adaptation to a concavity in the base of the middle meta-
carpal, which likewise is so extended as to interpose itself between the fourth meta-
carpal and the carpus.

The atrophy of the fifth finger, which has proceeded in the Megatherium to cause
the absence of the ungual phalanx, and which atrophy similarly affects both the
fifth and fourth digits in the Mylodon and Scelidotherium, has proceeded in the

• Ost6ographio de Pareaseux, 4to, p. 22.

61

Unau to the removal of all the. bones of those digits, save the metacarpal of the
fourth, which is reduced to a rudiment of even smaller size than that which forms
the vestige of the thumb on the radial side of the hand : it rests, as a great part of the
fourth metacarpal in the Megatherium does, upon the expanded base of the third meta-
carpal.

In the Ai (Bradypus tridactylus) the metacarpal rudiment of the pollex is anchylosed
at its lateral joint to the base of the metacarpal of the index, but it retains its free arti-
culation with the scapho-trapezium. The chief modifications of both hand and foot in
the Three-toed Sloth are the extensive anchyloses of different bones : this character is
shown by the coalescence of the trapezoides with the os magnum, such compound bone
supporting the base of the second metacarpal and a great part of that of the middle
metacarpal; thus fulfilling the same relations to the metacarpus as do the separated
bones in the Unau and Megatherium.

The great extent to which the metacarpals are suturally united to each other in the
Megatherium, is a character repeated in those of the Ai, but the suture is speedily, in
the living Sloth, converted into bony union, and the three metacarpals, like the three
metatarsals, thus form one compound bone, as in Birds. The unguiculate digits which
this bone supports in the fore-paw, are the homologues of the three claw-bearing toes in
the Megatherium. The rudiment of the fifth finger appears as a mere process from the
outside of the base of the metacarpal of the fourth : the huge terrestrial predecessor of
the small leaf-eating and tree-dwelling quadrupeds retained the fifth toe, minus its ter-
minal phalanx, yet of great size and strength, and modified expressly for the purpose of
supporting the ponderous body in terrestrial progression.

The fore-foot of the Mylodon more closely conforms, in its essentials, to the type of
that of the Unau, inasmuch as the two outer digits (fourth and fifth) were mutilated
and clawless ; they were, however, developed to the same degree as the fifth digit is in
the Megatherium, and for the same end, but probably made little show, externally, in
the entire foot. The pollex, however, instead of being rudimental, was fully developed,
though small, in the Mylodon. In the Megatherium this digit is rudimental, as in both
forms of existing Sloth ; but the bones of the fore-foot correspond more closely with
the type of the manus in the Ai ; there being, indeed, as, regards the digits, only this
essential difference, that the fifth, instead of being, like the first, a mere rudiment, was
developed to be adapted to progression on the ground. It is most interesting, however,
to trace the interchangeable relations between the two above-cited great extinct Mega-
therioids and the two existing forms of Sloth, respectively.

In regard to other existing Edentata, the Myrmecophaga jubata, by reason of the
clawless condition of its fifth digit, and the Myrmecophaga didactyla, by that of the
rudimental pollex as well as fourth and fifth digits, ought to succeed the Sloths as next
of kin to the Megatherioid quadrupeds, the interval being due to the difference of carpal
structure.

Cuvier has observed that the fore-foot of the Dasypus gigas is one of the most extra-

62

ordinary among quadrupeds ; and, he adds, that it alone would give the key to all the
anomalies in that of the Megatherium*. But this could only have been affirmed under
a misconception of the real nature of those anomalies. In the Dasypus gigas the fore-
foot is pentadactyle ; all the digits are unguiculate, and, in three of them, the claw-
phalanges furnish a bony sheath as well as core to the claws ; but these belong to the
third, fourth and fifth toes, not, as in the Mylodon, to the first, second and third, or, as
in the Megatherium, to the second, third and fourth toes ; they moreover successively
decrease in size from the radial to the ulnar aspect, instead of the reverse proportions
which they present in the Mylodon. No doubt the claw on the middle digit is the most
developed, as in the Megatherium, and the first and second phalanges of this digit have
coalesced ; but here ends the particular resemblance between the Megatherium and the
great Armadillo, in regard to the bony structure of the fore-foot.

"We can state with confidence, what M. Laurillard suggests f, viz. that the fore-feet
of the Megatherium, as represented by the skeleton in the Madrid Museum, are not
transposed, the right being on the left and the left on the right side, as Cuvier was led
to suspect ; but that the articulation of those complex parts by the laborious Prosector
and Curator Bru, was in the main correct.

The bony structure of the fore-limb of the Megatherium is now, indeed, as completely
understood, and the homologies of every constituent bone can be as exactly defined, as
in any existing species of quadruped. And, to the degree in which so important a part
of the frame throws light on the whole; the Naturalist may thereby trace the affinities,
and the Physiologist infer the habits, of the great extinct beast.

§8. Of the Bones of the Posterior Extremity.

In the description of the bones of the hind-limbs I commence with the ilium, as being
the homotype or correlative of the scapula in the fore-limb. The ischium, which is the
homotype of the coracoid, is confluent with the ilium, as the coracoid is with the sca^
pula ; the pubis, which is the homotype of the clavicle, is confluent with both the ilium
and ischium. All the three bones on both sides become confluent with the sacrum, and
form therewith, in the full-grown Megatherium, a single bone — the pelvis — which is the
largest known among terrestrial Mammals, and forms the most striking feature in the
skeleton of the present gigantic extinct Sloth.

As a like progressive ossification brings to pass a similar state of the pelvis in the small
existing Sloths, the limits of the primitive bony constituents of that of the Megatherium
can only be determined, at present, by analogy with the pelvis of its modern congeners,
studied at a period of immaturity. This I have had the opportunity of doing in the

• " La main du taiou giant est une des plus extraordinaires qu'il y ait parmi les quadruples, et a elle
aeule elle expliquerait toutes les anomalies que nous verrons dans celle du Megatherium." — Ossemens Fos-
■ue», ed. cit. torn. viii. p. 242. No qualifying note is appended by the editors to this statement.

t See the note (1) appended by that able anatomist to the chapter on the Megatherium, in the posthu-
mous edition of the ' Ossemens Fossiles,' 8vo, t. viii. p. 355.

63

young of the Bradypva tridactylus, prior to the completion of the coalescence of the
several bones.

The five vertebrae composing the sacral part of the pelvis of the Megatherium have
been described in § 2, p. 22, treating of the vertebral column to which they belong.

The ib'ac bones (Plates I. and XXII. 62), as they extend from their place of anchylosis
with the sacrum, expand in depth and breadth ; their anterior plane is directed forward,
being almost vertical and at right angles with the axis of the spine. Each ilium, after
contributing its share to the acetabulum (Plate XXII. fig. 1, a), rapidly contracts to an
obtuse point bent downward and outward. The two bones, in a front view, resemble
a pair of broad outstretched wings at the sides of the fore part of the sacrum. The
anterior surface is slightly concave, but is undulated, with many sharp ridges that have
penetrated between the fasciculi of the muscles thereto attached. The 'labrum,' or
upper and outer convex border of the ilium, is unusually thick and rugged ; the under
concave border is also rugged, but is thin, and in some parts sharp. On the inner side
of the acetabulum there is a well-defined, raised and very rough, oblong surface (p) for
the insertion of the tendon of a powerful 'psoas' muscle.

The outer surface of the ilium is slightly concave near the sacrum, and is then convex
in the direction of its longest diameter, which is from within outwards ; in other directions
it is nearly flat: its surface is much broken by numerous intermuscular ridges.

The pubis (Plates I., XXII. & XXVII. <u) is very slender where it forms the anterior
border of the 'foramen ovale' (Plate XXII. fig. 1, 0), but expands at its extremities,
and especially where it coalesces with the ischium to form the produced and pointed
'symphysis.' The extent of this symphysis is 10 inches in a straight line.

The pelvis, as in the Sloths and other members of the Order Bruta, shows the con-
version of the ischiadic notches (ib. i) into foramina by the anchylosis of the ischia
with the posterior sacral vertebrae (ib. s). Each ischium (Plates I. & XXII. «s), as it
extends from its confluence with the sacrum, expands into a broad smooth plate of
bone, bent outward and forward, then contracting as it converges inward towards its
fellow, to combine with the pubic bones at the symphysis. The hinder border, forming
the tuberosity, <&, is thick and rugged ; and two or three perforations here indicate the
original line of its separation from the sacrum. The part of the ischium which joins
the pubis on the sacral side of the foramen ovale presents on its inner surface the usual
oblique channel leading to that foramen.

Among the existing species of the Order Bruta the Sloths alone resemble the Mega-
therium in the expansion of the iliac bones, but this is much less in comparison with
the length of the trunk ; the iliac expansion is relatively greater in the Megatherium
than in the Elephant, and is associated with a much greater proportionate size of the
whole pelvis and of its cavity or channel. The extreme breadth of the pelvis of a large
Asiatic Elephant is 3 feet 8 inches, whilst in the Megatherium it is upwards of 5 feet.

The pelvis which Cuvier was led to suspect, from the defective condition of its fore
part in the Madrid skeleton, to be naturally open anteriorly, as in the Myrmeco-

I

64

phaga didactyla, is closed anteriorly, as in the Sloths, by a 'symphysis pubis' of short

extent.

The acetabulum (Plate XXII. fig. 2) presents a full oval shape, with the lower
margin bisected by a narrow and deep 'Haversian' groove, which extends, slightly
expanding and becoming more shallow, to near the bottom of the cavity ; the outer
division of the lower border of the groove is most produced. The large and deep
acetabula look downward and a little outward. One diameter of the hemispheroid
canty is 8 inches, the other diameter is 7 inches ; it therefore presents a plane surface
of 43*9824 square inches, which, multiplied by 15, with the barometer at 30 inches,
gives about 660 pounds atmospheric pressure upon the hip-joint of the Megathe-
rium.

The size and strength of the ordinary processes of the pelvis, the breadth of the
rough labrum of the iliac bones, and the numerous and well-defined intermuscular
crests, indicate the unusual size and vigour of the muscular masses which proceeded
from the pelvis in different directions to act upon the trunk and fore limbs and upon the
hind limbs and tail. They lead to the conviction that the resistance which demanded
such forces for its overcoming must have been of a very different nature and degree
from any that now opposes itself to the labours of the existing vegetable feeders
when engaged in supplying their daily wants; whence it may be inferred that the
exertion of such forces was associated with equally peculiar habits in the megatherioid
animals.

