for its own mere support requiring therefore no such mechanism, was used in aid
o f the fossorial actions o f the extremities.
As the cervical vertebræ o f the Megatherium have their processes comparatively
weaker than in the Scelidotherium, and the anterior dorsal spines are relatively
shorter, it may be concluded, that whatever were the ex tent or nature o f the
fossorial labours o f the enormous claws with which it was provided, the head did not
co-operate with the digging implements in their especial task in the same degree
as in the Scelidothere and Orycterope. At the same time there is no modification
o f the cervical region o f the spine o f the Megathere corresponding with those which
we have seen to be subservient to the arboreal habits o f the sloth, a remark vvlfich
will not be deemed superfluous by those who have perused the acute observations
and arguments adduced by M. Lund in favour o f the scansorial character o f the
extremities o f the Megatherium and Megalonyx.
The fragments o f the dorsal vertebræ and ribs o f the Scelidotherium, which are
figured in Plate X X , offer no modifications which need detain our attention ; they
closelyconform, ex c epting in the greater relative height o f the anterior dorsal spines,
already noticed, with the Megatherioid type. The sacrum manifests in its vast
expanse, the great development o f the posterior transverse processes to join the
ischium, the capacious medullary cavity, and wide nervous foramina, a like conformity
with the Megatherium, and a corresponding harmony with the disproportionate
bulk o f the bind legs.
B o n e s o f t h e E x t r e m i t ie s .
The Scapula in its double spine, the osseous arch formed by the confluence
of the acromion with the coracoid process, and the substitution o f a distinct foramen
for the suprascapular notch, agrees with that o f the Megatherium : but the span
o f the acromial arch is relatively wider, and the surface for the articulation o f the
clavicle is better marked. This articular surface, which is distinctly shewn upon
the acromion o f both the scapulæ in PI. X X . is the more interesting, as being
the only evidence o f the clavicle o f the Scelidothere which we at present possess;
but it is enough to prove that this quadruped enjoyed all the advantages in
the actions o f the fore-extremity, which arise out o f the additional fixation o f the
shoulder-joint afforded by the clavicle—a bone which the ex tinc t Megatherioids
are the largest o f the mammiferous cla ss to possess in a completely developed
state. The form, position, and a spe ct o f the glenoid cavity for the humerus
c lose ly correspond with the condition o f the same part in the Megatherium. The
limits o f the acromial and coronoid portions o f the arch were still defineable in the
present skeleton, which indicates the nonage o f the individual in the unanchylosed
condition o f most o f the epiphyses.
In regard to the presence of a clavicle in the Megalonyx M. Lund has
deduced certain conclusions, which, if well founded, would be equally applicable
to the present allied species, and to the great Megatherium. I am induced,
therefore, to offer a few physiological observations on that bone, which appear to
me to lead to a more correct interpretation o f its use s and relations in the great
mammiferous animals now under consideration.
When the anterior extremities in mammalia are used simply for the purpose
o f progressive motion on dry land, as in the Pachyderms and Ruminants, or in
water, as in the Cetaceans, there is no c la v ic le ; this bone is introduced between
the sternum and acromion, in order to give firmness and fixity to the shoulder-
jo int when the fore-leg is to discharge some other office than that o f locomotion.
In these cases, however, the clavicle ex ists in various degrees o f development,
and even its rudiment may be dispensed with in some o f the actions which
require a considerable extent o f lateral or outward motion, and o f freedom of
rotation o f the fore-limb. When, therefore, we find the clavicle fully developed
in the skeleton o f an extinct mammiferous animal, and so placed as to give the
humeral articulation all the benefit o f this additional mechanism, we may confidently
exp ec t that it will afford an insight into the habits and mode o f life o f such
ex tinc t species. M, Lund* has argued from the clavicle o f the Megalonyx, that
it climbed like a Sloth. “ Animals,” sa y s Sir C. B e ll,t “ which fly or dig, or
climb, as Ba ts, Moles, Porcupines, Squirrels, Ant-eaters, Armadilloes, and Sloths,
have this bone; for in them, a lateral or outward motion is required.” But in regard
to the present problem, we have to enquire whether the clavicle manifests any
modifications o f form, o f strength, or development in relation to the special differences
of these several actions, with which its presence is asserted to be associated ?
In mammals which fly, the clavicle is always complete : the rabbit, the
fox, and the badger are instanc es o f burrowing animals in which the clavicle is
absent or rudimental. The presence o f a perfect clavicle is not more constant in
climbing quadrupeds. The Ai, for example, has an incomplete clavicle, which is
attached to the acromion process, and terminates in a point about one-fourth o f
the distance between the acromion and the top o f the sternum, to which the
clavicular sty le is attached by a long slender lig am en t: the advantage, therefore,
which a perfect clavicle affords in the fixation o f the shoulder-joint, is lost to this
c l i m b e r excellence. Again, the Bears, which are the bulkiest quadrupeds that
are gifted with the faculty o f climbing, and this in so perfect a degree that the
Sun-bears o f the Eastern Tropics may be termed arboreal animals,—these scan-
* Loc. cit. t Bridgewater Treatise, p. 46.