The femur of the Megatherium (Plates I. & XXVII. es, and Plate XXIII. fig. 1) is one
of the most massive limb-bones in the Mammalian class ; from its proportions it might
rank with the 'flat' instead of the 'long' bones, but that its thickness would rather
bring it into the category in which the carpal or tarsal bones are placed according to the
old anatomical character derived from shape.

The head (Plate XXIV. fig. 1) would be a smooth hemisphere, but that the antero-
posterior diameter somewhat exceeds the transverse one. Its surface is unimpressed,
and its periphery uninterrupted, save by a small entering notch at the middle of its back
part (Plate XXIII. fig. 1), into which possibly some thickened band of the capsule, like
a ' ligamentum teres,' may have been implanted. The neck of the femur is short and
ill-defined ; the upper contour passes from the head to the summit of the great trochanter
(ib. t), which is on the same level, in a slightly concave line ; the inner contour of the
bone descends with a somewhat deeper concavity from the lower periphery of the head
to the shaft. This is flattened from before backward, and presents a slightly oblique
twist, the head and the outer condyle being on a plane anterior to the trochanter and
inner condyle. The great trochanter presents a broad rugged surface, flattened obliquely
from above downward and inward, divided into two somewhat flattened facets by a
transverse ridge arching upward. The lower facet contracts as it descends, and is con-
tinued into the strong outer ridge which descends to the external condyle. The thick
rough border of the rest of the trochanter stands out beyond the contiguous parts of the

65

femur ; least so at the upper, and most at the back part of the process, where the border
defines outwardly a small but deep trochanterian fossa (ib. f). This fossa is bounded
below by a small tuberosity. The small trochanter is represented by a rough ridge
6 inches long, 2 inches broad, occupying the middle third of the shaft a little anterior to
its inner border. A few shorter longitudinal ridges occur on the fore part of the shaft
between the upper end of the small, and the upper and fore part of the great trochanter-
The rest of the anterior surface of the femur is smooth, concave lengthwise, slightly
convex transversely.

The back part of the femur presents a small low tuberosity below its middle part,
near the inner border ; the triangular surface between this tuberosity, the head, and the
great trochanter, is smooth and flat. The contour of the back part of the femur, from
the head to the outer condyle, is convex ; that from the trochanter to the inner condyle
is concave ; the lower half of the back part of the shaft is convex transversely. The
lower end of the femur (Plate XXIV. fig. 2) presents two articular surfaces, the inner
one (ib. i) being that of the internal condyle, the outer one being the combined ectocon-
dyloid (e) and rotular (r) surfaces. The latter is extensive, and describes a semicircle
from before backward, but is narrow from side to side : in this direction the rotular portion
is slightly concave ; its limits are indicated by a notch on the inner side : the condyloid
portion is slightly convex transversely, in which direction the extent is scarcely 2-J inches.
The entocondyloid surface (i) is of nearly twice that breadth ; is of a reniform shape,
and is convex in every direction. The intercondyloid channel is roughened by decus-
sating ligamentous impressions and ridges, with some vascular pits. Its narrowest part
is anterior, and measures an inch and a half across ; posteriorly it is 3 inches across.
The intercondyloid border of the inner condyle is sharply defined and projects below
the level of the canal, that of the outer condyle is mostly on a level with the canal.
The surface of the canal meets the hind surface" of the femur at almost a right angle,
the intervening ridge being rounded off. The fore part of the inner condyle stops short
of the fore part of the shaft ; the back part of the condyle projects 1 inch beyond the
back part of the shaft. The process or prominence above the inner condyle (eminentia
entocondyloidea, Plate XXIII. fig. 1, ie) forms an obtuse angle, the lower side of which
is rough, rather flattened, and expands as it descends towards the articular part of the
condyle. The outer condyle does not project so far back as the inner one : its supra-
condyloid prominence (ec) is larger, of a similar angular form, but is bent forward as
well as outward.

In the Elephant, Mastodon, and Diprotodon, the shaft of the femur is flattened from
before backward ; but the length of the femur so far exceeds its breadth, that, strong
as the thigh-bones of these quadrupeds are and well-proportioned to the weight they
had to sustain, they appear weak and even slender when placed by the side of the femur
of the Megatherium. The Rhinoceros, which has the thigh-bone relatively broader
and flatter than in the proboscidian pachyderms, differs more markedly from the Mega-
therium in the presence of the third trochanter.

k2

66

The shaft of the femur is more or less flattened in all the species of Bruta ; but the
Orycteropus and Armadillos, in which this character is conspicuous, differ, like the Rhino-
ceros, from the Megatherium in having the third trochanter. This process is not present
in the Pangolins (Manis), Anteaters (Myrmecophaga) or Sloths (Bradijpus) ; but in all
these genera the femur is relatively longer and more slender than in the Megatherium,
and only the Sloths amongst existing Mammals, not marine, repeat the remarkable mega-
therioid character of the absence of a medullary cavity in the shaft of the femur.

The general characters of the femur of the Megatherium are most closely repeated in
that of the Mylodon* and Scelidotheriumf. In these genera, however, instead of the
shallow notch, there is a deep and prolonged fossa for the ligamentum teres on the
middle of the hind border of the head of the bone. The post-trochanterian depression
is relatively larger in the Mylodon, the small trochanter is relatively less and higher
placed ; the whole femur is longer in proportion to its breadth, and the distal expansion
is relatively less. Here, also, the articular surfaces offer a well-marked character of
distinction ; the rotular articular surface is continuous with that of both condyles, and
unites them anteriorly.

From the cast of a distal epiphysis, transmitted by Dr. Harlan to the Museum of
the Royal College of Surgeons, London, as of the Megalonyx, it would appear that this
extinct megatherioid offered a third modification of the knee-joint, the rotular surface
being distinct from those of both condyles. Thus the knee-joint of the Mylodon must
have had one large synovial capsule, that of the Megatherium two, and that of the
Megalonyx three such sacs.

The patella (Plate I. w, and Plate XXIII. fig. 2) is a strong, thick, subtrihedral conical
bone, with the base rounded, and also the angle between the two rough outer sides. In
the natural position of the bone the base is uppermost, and chiefly composed of a strong
tuberosity coarsely and irregularly striated : at the lower half of the outer surface the
stria? have a longitudinal and subparallel direction, giving that part of the bone the
appearance of an ossified fibrous ligament. On the inner and broadest side the articular
surface occupies the upper two-thirds: it is less distinctly divided by a median longi-
tudinal rising into two channels than in the Mylodon, being more nearly level and
uniform. The non-articular surface below the joint is irregularly grooved and perfo-
rated by vascular canals.

The fabellaj, or post-tibial sesamoid bone (Plate I. «7'), is a smaller subhemispheric
bone, with a circular, slightly concave articular surface, which was applied to part of the
outer condyle of the femur : the rest of the surface is rough and fibrous, indicative of
the imbedding of the bone in a flexor tendon of the leg.

The tibia and fibula (Plates I. and XXVII. «s, «?) become anchylosed together at both
rxtremities in the Megatherium: they are both short, and the tibia presents massive

• Owes ' On the Mylodon robustus,' 4to. p. 111. pi. 17.

t lb. 'Fossil Mammalia' of the ' Voyage of the Beagle,' 4to. pi. 25. fig. 5.

X lb. ' Archetype of the Vertebrate Skeleton,' 8vo. p. 190.

67

proportions corresponding with those of the femur. The proximal end of the tibia
(Plate XXIV. fig. 3) presents two distinct and well-marked articular surfaces ; the inner
one (?) is concave, the outer one (e) is convex : the extent of these surfaces corresponds
with the breadth of the articular part of the outer and inner femoral condyles respect-
ively. The back part of the outer facet which bends downward affords an articulation
(f) to the fabella. The rough interspace between the articular surfaces is a little con-
cave transversely, and convex from before backward ; its breadth equals that of the outer
surface : it developes no intercondyloid process for crucial ligaments.

The fore part of the proximal end of the tibia presents a large irregularly triangular
rugged protuberance for the ligamentum patellae; the back part, below the outer con-
dyle, developes a smaller but more prominent rugged process, the apex of which over-
hangs the upper part of the interosseous space: between this 'post-tibial' process and
the rough inner border of the bone there is a deep and wide longitudinal channel
inclining a little obliquely to the interosseous space. The shaft of the tibia gradually
contracts to its middle, and as gradually expands to its distal end. It is subcompressed
from before backward ; is smoother behind than in front : there is a longitudinal
channel on each side the back part of the lower end of the tibia, which forms a con-
vexity between them.

The anterior surface is divided by a ridge extending obliquely from the rotular pro-
tuberance to the inner malleolus : the surface on the fibular side of this ridge is smoother
than the other, which seems to have been wholly given up to musculo-tendinous attach-
ments. The inner malleolus (Plate XXIV. fig. 4, m) is a slight expansion below the
confluence of the inner and oblique anterior ridges: it does not project below the level
of the distal articular surface. This is deeply concave, and is divided into two facets by
the deeper hemispherical excavation near its inner side for the reception of the inner
protuberance of the astragalus: this excavation (e^ gives to the larger and shallower
facet (?) a full crescentic figure. The smooth surface is sometimes continued upon,
sometimes interrupted by, a narrow tract from the vertical surface upon the malleolar
end of the fibula, which surface is applied to the outer facet of the astragalus. There
is a small orifice for a medullary artery at the middle of the back part of the tibia, but
it does not open into any medullary cavity : the bone is cancellous throughout.

The fibula is thickest at its upper end, where it has a trihedral form ; the outer surface
is convex and rough, the inner and hinder surfaces are concave and smooth, meeting at
a sharp interosseous border directed obliquely backward : this border is slightly thickened
and produced at its middle part, where the shaft of the fibula is compressed: it
augments in thickness and resumes its trihedral shape as it descends, and terminates
in a moderately produced outer malleolus (/) with a very rugged surface, except where it
articulates with the astragalus.

In the Mylodon, as well as in the Megalonyx and Scelidotherium, the tibia and fibula
continue separate, — a fact affecting the value of the evidence which Cuvier deduced from
their anchylosed condition in the Megatherium in favour of its affinities to the Arma-

68

dittos, to which this structure is peculiar amongst existing Mammals. But, since it is
known only in the Order Bruta, it forms an interesting additional proof of the essential
relations of the huge extinct animal under description to that now anomalous group of
Mammals. The tibia of the Mylodon is proportionally shorter and thicker than in the
Megatherium: the outer articular surface at the upper end is of a subcircular form
and slightly concave : in the Megalonyx it is convex, but in a less degree than in the
Megatherium. The lower articular surface in both Mylodon, Scelidotherium, and Mega-
lonyx, presents the additional facet for the fibula. The hemispheric excavation on the
inner side of the distal articulation is relatively larger in the Mylodon than in the Mega-
therium. This excavation, with the concomitant protuberance of the astragalus, is pecu-
liar to the great extinct Sloth-like quadrupeds ; in which so secure an interlocking of
the foot with the leg bespeaks some habits peculiar to them, connected with the require-
ment of unusual resistance in the foot to the forces acting upon it from the leg and
thigh.

In the series of existing animals Man presents the plantigrade foot in which the
weight of the body presses most nearly upon the crown of the tarsal arch ; but, in the
Megatherium, owing to the length of the heel and the shortness of the toes, the leg
transmits the superincumbent weight nearly upon the middle of the foot. So singularly
shaped and adjusted, however, are the tarsal bones in the Megatherium, that the tibia
articulates with the side instead of the summit of the tarsus, so that the whole foot is
turned inward and rests upon its outer edge instead of its sole (Plate XXVI. fig. 1, and
Plate I. cl, a).

The number of tarsal bones is reduced to six, through the absence of the entocunei-
form*; but the astragalus, and especially the calcaneum, are developed to a great
size.

The astragalus (Plates XXV. & XXVI. fig. 1, a) is of a peculiarly irregular form : if
the foot be placed with the sole flat on the ground, as in Plate XXV., the chief articular
surface (a) for the tibia looks inward, and the small fibular facet, at right angles there-
with, is uppermost. The extensive surface by which it articulates with the bones of the
leg is divided into three parts, the planes of which are at right angles to each other.
The middle and largest division (a), answering to the outer ridge of the trochlear
surface in the common form of astragalus, is here expanded into a broad reniform
smooth tract, horizontal in the ordinary position of the Megatherium's foot (Plate XXVI.
fig. 1, a), almost flat from before backward, convex from side to side. This surface is
continued over the outer edge upon the outer side of the bone, in a triangular form, with
the apex rounded off, to form the facet (o) for the fibula. The surface answering to the
main part of the trochlea and its inner malleolar facet in ordinary astragali, is here
reduced to a small triangular convexity (ib. & Plate XXV. fig. 1, i), forming the third
and internal division of the surface, and supported on what appears to be an obtuse
pyramidal process from the inner and lower part of the bone. This convexity is wedged

• ' Os cuneiforme internum' of S<EMMEBBi2fG.

69

into the deeper excavation on the inner part of the tibial articular surface, and forms a
kind of pivot on which the foot worked.

The under or calcaneal side of the astragalus slopes from behind downward and
forward, and is divided by an oblique groove, about an inch broad, into two facets ; the
outer and posterior one is for the calcaneum exclusively ; the inner and anterior one is
for the calcaneum, but is continuous with the surface for the naviculare. The outer
calcaneal surface is ovate, concave lengthwise, convex across its posterior broader end,
nearly flat at its anterior end. The inner calcaneal surface is of much less extent and
is nearly flat ; its anterior end suddenly bends forward and upward to be continued into
the outer convex part of the navicular surface, which surface is divided into this convex
portion and a contracted subcircular concavity. The lower part of the convex facet rests
upon an articular concave surface on the cuboides.

Only a small proportion of the outer surface of the astragalus is non-articular, and this
is chiefly on the outer, or, in the megatherian position of the foot, the upper surface
(Plate XXVI. fig. 1). The groove dividing the calcaneal surfaces begins at their lower
part and passes backward and inward, with slight terminal bends, which give it the form
of the italic f. It is divided by a ridge from a wider channel, excavating the under part
of the inner process ; from which a third channel extends backward, dividing part of the
inner calcaneal facet from the naviculo-cuboid surface. Both this channel and the
calcaneal one are perforated by conspicuous vascular canals.

The calcaneum (Plates XXV. & XXVI. fig. 1, b) is a long irregular pyramidal bone,
with an obtuse apex : the base is obliquely truncated, forms the fore part of the bone, and
supports two articular surfaces (Plate XXVI. fig. 2) : the upper and outer surface (as)
is ovate, concave where that on the astragalus is convex, and vice versa: the oblique
channel which divides this astragalar surface from the astragalo-cuboidal surface (as1) is
as deep as in the astragalus, and, like it, is perforated by vascular canals. The slightly
concave triangular inner astragalar facet (as!) is continued below, at an obtuse angle,
into the semielliptic slightly concave surface (cb) for the os cuboides. External to the
astragalar facet a strong vertically extended tuberosity (Plate XXVI. fig. 1, t) forms
the fore part of a wide and deep tendinal canal (u) ; a narrow and feeble ridge bounds it
behind ; a second and more shallow groove (v) succeeds ; and, behind this, is a wide and
deep vascular perforation (w). The under part of the calcaneum is very rugged and flat.
The obtuse hinder extremity of the heel-bone shows by its sculpturing and the out-
standing osseous spiculae, the force with which the attached cable-like ' tendo Achillis'
must have acted on so unusually produced a calcaneal lever.

The os naviculare (Plates XXV. & XXVI. tig. 1, c) is a transversely oblong bone, corn-
ed from before backward. Its posterior surface is occupied by the articulation for
the astragalus, which is equally divided into an inner concavity and an outer convexity,
the latter approaching the conical form. From the lower part of this is continued at a
right angle a small flat triangular surface for the cuboides (Plate XXV. fig. 3, cb").
The upper non-articular surface is narrowest at its middle, and is developed into a low

70

oblong tuberosity on each side. On the fore part of the bone (ib. fig. 3) is the articular
surface, divided into the narrow oblong tract (cm), with two slight convexities, for the
mesocuneiforme (ib. fig. l,f), and into the almost flat triangular tract (fig. 3, ci) for the
ectocuneiforme (fig. 1, e). The latter surface (fig. 3, ci) is divided by a very narrow non-
articular tract from the cuboidal surface (fig. 3, cb"). The inner (tibial) part of the
anterior surface is non-articular, and extends beyond the mesocuneiform articulation to
where the entocuneiforme would have been, had that bone existed in the tarsus of the
Megatherium.

The mesocuneiforme* (Plates XXV. & XXVI. fig. 1, /) is a laterally compressed
reniform bone, with a thick rough convex inferior border: the inner (tibial) surface
(Plate XXV. fig. 1, f) is irregular and flattened: the similarly modified outer (fibular)
surface (Plate XXV. fig. 2, f) is varied by a flat elliptic articular facet (Hi) near its
upper part, for a similar surface on the side of the metatarsal of the 'digitus medius.'
The back part of the bone is chiefly occupied by the narrow surface for the naviculare.
The fore part of the bone is obtuse and rough. Not a vestige of the toe (digitus secundus),
usually supported by the mesocuneiforme, is developed in the Megatherium.

The ectocuneiforme f (Plates XXV. & XXVI. fig. 1, e) presents the normal wedge-like
figure of the tarsal cuneiform bones. It is flattened from before backward ; with its
thick base (e) rough and convex. The posterior surface presents the almost flat, slightly
concave articular surface for the naviculare. The outer border or surface is rough and
tuberous; the inner one less rough and flat. The anterior convex facet (Plate XXV.
fig. 2, Hi) for the base of the metacarpal does not extend to the upper or under borders
of the bone. The articulation with the contiguous cuneiforme, as with the cuboides, is
by syndesmosis.

The cuboides (Plate XXVI. fig. 1, d) presents an articular surface divided into three
facets on its upper, or tibial, and back part ; the anterior and smallest facet is for the
naviculare (c), the middle and largest for the astragalus (a), and the posterior for the
calcaneum (ib) : the latter surface is at almost a right angle with the astragalar one, and
looks backward. On the under, or fibular, and fore part of the bone is the articular
surface for the two outer metatarsals ; that for the fourth toe (Plate XXV. fig. 2, iv)
being concave transversely and slightly convex lengthwise ; that for the fifth toe (ib. v)
being uniformly but very slightly convex.

A broad non-articular surface, rough and with two oblique low ridges on the upper
and outer part of the bone, divides the back from the front articular surface ; a narrower
non-articular tract, but produced into a strong obtuse ridge, divides the same surfaces
on the inner or under side of the bone. The fore part of the bone is produced into an
angular process (Plate XXV. fig. 2, p), which forms the inner part of the articular
channel for the fourth metatarsal. The under part of the bone is impressed by the
broad tendinal groove continued from that which impresses the outer part of the calca-
neum.

• ' Os cuneiforme medium ' of Sosmmerbixg. f ' Os cuneiforme externum,' ib.

71

The above-described composition of the tarsus of the Megatherium has been deduced
from the study of three entire specimens of the bones of the hind foot. The distal
articular surfaces demonstrate that the digits of that foot were but three in number,
and that they answered to the 'third,' 'fourth,' and 'fifth' of the pentadactyle type.
Not a rudiment of the ' second ' exists independently, and every vestige of the first,
together with the cuneiform bone supporting it, is absent. There are no little bones
missing on the inner side of the ' mesocuneiforme,' as Dr. Pander conjectured might be
the case in the Madrid skeleton ; and there is no ' os cuneiforme ' for the hallux (' grand
doit du pied '), as Cuvier supposed.

Metatarsus.— The metatarsal of the 'third' toe (Plates XXV. & XXVI. fig. 1, m3)
resembles rather an ' os cuneiforme,' by reason of its extreme shortness, or fore-and-aft
compression. Its upper non-articular surface is the broadest, and is rough and convex,
like that of the ectocuneiforme. The proximal or posterior surface for that cuneiform
bone is triangular and slightly concave (Plate XXVI. fig. 3, ci) ; the bone is prolonged
into a tuberous process beneath it, forming a lever of advantage for the insertion of a
flexor tendon. On the inner side of the bone, at its upper part, is the small flat surface
(cm) adapted to that on the mesocuneiforme ; on the outer side of the bone is a larger
surface, partly convex, partly concave, for articulating with the side of the base of the
fourth metacarpal (Plate XXVI. fig. 1, m <) ; the rest of the outer surface is very irre-
gular, as if honeycombed. The distal or anterior articular surface presents a vertical
median prominence, passing into a partly flat surface internally, and into a vertically
concave surface externally. The lower part of the vertical prominence is most produced
(ib. fig. 2, p).

The fourth metatarsal (Plate XXVI. fig. 1, m *) has its base compressed laterally, and
expanded vertically ; but the distal end is still more produced in that direction, so as
to be almost hammer-shaped. The vertically extended articular surface at the base of
this metatarsal is narrow, convex transversely, and adapted to the channel in the
cuboides (Plate XXV. fig. 2, p iv). On the inner or tibial side of the base is the articular
surface, concave posteriorly, less convex anteriorly, for the interlocking joint with the
' third ' metatarsal. In front and below this articular surface is a very rough honey-
combed tract of bone for firm syndesmotic junction with the similarly modified surface
of the third metatarsal.

On the outer or fibular side of the base are two articular surfaces for the fifth meta-
tarsal : the hinder and smaller one is flat, and is continued, at right angles, with the
basal surface ; the larger surface is subcircular and rather undulating : in front and
below these surfaces is a narrow rugged tract for ligamentous junction with the fifth
metatarsal. The part of the shaft of the fourth metatarsal which stands out free is
smooth upon its inner and upper sides, is traversed by a wide oblique tendinal groove
below, and is rather rough and irregular externally.

The vertically produced distal surface presents a large rough protuberance at its upper
part (Plate XXV. fig. 1, iv), and three protuberances at its under part, of which the

72

innermost is the most produced, the outermost the least. The articular surface resem-
bles in character that on the end of the third metatarsal, but is relatively smaller ; it
consists of a longitudinal median prominence, continuous with a smooth narrow tract on
the inner side, on which surface articular cartilage and synovial membrane seem to have
existed at only a small part at its lower end. On the outer side, the smooth tract is
limited to the upper half of the prominence ; this is slightly concave vertically or length-
wise. It is a form of articulation calculated rather for firm and unyielding junction
than for flexibility.

The outermost metatarsal (Plate XXVI. fig. 1, m s), answering to the fifth of the
pentadactyle foot, is the longest, but from its more backward articulation with the
tarsus it does not reach so far forward as the fourth.

The proximal end is a free rough tuberosity, somewhat more than an inch anterior
to which, on the tibial side, is the oval slightly concave articular surface adapted to the
cuboides (Plate XXV. fig. 2, v), which surface is continuous with the short transverse
flattened surface for the fourth metatarsal ; and, in advance of this, is the larger, sub-
circular, slightly undulated surface for the corresponding surface on the fourth meta-
tarsal. The second surface is on the middle of the inner surface of the fifth metatarsal,
and is bordered in front and beneath by the very rough honeycombed surface for
ligamentous junction with the adjoining metatarsal. This combination of synovial with
syndesmotic joints admits of that degree of slight elastic movement of the very firmly
attached fourth and fifth metatarsals, which must have facilitated the heavy tread of the
ponderous quadruped, and have alleviated the effect of the enormous pressure on the
two gradatorial toes. The whole outer and under part of the non-articular surface of
the fifth metatarsal is strongly sculptured by irregular ridges, tubercles, grooves, and
foramina, indicative of the hoof-like callosity of the outer border of the sole in which it
was chiefly imbedded. The upper and inner non-articular surfaces are comparatively
smooth. The inner side is flat, and is traversed by an oblique shallow (tendinal 1) groove.
On the distal surface are two small articular facets ; the inner one, which is the best
marked, is subcircular, about 8 lines in diameter.

Phalanges. — The bones of this class are unusually reduced in number in the foot of
the Megatherium, even admitting the accuracy of the figures of the hind foot of the
Madrid skeleton, in which two stunted phalanges appear to terminate both the fourth
and fifth toes. For, as the great unguiculate toe, like that of the fore-foot, has only
two moveable phalanges, the total number of these bones is but six, not exceeding that
of the tarsal bones of the same foot.

The phalanx (Plate XXVI. figs. 1 & 3, i *a) of the innermost toe (Hi), answering to the
third of the pentadactyle foot, represents, as in the corresponding digit of the fore foot,
the proximal and middle phalanges connate. The compound bone (ib. fig. 3, i & i) is
shaped like a wedge with an oblique edge, deeply notched. It has articular surfaces
not only on its proximal and distal ends, but upon its upper or dorsal surface. The two
former surfaces, which are the ' ends ' of the bone in ordinary phalanges, here form the

73

' sides ' of the wedge : the sides of the phalanx are the ' margins ' of the wedge : the
dorsal surface forms the base, the plantar or under surface is represented by the two
processes of the cleft apex or edge of the wedge. The proximal articular surface presents
a longitudinal channel, convex vertically, concave transversely, from which a flat surface
extends from nearly the whole of the tibial side, and a convex surface from the upper
part of the fibular side ; the whole articular surface being the counterpart of that on
the metatarsal (ib. m), with which the present remarkable bone is, by this interlocking
joint, firmly united. The lower prominence of the median rising of the distal joint of
the metatarsal (p) protrudes through the lower notch in the phalanx (»). A yielding,
elastic, slightly-sliding movement was all that could take place between these bones.
The complex distal articulation is adapted to an equally restricted junction with the
enormous terminal phalanx ( 3 ) ; it consists of four distinct articular surfaces, two on the
anterior and two on the upper part of the connate phalanges (1 & a). The internal of
the two distal surfaces is the largest and is slightly concave, the external one, near the
upper and outer angle of the bone, is slightly convex ; they are divided by a rough tract,
indicating strong ligamentous union with the claw-phalanx, of half an inch in breadth ;
and they cover the smaller proportion of the distal surface of the bone. Below the
outer articular surface there is a protuberance, with a smooth but non-synovial surface
in the present bone, which is adapted to a definite smooth surface upon the claw-
phalanx. The two upper surfaces are subcircular, each about half an inch in diameter,
very slightly convex, and about 5 lines apart.

The great terminal phalanx of the present toe (Plates XXV. & XXVI. in, ») is
modified, like its homotype on the fore foot, for the firm fixation and development of a
powerful claw, the hollow base of which covered the bony core, and was encompassed
by a bony sheath of 5 inches in length.

The base of the phalanx, whence both core and sheath extend forward, is a surface of
a long, narrow, vertically elliptical form, with the upper third produced backward so as
to overhang or cover the confluent supporting phalanges ; and the lower third sloping
forward at nearly the same angle with the vertical middle third, but terminated by a
backwardly projecting ridge. There are two articular surfaces on the middle division,
two on the upper division, and one on the lower division of the base. The innermost
of the middle articulations is slightly convex, the outermost as slightly concave ; they
both look directly backward. The lower articular surface is flat and subcircular ; it is
on the hinder and outer angle of the lower third of the base, and looks downward and
a little backward ; the two small surfaces on the upper third of the base are subcircular
and flat, and look downward : these different articular surfaces are counterparts of those
on the distal and upper parts of the connate phalanges. On each side of the lower
third of the base of the claw-phalanx is a large canal, leading forward to the interspace
between the bony core and sheath of the claw, and giving passage to the vessels and
nerves of the formative matrix of that instrument. The tibial side of the sheath
(Plate XXV. fig. 1, 3) is gently convex ; the fibular side (Plate XXVI. fig. 3, s) is flatter :

l2

74

a similar modification affects the claw-core, the narrow basal part of which is divided by
sharp borders from the sides. The point of the core projects about 3 inches in advance
of the lower border of the sheath: its form, broken in the specimen, is restored in
dotted outline in the figures.

The distal articular surface of the fourth metatarsal shows that it supported a phalanx
so articulated with it as to have no movements of flexion or extension, and only a slight
degree of bending from side to side. The illustrations of the Madrid skeleton, especially
plate 5, fig. 5, show that this phalanx (Plate XXVI. fig. 1, iv, i) was very short, and that
it supported a second phalanx (ib. iv, s) of a subhemispheric form, terminated obtusely.

The distal articular surface of the fifth metatarsal indicates that it supported a pha-
lanx smaller than the proximal one of the fourth toe ; and the figure, above cited, of the
Madrid skeleton shows such a phalanx (ib. v, i), and also a second small stunted hemi-
spheric phalanx (ib. v, i)* ; and this, from the analogy of the Mylodon, is most probably
the true structure.

If we contemplate the bones of this singularly constructed foot in their natural
co-adaptation, the same relation of the osseous masonry to the transference of pressure
from the leg to the outer border of the foot will be appreciated, as has been pointed out
in the fore limb. The broad surface of the astragalus — the great keystone of the tarso-
metatarsal arch (Plate XXVI. fig. 1, a) — transmits the superincumbent weight in two
chief directions, backward upon the massive heel-bone (b), forward upon the metatarsus.
By the naviculare (c) it is transmitted through the ectocuneiforme (e) and the produced
outer angle of the base of the mid-metatarsal (m s) to the fourth (m <), and thence to
the fifth metatarsal (m t). The cuboides (d), receiving the weight from both astragalus
and naviculare, transmits it by its produced fore part to the base of the fourth meta-
tarsal ; and partly by that medium, but chiefly by direct articulation, to the side of the

A

base of the fifth metatarsal. The tendency in the cuboides to yield under this pressure
and slip back, is resisted by the abutment of the calcaneum (b, t) against its back part.

§ 9. Comparison of the Bones of the Hind Foot.

No known recent Mammal offers in its astragalus any repetition of the peculiarities
of that bone in the Megatherium. In the Anteaters and Armadillos the upper surface
of the astragalus has the usual configuration, and is received into a deep tibio-peroneal
mortice. In the Sloths the outer part of the astragalus is excavated by a deep cell, in
which the pivot-shaped end of the fibula rotates: the inner side is applied, as usual,

• In the original memoir, by Beu and Gakeiga, on the Skeleton of the Megatherium at Madrid, the
metatarsus and toes of the hind foot are said to agree with those of the fore foot, except that there is only
one toe with a claw, instead of three (" Tambien se advierte que en este hay solo un Dedo con una, quando
en la Mono Be registran tres ; en lo demas convienen en un todo," p. 16). In the description of the outer-
most (fourth developed) toe of the fore foot it is stated that it has two phalanges, and that there is
nothing to be remarked except that they are rounded (" Los dos Falanget del quarto que se reconocen en
J. y L. no tienen cosa que advertir mas que son casi redondas," p. 13).

75

against a malleolar process of the tibia. There is an analogy to the megatherian
structure in the pivoted part of the articulation, but the process and the cavity are on
reverse parts of the ankle-joint.

The convex protuberance or pivot on the inner half of the tibial surface of the astra-
galus is common to all the extinct Megatherioids hitherto discovered, but it is asso-
ciated with well-marked modifications of the bone in each genus, and with minor
differences in different species. In the Mylodon * the calcaneal surface is single, and is
continuous with the navicular one; no part is insulated by a bisecting groove, as in
the Megatherium : the middle division (a) of the upper articular surface is less convex ;
the upper half of the navicular surface is flat, instead of being concave.

The astragalus of the Scelidotheriumf agrees with that of the Mylodon in the less
depth of the middle division of the upper surface, and the more open angle at which it
joins the inner convexity ; it agrees with that of the Megatherium in the division of
the calcaneal surface ; but it differs from both in the presence of two deep concavities
upon the naviculo-cuboid surface, the portion to which the cuboides articulates being
concave instead of convex.

As the modification of the calcaneal surface of the astragalus governs that of the
co-adapted surface in the calcaneum, this bone, in the Mylodon, is distinguished by the
uninterrupted continuity of the articulation presented to the astragalus, with which
that for the cuboides is continuous ; the posterior part of the astragalar surface is less
convex than in the Megatherium : the posterior prolongation of the calcaneum is rela-
tively shorter and is less pointed : the posterior wall of the great outer tendinal groove
is more developed in the Mylodon. In the Scelidotherium the posterior termination of
the calcaneum is broader, and terminated by a less angular convexity than in the Mylo-
don ; but in the separation of the posterior from the anterior part of the astragalar
surface it agrees with the Megatherium and with the Sloths. These, of all recent
Bruta, most resemble the Megatherioids in the posterior prolongation of the calca-
neum, and more especially the Megalonyx in the compression and distal expansion of
that lever ; but, in the Megalonyx, this expansion in the vertical direction is extreme,
and gives the heel-bone more the form of an os ilium than of a tarsal bone.

In the os cuboides of the Mylodon the two surfaces for the fourth and fifth metatarsals
are nearly on the same plane; in the Megatherium they are nearly at a right angle.
In the Scelidotherium the surface by which the cuboides joins the astragalus is convex,
instead of being concave, as it is in the Megatherium and Mylodon. In the latter genus
the surface on the os naviculare for the mesocuneiform bone is relatively broader than
in the Megatherium ; and the upper division of the astragalar surface is flat, instead of
being convex.

With respect to the digits, it is significant of the true affinities of the Megatherium

• " Description of the Skeleton of the Mylodon robuttus," 4to, 1842, p. 117. pis. 21, 22, & 23.
figs. 1 & 2.
t "Fossil Mammalia," Voyage of the Beagle, 4to, pi. 26.

76

to find that the Sloths alone amongst the existing Beuta show a suppression of certain
toes: in all the other existing genera of the order the hind foot retains the typical
number of digits.

The Armadillos, including the Chlamyphorus, resemble the Megatherium in the ter-
minal confluence of the tibia and fibula ; but these bones articulate in the Dasypodidce
in the normal way with the astragalus, and the foot is planted firmly on the ground by
its flat surface. The hallux is not only present in the Chlamyphorus, but is longer than
the fifth toe. All the five toes are armed with claws ; and anomalous as the three-toed
hind feet of the Sloths may appear, they have more real resemblance with the feet of
the Megatherioids than have those of any of the Armadillo family. The great extinct
Glyptodm agrees with the Chlamyphvrus in the pentadactyle condition of the hind foot,
the OS naviculare having three facets for as many cuneiform bones, the innermost of
which supports a well-developed hallux. This toe is abortive in existing Sloths : it is
represented, in the Ai, by a small compressed bone, consisting apparently of the connate
en to-cuneiform and metatarsal : it retains its distinctness in the Unau (Plate, Bradypus
didactylus, i), as the similar representative of the second toe does in the Megatherium ;
and the meso- and ecto-cuneiform bones are also distinct ; but in the Ai each cuneiform
bone is confluent with its metatarsal, with each other, with the naviculare, and with the
cuboides.

In the Megatherium, the mutilation of the foot has commenced on its outer side by
the removal of the ungual phalanx from the fifth and fourth toes ; in the Sloths the
mutilation is restricted to the fifth toe, but is carried further, viz. to the removal of all
the phalanges and the reduction of size of the metatarsal (Plate, Bradypus didactylus, v) :
nevertheless, the Sloths alone, amongst existing Bruta, manifest in any degree the cha-
racteristic megatherioid condition of the marginal toes, first and fifth, of the penta-
dactyle series : and the minor modifications of this character, differentiating the existing
from the extinct phyllophagous Bruta, are such as intelligibly relate to the different
powers and habits of creatures so different in size and mode of progression.

The mutilation of the two outer toes in the Megatherium is accompanied by modifi-
cations which adapt them to the important office of the support and progression of the
body on level ground: in the scansorial Sloths, the three middle digits being equally
developed for prehensile purposes, and none being needed for walking, one toe on the
outer and one on the inner side of the foot is reduced to the metatarsal basis, and is
concealed beneath the skin. In the Megatherium the hallux and its cuneiform bone
are wanting : the second toe is represented by its cuneiform bone, with, perhaps, a con-
nate rudiment of a metatarsal : only the third toe can be compared, by its size and the
claw it supports, with the condition of the three unguiculate toes in the Sloths ; and,
like those of the Ai, it has but two moveable phalanges. The reduction of the claws
to one in the hind foot of the Megatherium, its enormous size and strength, its secure
attachment to the phalanx, and the solidity of the articulations of the constituent bones
of the whole toe, relate to the contrasted mode of obtaining the leafy food in the

77

colossal extinct Sloth as compared with the diminutive climbing species which still
exist. If the hind foot were put by the Megatherium to the preliminary work of
exposing and loosening the roots of the tree about to be prostrated, its efficiency for
that purpose would be greater by having but one claw, than if it had two or three : for
the single claw, like a pickaxe, would clear away the soil from the interstices of the
root-ramifications, the more easily by not being associated with a second contiguous
claw, impeding such operation by striking upon the root itself.

§10. Physiological Summary.

To sum up the results of the foregoing descriptions of the known fossilized parts of the
extinct giant of the Order Bruta, as indicative of its habits and affinities. The teeth
agree in number, kind, mode of implantation, and growth, with those of the Sloth
(Bradypus), and their structure is a modification of that peculiar to the same genus.
All the modifications of the skull relating to the act of mastication, especially the large
and complex malar bones, repeat the peculiarities presented by the existing Sloths.
There are the same hemispheric depressions for the hyoid bone in the Megatherium as
in the Sloth. In the number of cervical vertebrae, the Megatherium, like the Two-toed
Sloth, agrees with the Mammalia generally *. In the accessory articular surfaces afforded
by the anapophyses and parapophyses of the hinder dorsal and lumbar vertebrae f, the
Megatherium resembles the Anteaters (Myrmecophaga%); but it does not resemble the
Armadillos (Dasypm^) in having long metapophyses, the peculiar development of which
in those loricated Bruta has a direct relation to the support of their bony dermal armour.
In the mesozygapophyses of the middle dorsal vertebrae ||, the Megatherium appears to
be peculiar amongst Mammalia In the small extent of the produced and pointed
symphysis pubis it resembles the Sloths ; and in the junction of both ilium and ischium
with the sacrum, it manifests a character common to the Edentate order ; but in the
expanse and massiveness of the iliac bones, it can only be compared with other extinct
members of its own peculiar family of phyllophagous Bruta. The habits of the Mega-
therium necessitating a strong and powerful tail, we find this resembling in its bony
structure that of other Bruta with a similar appendage, especially in the independency
of the two haemapophyses of the first caudal, a character which obtains in the Ant-
eaters % and in some Armadillos; but this is no evidence of direct affinity to either of
those families : the habits of the small arboreal Sloths render their eminently prehensile
limbs sufficient for their required movements, and the tail is wanting. Had that
appendage been proportionally as large as in the Megatherium, we cannot suppose that
the caudal vertebrae would have materially differed from those of other Bruta.

In the coalescence of the anterior vertebral ribs with the bony sternal ribs, the Mega-
therium resembles the Sloths. This essential affinity is still more marked in the pecu-

* Plate I. and Bradypu* didactylus, C 1...7. t Plate III. figs. 4 & 5, a and^.

% See Philosophical Transactions for 1851, Plate XLIX. fig. 20. § Ibid. Plate XLIX. figs. 18, 19.
|| Plate III. mz. % Philosophical Transactions for 1851, Plate LIII. fig. 60.

78

liarities of the scapula and of the carpus. In the Myrmecophaga jubata, the scaphoid is
distinct : in the Manis it coalesces with the lunare : in the Dasypm gigas the trapezoides
is anchylosed to the second metacarpal : in the Das. sexcinctus it has coalesced with the
trapezium. Not any of these characteristics are manifested by the Megatherium : its
carpus repeats the peculiarities of that in the Sloths, viz. the reduction of the number
of carpal bones to seven by the coalescence of the scaphoid with the trapezium *. The
first digit (pollex), which is retained in the Anteaters and Armadillos, is obsolete in the
Megatherium, as in the Sloths and Orycteropus : three digits are fully developed and
armed with claws, as in the Bradypus tridactylus ; and the fifth, though incomplete in
the Megatherium, is better developed, because it was required in the ponderous terres-
trial Sloth for its progression on level ground. In no existing ground-dwelling member
of the Bruta is the fifth digit deprived of its ungual phalanx, as in the Megatherium.
The bones of the fore foot of that extinct animal are thus seen to be modified mainly
after the type of the Bradypodidce.

The long bones of all the limbs are devoid of medullary cavities, as in the Sloths. The
femur lacks the ligamentum teres, as in the Sloths. The fibula is anchylosed to the tibia
at both ends in the Megatherium, as in Dasypus ; but this is not the case in the closely-
allied extinct Megatherioids called Mylodon, Megalonyx, and Scelidotherium, a fact
which diminishes the force of the argument which Cuvier deduced from the coalesced
condition of the bones in favour of the affinities of the Megatherium to the Armadillos.
The semi-inverted but firm interlocking articulation of the hind foot to the leg shows
the peculiarities of that joint in the Sloths exaggerated, and departs further from its
characteristics in other Bruta. In all the existing members of the order, save the
Sloths, the hind foot is pentadactyle, and four of the toes have a long claw ; this is even
the case with the little arboreal Myrnxecoplmga didactyla. The departure by degrada-
tion from the pentadactyle type is a peculiar characteristic of the Sloth-tribe in the
order: it is carried further in the same direction in the Megatherium and other great
extinct terrestrial Sloths.

Guided by the general rule that animals having the same kind of dentition have the
same kind of food, I conclude that the Megatherium must have subsisted, like the Sloths,
on the foliage of trees ; but that the greater size and strength of the jaws and teeth,
and the double-ridged grinding surface of the molars in the Megatherium, adapted it to
bruise the smaller branches as well as the leaves, and thus to approximate its diet to
that of the Elephants and Mastodons. The Elephant and the Giraffe are specially modi-
fied to obtain their leafy food ; the one being provided with a proboscis, the other with
much elongated cervical vertebrae ; the entire frame of the lofty ruminant being con-
currently modified to adapt it to browse on branches above the reach of its largest con-
geners. If the Megatherium had possessed, as Cuvier conjectured, a proboscis, it could
not, judging from the suborbital foramina, have exceeded in size that of the Tapir, and
must have been able to operate only upon branches brought near the mouth. Of the

* Plate, Bradypus didactylus, st.

79

use of such a proboscis in obtaining nutritious roots, on the Cuvierian hypothesis that
such formed the sustenance of the Megatherium, it is not easy to speculate : the Hog's
snout might be supposed to be more serviceable in obtaining those buried parts of
vegetables ; but no trace of the prenasal bone exists in the skull of the Megatherium.
A short proboscis might be useful in rending off the branches of a tree when prostrated
and within reach of the low and broad-bodied Megatherium, but this office has been
provided for by the organization of the tongue, of which, both the hyoid skeleton by its
strength and articulation, and the foramina for the muscular nerves by their unusual
area, attest the great size and power.

As regards the limbs, the Megatherium differs from the Giraffe and Elephant in the
unguiculate character of certain of its toes, in the power of rotating the bones of the
fore arm, in the corresponding development of supinator and entocondyloid ridges on
the humerus, and in the possession of complete clavicles. These bones are requisite to
give due strength and stability to the shoulder-joint for varied actions of the fore arm,
as in grasping, climbing, and burrowing. But they are not essential to scansorial or
fossorial quadrupeds : the Bear and the Badger have not a trace of clavicles, and merely
rudiments of these bones exist in the Rabbit and the Fox. We must seek, therefore,
in the other parts of the organization of the Megatherium, for a clue to the nature of
the actions by which it obtained its food. In habitual burrowers the claws can be
extended in the same plane as the palm, and they are broader than they are deep. In
the Megatherium the depth of the claw-phalanx exceeds its breadth, especially in the
large one of the middle finger ; and not any of the claws can be extended into a line
with the metacarpus, but they are all more or less bent inward and downward. Thus,
although they might be used for occasional acts of scratching up the soil, they are
better adapted for grasping ; and the whole structure of the fore foot militates against
the hypothesis of Pander and D' Alton*, that the Megatherium was a burrowing
animal.

The same structure equally shows that it was not, as Dr. LuNDf supposes, a scansorial
quadruped ; for, in the degree in which the fore foot departs from the structure of that
of the existing Sloths, it is unfitted for climbing ; and the outer digit is modified, after
the ungulate type, for the exclusive office of supporting the body in ordinary terrestrial
progression. It may be inferred from the diminished curvature and length, and from
the increased strength and the inequality of the claws, especially the disproportionately
large size of that weapon of the middle digit, that the fore foot of the Megatherium was
occasionally applied by the short and strong fore limb in the act of digging : but its
analogy to that of the Anteaters teaches that the fossorial actions were limited to the
removal of the surface-soil, in order to expose something there concealed, and not for
the purpose of burrowing. Such an instrument would be equally effective in the dis-
turbance of roots and of ants ; it is, however, still better adapted for grasping than for

• Das Riesen-l'aulthier, Ac. fol. 1821, p. 16.

t Blik paa Brasiliena Dyreverden for sidste Jordomva-ltning, at' Dr. Lund, 4to. Kjobenbavn, 1838, p. 21.

M

80

delving. But to whatever task the partially unguiculate hand of the Megatherium
might have been applied, the bones of the wrist, fore arm, arm, and shoulder, attest the
prodigious force which would be brought to bear upon its execution. The general
organization of the anterior extremity of the Megatherium is incompatible with its
being a strictly scansorial or exclusively fossorial animal, and its teeth and jaws decidedly
negative the idea of its having fed upon insects; the two extremes in regard to the
length of the jaws are presented by the phyllophagous and myrmecophagous members
of the order Bruta, and the Megatherium in the shortness of its face agrees with the
Sloths.

Proceeding, then, to other parts of the skeleton for the solution of the question as to
how the Megatherium obtained its leafy food, it may be remarked that the pelvis and
hind limbs of the strictly burrowing animal, e. g. the Mole, are remarkably slender and
feeble, and they offer no notable development in the Rabbit, the Orycterope, or other less
powerful excavators. In the climbing animals, as, e. g., the Sloths and Orangs, the hind
legs are much shorter than the fore legs ; and even in those Quadrumana in which the
prehensile tail is superadded to the sacrum, the pelvis is not remarkable for its size or
the expansion of its iliac bones. But, in the Megatherium, the extraordinary bulk and
massive proportions of the pelvis and hind limbs arrest the attention of the least curious
beholder, and become to the physiologist eminently suggestive of the peculiar powers
and actions of the animal. The enormous pelvis was the centre whence muscular
masses of unwonted force diverged to act upon the trunk, the tail, and the hind legs,
and also by the ' latissimus dorsi ' on the fore limbs. The fore foot, being adapted for
scratching as well as for grasping, may have been employed in removing the earth from
the roots of the tree and detaching them from the soil : but the hind foot, which, like a
pickaxe, had but one strong perforating and digging pointed weapon, was more pro-
bably the instrument mainly employed in removing the earth from the ramifications of
the root The fore limbs, terminated each by three claws, appear to have been more
especially adapted for grasping the trunk of a tree ; and the forces concentrated upon
them from the broad posterior basis of the body must have cooperated with them in the
labour, for which they are so amply organized, of uprooting and prostrating the tree.
To give due resistance and stability to the pelvis, the bones of the hind legs are extra-
ordinarily and massively developed, and the strong and powerful tail must have con-
curred with the two hind legs in forming a tripod, as a firm foundation for the vast
pelvis, thus providing adequate resistance to the forces acting and re-acting from and
upon that great osseous centre. The large processes and capacious spinal canal indicate
the strength of the muscles which surrounded the tail, and the vast mass of nervous
fibre from which those muscles derived their energy. The natural co-adaptation of the
articular surfaces shows that the ordinary inflection of the end of the tail was backward,
as in a Cauda fulciens, not forward, as in a cauda preliensilis. Dr. Lund's hypothesis,
therefore, that the Megatherium was a climber and had a prehensile tail, is destroyed
by the now known structure of that part.

81

But viewing the pelvis of the Megatherium as being the fixed centre towards which
the fore legs and fore part of the body wTere drawn in the gigantic leaf-eater's efforts to
uprend the tree that bore its sustenance, the colossal proportions of its hind extremities
and tail lose all their anomaly, and appear in just harmony with the robust claviculate
and unguiculate fore limbs with which they combined their forces in the Herculean
labour.

Finally, with reference to the hypothesis of the German authors and artists* of the
gradual degeneration of the ancient Megatherioids of South America into the modern
Sloths, it may be admitted that the general results of the labours of the anatomist in
the restoration of extinct species^jviewed in relation to their existing representatives!)?
the different continents and islands, have been such as might naturally suggest the idea
that the races of animals had deteriorated in point of size. Thus the palmated Megaceros
is contrasted, by its superior bulk, with the Fallow-deer, and the great Cave-bear with the
actual Brown Bear of Europe. The huge Diprotodon and Nototherium afford a similar
contrast with the Kangaroos and Koalas of Australia, as do the towering Dinornis and
Palapteryx with the small Apteryx of New Zealand. But the comparatively diminutive
aboriginal animals of South America, Australia, and New Zealand, which are the nearest
allies of the gigantic extinct species respectively characteristic of such tracts of dry land,
are specifically distinct, and usually by characters so well marked as to require a sub-
generic division, and such as no known outward influences have been observed to pro-
duce by progressive alteration of structure. Moreover, as in England, for example, our
Moles, WateT^volesv Weasels, Foxes, and Badgers, are of the same species as those that
coexisted with the Mammoth, Tichorhine Rhinoceros, Cave Hyaena, Bear, &c. ; so likewise
the remains of small Sloths and Armadillos are found associated with the Megatherium .
and Glyptodon in South America ; the fossil remains of ordinary Kangaroos and Wom-
bats occur together with those of gigantic herbivorous Marsupials in Australia; and
there is similar evidence that the Apteryx coexisted with the Dinornis in New Zealand.
I have been led, therefore, to offer the following suggestions as more applicable to, or
explanatory of, the phenomena than Lamaeck's progressive hypothesis of the origin of
species by transmutation f, or Buffon's retrograde hypothesis by degradation J.

■• In proportion to the bulk of an animal is the difficulty of the contest which, as a
living being, it has to maintain against the surrounding influences which are ever tending
to dissolve the \ital bond and subjugate the organized matter to the ordinary chemical
and physical forces. Any changes, therefore, in the external circumstances in which a
species may have been adapted to exist, will militate against that existence in probably
a geometrical ratio to the bulk of such species. If a dry season be gradually prolonged,
the laiire Mammal will suffer from the drought sooner than the small one; if such
alteration of climate affect the quantity of vegetable food, the bulky Herbivore will first
feel the effect of the stinted nourishment ; if new enemies are introduced, the large and

* Paadee and D'Altox, loc. cit. t Philosopbie Zoologique, 8vo 1809, torn. i. ch. vii.

X Histoire Naturelle, 4to. torn. iv. (1766), "Degeneration des Animaux," p. 311.

m2

82

conspicuous quadruped or bird will fall a prey, whilst the smaller species might conceal
themselves and escape. Smaller quadrupeds are usually more prolific than larger ones.
The actual presence, therefore, of small species of animals in countries where the larger
species of the same natural families formerly existed, is not to be ascribed to any gradual
diminution of the size of such larger animals, but is the result of circumstances which
may be illustrated by the fable of the ' oak and the reed ;' the small animals have
bent and accommodated themselves to changes under which the larger species have
succumbed *."

§ 11. Geological Summary.

The first evidences of the great extinct quadruped which forms the subject of the
present Work, were received in Europe in 1789 ; they consisted of a considerable pro-
portion of the skeleton of a full-grown individual, which had been discovered " in some
excavations on the banks of the river Luxan, which flows close by the town of the same
name, about thirteen leagues W.S.W. of Buenos Ayres, in a ravine ten yards in depth,
which lies at a league and a half to the S.W. of the said townf:" and were transmitted
by His Excellency the Marquis of Loreto, Viceroy of Buenos Ayres, to the Koyal Mu-
seum of Natural History at Madrid.

A considerable portion of a second skeleton of a Megatherium was transmitted from
Lima to the same museum in 1795. Parts of a third skeleton were, in the same year,
sent from Paraguay to the Padre Fernando Scio, of the 'Escuelas Pias^.' The parti-
cular circumstances attending the discovery of the last two specimens are not recorded.
The skeleton, mounted, from the foregoing materials, forms the subject of the work by
Bru and Garriga above cited, of the memoirs by Cuvier, and of the illustrations by
Pander and D' Alton, cited at p. 5.

In 1823, a Prussian traveller in South America, Sellow, discovered in the bed of the
Queguay, Uruguay, part of the femur of a Megatherium : it is deposited in the Royal
Museum of Natural History at Berlin, and is noticed by Professor Weiss, in his Memoir
in the ' Berlin Transactions' for 1827, quoted at page 6 of the present Work.

In 1824, Messrs. Mitchell and Cooper gave a brief notice of some bones and teeth
of the Megatherium, discovered in the pleistocene marl of the Skidaway Marshes, in the
State of New York, North America §. In the same year, Messrs. Waring and Haber-
shaw || obtained a few bones of the Megatherium from a like recent formation at l White-
Bluff,' Savannah. In 1831 an instructive proportion of the skeleton of a Megatherium,
including parts not preserved in the Madrid specimen, were discovered in the bed of

• " On the genus Dinorni*'" (part 4), Transactions of the Zoological Society, vol. iv. p. 15
t " En las escavaciones que se hacian en las orillas del Eio Luxan, que corre immediato a la Villa de este
nombre (que esta a unas trece leguas O.S.O. de Buenos Ayres) en una baranca de diez varas de alto, que
e»ta a legua y media al O.S. de dicha Villa." — Descripcion del Esqueleto de un Quadrupedo muy corpulento
y raro, Ac., Don Joseph Gabbioa, Madrid, fol. 1796, ' Prologo.'

X Ibid. § Annals of the Lyceum of New York, vol. i. p. 58. || Ibid.

83

the river Salado, which flows through the flat alluvial plains to the south of tne city of
Buenos Ayres. They were placed at the disposal of Sir Woodbine Parish, K.H., by
Don Hilario Sosa, the owner of the property on which they were found, and were pre-
sented by Sir Woodbine to the Eoyal College of Surgeons. They form the subject of
the Memoir by William Clift, Esq., F.R.S., published in the ' Transactions of the Geo-
logical Society,' Second Series, vol. iii. p. 437.

In 1834 Mr. Charles Darwin discovered remains of the Megatherium on the shores
of the bay called ' Bahia Blanca,' in latitude 39° S., about 250 miles south of the Rio
Plata : they were impacted in a mass of quartz shingle, forming an irregularly stratified
mass divided by curved layers of indurated clay, and constituting the lower bed of the
cliff called ' Punta Alta,' about 18 miles from Monte Hermoso, North Patagonia. The
pebbles are cemented together by calcareous matter, probably resulting from the decom-
position of numerous imbedded shells. These remains are preserved in the Museum of
the Royal College of Surgeons, and are described in the volume of the ' Fossil Mam-
malia of the Voyage of the Beagle,' &c, 4to. 1840, p. 100. pi. 30.

In 1837 a second nearly entire skeleton of the Megatherium was discovered by Signor
Muniz in the river Luxan, near the locality of the first discovered specimen : this was
deposited in the Museum at Monte Video. In 1838 Dr. Decalzi discovered on the
estate of Don Manuel Rosas, called ' Las Averias,' north of the Rio Salado, various
bones of the Megatherium. Other specimens, subsequently discovered at Luxan, were
transmitted by Mr. Falconet, in 1845, to London, and were purchased by the Trustees
of the British Museum : they are noticed at p. 11, and form, with the specimens pre-
viously transmitted to London, the subjects of the descriptions and figures in the
present work.

From the foregoing summary it appears that the Megatherium ranged from the State
of New York to Northern Patagonia, being restricted by the nature of its food, its
habits and organization, to the latitudes most favourable to the growth of trees.

The geological and palseontological evidences concur in showing that the formations
in which its remains have been found are of the newer pliocene period, coeval apparently
with those which contain the remains of the Mammoth and Megaceros in Europe. Of
twenty-three kinds of shells determined by Mr. Sowerby in the collection brought by
Mr. Darwin from the Megatherium-shingle at Punta Alta, twelve are certainly iden-
tical with existing species, and four are probably so, the doubt partly arising from the
imperfect condition of the specimens. These species, moreover, still inhabit the bay on
the shores of which they are found fossil.

The conclusion to which Mr. Darwin arrived as to the comparatively recent period
of the shingle strata at Punta Alta, is nearly the same as that to which he was led by
observations on the beds in which the megatherian remains were imbedded in the
Pampas and river-courses of the province of Buenos Ayres. "We may suppose," he
writes, "that whilst the ancient rivers of the Plata occasionally carried down the
carcasses of animals existing in that country, and deposited them in the mud of the

84

«->tuar\. other animals inhabited the plains round the Sierra de la Ventana, and that
Lmmc streams acting together with the currents of a large bay, drifted their remains
towards a point where sand and shingle were accumulating in a shoal. The whole area
has since been elevated ; the estuary mud of former rivers has been converted into wide
and level plains ; and the shoals of the ancient Bahia Blanca now form low headlands on

the present coast everything indicates a former state of tranquillity, during which

various deposits were accumulating near the then existing coasts, in the same manner as we
may suppose others are at this day in progress. The only physical change, which we
kimw has taken place, since the existence of these ancient Mammalia, has been a small
and gradual rising of the continent*."

* Fossil Mammalia of the Voyage of the Beagle, p. 11.

.

PLATE I.

Skeleton of the Megatherium, on the scale of one inch to a foot. (The vertebrae

concealed by the scapula are added in outline.)

C i-7. Cervical vertebrae. c.

D 1-16. Dorsal vertebrae. p.

L 1-3. Lumbar vertebrae. d.

S. Sacrum. m.

Cd 1-ib. Caudal vertebrae. u.

38. Stylohyal. i.

hJ Scapula.

63. Humerus. n.

"■ Ulna. in.

J*. Radius. iv.

s. Scaphoid part "l of Scaphotrape- v.

/. Trapezial part/ zium. <&•

/. Lunare. us.

<n- Pubis. ce.

«s. Femur. b.

m Tibia. n.

w. Patella.

«7. Fibula. in.

«r Fabella. iv.

a. Astragalus. v.

cl. Calcaneum.

*. Scaphoides.

Cuneiforme.

Pisiforme.

Trapezoides.

Magnum.

Unciforme.

Rudimentary metacarpal of first digit

or. pollex.
Second digit, or index.
Third digit, or medius.
Fourth digit, or annularis.
Fifth digit, or minimus.
Ilium.
Ischium.
Ectocuneiform.
Cuboides.
Rudimentary metatarsal of second

digit.
Third digit.
Fourth digit.
Fifth digit. (There is no rudiment

of the first or innermost toe of the

hind foot in the Megatherium.)

PLATE II.

Typical vertebrae in the Megatherium. One-fourth natural size.

Fig. 1. Fifth dorsal vertebra, or natural segment of the skeleton, c. Centrum, d. arti-
cular tubercle for head rib ; n, neurapophysis, n', neurapophysial surface
for neck of rib ; d, diapophysis, d', diapophysial surface for tubercle of rib ;
*, anterior zygapophysis ; m, metapophysis ; ns, neural spine ; pi, pleurapo-
physis or 'vertebral rib'; c", head; n", articular surface on upper part of
the neck ; d", articular surface on tubercle ; h, haemapophysis or ' sternal
rib,' s1, V, its condyles ; hs, haemal spine, or ' sternal bone;' /, *", articular
surfaces for haemapophysial condyles.

Fig. 2. First caudal vertebra, n. Neural arch ; p, parapophysial part, d, diapophysial
part, pi, pleurapophysial part, of compound transverse process ; hy, hyp-
apophyses; h, haemapophysis, hy1, articular surface for its own vertebra,
which is in advance; hy", articular surface for succeeding vertebra.

PLATE .II.

Sfy DcrvaJ Segment

AvtUd. ty Jfann

PLATE, m.

S.fcJt^jtl * luA

/h/i'fff fiv •> ' Rojtrt:

PLATE III.

Fig. 1 . Seventh dorsal vertebra, front view.

Fig. 9, Seventh dorsal vertebra, back view.

Fig. 3. Seventh dorsal vertebra, side view.

Fig. 4. Sixteenth dorsal vertebra, front view.

Fig. 5. Sixteenth dorsal vertebra, back view.

All the figures are drawn, minus the haemal arch, one-fourth natural size.

d. Articular tubercle for the head of the rib ; «', neurapophysial articular
surface for the neck of the rib ; d, diapophysis, d! , diapophysial articular
surface for tubercle of rib ; m, metapophysis, mz', inetapophysial arti-
cular surface ; a, anapophysis, a', anapophysial articular surface ; /;, par-
apophysis, pa, parapophysial articular surface ; 2, anterior, z', posterior,
mz, mid-anterior, mz', mid-posterior, zygapophyses ; ns, neural spine.

PLATE IV.

Fig. 1 . The atlas, upper view.

Fig. 2. The atlas, under view.

Fig. 3. The atlas, back view.

Fig. 4. The atlas, side view.

Fig. 5. The seven cervical and first dorsal vertebrae, upper view.

Fig. 6. The seventh cervical vertebra, back view.

Fig. 7- The seventh cervical vertebra, side view.

One-fourth natural size; c, centrum; n, neurapophysis ; n.v, neural spine ;
d, diapophysis, da-dr, of the third to the seventh cervicals inclusive;
pi, pleurapophysis ; pi', (fig. 7) articular surface for head of first dorsal
rib; m, metapophysis ; nz (fig. 2), anterior articular surface ; z', posterior
zygapophysis ; hy, hypapophysis ; o, (figs. 1 & 3) its articular surface for
the odontoid or true body of the atlas ; r, division of the foramen alare
posterius, for the passage of the posterior branch of the occipital artery;
s, division of the same foramen for the passage of a branch of the second
spinal nerve; v, foramen alare anterius, communicating with q the canal
for the vertebral artery.

PLATE. IV.

fig. 2.

Printed by J.Bcutrr.

PT.A1

JSfJ.

Fig. 2.

Ify.3.

,l*HA4,M.tH*

frinud by J./iaMs*

PLATE V.

Fig. 1. The axis, or vertebra dentata, front view.

Fig. 2. The axis, or vertebra dentata, side view.

Fig. 3. The third cervical vertebra, front view.

Fig. 4. The third cervical vertebra, side view.

Fig. 5. The sixth cervical vertebra, front view.

Fig. 6. The sixth cervical vertebra, side view.

One-fourth natural size; o, odontoid process (centrum of atlas) ; zn, ana-
logues of anterior zygapophyses, in the dentata ; z, anterior zygapo-
physis ; z', posterior zygapophysis ; d, diapophysis ; p, parapophysis ; pi,
pleurapophysis, pi, its anterior production; m, metapophysis ; ns, neural
spine.

PLATE VI.

First sacral vertebra, with part of its haemal arch (ilium and pubis).
One-fourth natural size ; c, centrum ; ## exogenous growths ; ma, metapophysial
process and articular surface.

PLATE, VU.
1.

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PLATE VII.

Fig. 1 . The five sacral vertebrae, upper view.
Fig. 2. The sacral vertebrae, back view.

One-fourth natural size ; c, centrum ; ni,»i, coalesced neurapophyses ; ns i,

ns 4, coalesced neural spines, of the four anterior vertebrae ; ns s, neural

spine of fifth vertebra ; d i, », a, <, s, diapophyses ; m a-m s, metapophyses ;

o \-o !, posterior outlets of nerve-canals ; z, anterior, z', posterior, zygapo-

physes.

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PLATE VIII.

Fig. 1. The second caudal vertebra, back view.

Fig. 2. The second caudal vertebra, under view, minus the haemal arch.

Fig. 3. The eleventh caudal vertebra, back view.

Fig. 4. The eleventh caudal vertebra, side view.

Fig. 5. The eleventh caudal vertebra, upper view.

Fig. 6. The eleventh caudal vertebra, under view, minus the haemal arch.

Fig. 7- The thirteenth caudal vertebra, upper view.

Fig. 8. The sixteenth caudal vertebra, upper view.

Fig. 9. The sixteenth caudal vertebra, under view.

Fig. 10. The seventeenth caudal vertebra, upper view.

Fig. 11. The seventeenth caudal vertebra, under view.

Fig. 12. The eighteenth caudal vertebra, back view.

Fig. 13. The eighteenth caudal vertebra, front view.

One-fourth natural size ; c, centrum ; hy, anterior, hy', posterior hypapo-
physes ; h, their haemal articular surface ; n, neural arch and neurapo-
physes ; ns, neural spine; d, diapophysis ; pi, pleurapophysis ; m, met-
apophysis ; z, anterior, z', posterior zygapophysis ; h, haemal arch, hy',
(fig. 1) its anterior hypapophysial articular surface, hy", its posterior hyp-
apophysial surface ; hs, haemal spine.

PLATE. VIIL

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PLATE. IX.

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PLATE IX.

Fig. 1. First dorsal rib.

Fig. 2. Second dorsal rib.

Fig. 3. Third dorsal rib ; one-fourth natural size.

Figs, la, 2a, 3a, their upper articulations.

Figs, lb, 2b, 3b, their lower articulations; one-half natural size.

c", articular surface on the head for the centrum ; n", articular surface on
the neck for the neurapophysis ; d", articular surface on the tubercle for
the diapophysis ; V, anterior condyle for sternum, s", posterior condyle
for sternum ; pi, pleurapophysial part, h, hsemapophysial part, of rib.

PLATE X.

Fig. 1. Head and neck of the fifteenth dorsal rib, upper view.
Fig. 2. Head and neck of the sixteenth dorsal rib, upper view.
Fig. 3. Head and neck of the fifteenth dorsal rib, side. view.

One-half natural size ; n", neurapophysial articular surface.
Fig. 4. The thirteenth dorsal vertebra.
Fig. 5. The sixteenth dorsal vertebra.

Fig. 6. The first caudal vertebra, under view, minus the haemal arch.
Fig. 7- The clavicle.

One-fourth natural size ; c', articular surface for the head of the rib ;

«', neurapophysial surface for the neck of the rib ; p, parapophysis ;

d, diapophysis ; a, anapophysis, a', its articular surface ; pi, pleurapo-

physis ; hi/, hypapophysis ; z, anterior, *', posterior zygapophysis ; ns,

neural spine.

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PLATE. X.

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PLATE XI.

Haemapophyses of dorsal vertebrae or bones of the sternum.

Fig. 1 . Manubrium sterni, outer or under view.

Fig. 2. Manubrium sterni, inner or upper view.

Fig. 3. Manubrium sterni, side view.

Fig. 4. Sixth sternal bone, inner or central surface.

Fig. 5. Sixth sternal bone, outer or peripheral surface.

Fig. 6. Sixth sternal bone, upper or anterior surface.

Fig. 7- Sixth sternal bone, side view.

Fig. 8. Eighth sternal bone, inner surface.

Fig. 9. Eighth sternal bone, outer surface.

Fig. 10. Eighth sternal bone, upper surface.

Fig. 1 1 . Eighth sternal bone, under surface.

One-fourth natural size ; ql, surface for the clavicle ; hp, articular surface
for haemapophysis, or sternal portion of rib ; * and s1, articular surface for
contiguous sternal bone ; ha, anterior outer haemapophysial surface, ha',
posterior outer haemapophysial surface ; hp, anterior inner, hp', posterior
inner haemapophysial surface.

PLATE XII

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PLATE XII.

One-fourth natural size.
Fig. 1. Side view of the skull of the Megatherium.

Fig. 2. Inner side view of the mandible and of the section of the symphysis.
The letters and ciphers are explained in the text.

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PLATE XIII.

One-fourth natural size.

Fig. 1. The opposite and more mutilated side of the same skull as PI. XII., showing

the surface of the temporal fossa.
Fig. 2. Upper view of the same skull.

The letters and ciphers are explained in the text.

PLATE XIII

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PLATE XIV.

One-fourth natural size.

Fig. 1 . Back view of the skull of the Megatherium.

Fig. 2. Front view of the same skull.

Fig. 3. Back view of the mandible.

Fig. 4. Front view of the same mandible.

The letters and ciphers are explained in the text.

PLATE XV.

Natural size.

Base view of the skull, and grinding surface of the upper teeth, of the Megatherium.

The letters and ciphers are explained in the text.

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PLATE XVI.

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PLATE XVI.

One-fourth natural size.

Fig. 1 . Upper view of the mandible, and grinding surface of the lower teeth, of the

Megatherium.
Fig. 2. Side view of the mandible.

The letters and ciphers are explained in the text.

PLATE XVII.

Fig. 1. The second upper molar tooth — natural size — of the Megatherium.

Fig. 2. Longitudinal section of the alveolar part of the upper jaw and of the five
upper molars, in situ — three-fourths of the natural size — of the Megathe-
rium.

Fig. 3. Transverse section of a portion of a molar tooth of the Megatherium : — mag-
nified 500 linear diameters.

The letters and ciphers are explained in the text.

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PLATE XVIII.

PLATE XVIII.

One-fourth natural size.
Fig. 1. Inner surface of the scapula.
Fig. 2. Glenoid cavity and acromio-coracoid arch.

PLATE XIX.

One-fourth natural size.

Fig. 1. Under surface of the right clavicle.

Fig. 2. Front view of the left humerus.

Fig. 3. Back, view of the left humerus.

Fig. 4. Head, or proximal articular surface of the left humerus.

Fig. 5. Distal articular surface of the left humerus.

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PLATE XIX.

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PLATE XX.

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PLATE XX.

One-fourth natural size.

Fig. 1. Antero-radial surface of the left ulna.

Fig. 2. Proximal end of the ulna.

Fig. 3. Distal end of the ulna.

Fig. 4. Anterior surface of the left radius.

Fig. 5. Proximal end of the radius.

Rg. '!. Distal end of the radius.

PLATE XXI.

Dorsal or upper surface of the bones of the right fore-foot: less than one-half the
natural size.

PLATE XXI.

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PLATE XXII.

One-fifth natural size.

Fig. 1. Side view of the pelvis of the Megatherium.
Fig. 2. The acetabulum.

The letters and ciphers are explained in the text.

Drawn, by permission of the President and Council, from the specimen presented by
Sir WooDBDJB Pakish. K.H., F.K.S.. to the Museum of the Royal College of Surgeons
of England.

PLATE XXIII.

One-third natural size.

Fig. 1. Back view of the left femur.
Fig. 2. Back view of the left patella.

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PLATE JHV.

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PLATE XXIV.

One-third the natural size.

Fig. 1. Proximal end of the femur.

Fig. 2. Distal end of the femur.

Fig. 3. Proximal end of the tibia.

Fig. 4. Distal ends of the anchylosed tibia and fibula.

PLATE XX V.

One-half the natural size.
Fig. 1. Tibial side of the bones of the hind-foot,
l'iir. 2. Front view of the two cuneiform and the cuboid bones.
Fig. 3. Front view of the navicular bone, and parts of the astragalus and calcaneum.

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PLATE XXVI.

One-half the natural size.
Fig. 1. Upper view of the bones of the hind-foot.
Fig. 2. Front view of the calcaneum.
Kg. 3. Fibular side of the bones of the great unguiculate toe.

PLATE XXVII.

Reduced and oblique front view of the skeleton of the Megatherium americaimm, in the
British Museum.

^*"«wiiM«a iiL.^,.

NOV 1 0 1978

PLEASE DO NOT REMOVE
CARDS OR SLIPS FROM THIS POCKET

UNIVERSITY OF TORONTO LIBRARY

QE
882
E2
084

Owen, (Sir) Richard

Memoir on the megatherium,
or giant ground-sloth of
America. 1861

P&A Sci

7/

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