With respect to the A i, or three-toed Sloth, “ an animal, great part o f whose life,
when not engaged in eating, is spent in sleeping on trees,—an easy attitude for repose
is most essential to its comfortable existenc e ; and accordingly we find, that the
auxiliary vertebræ at the base o f the neck contribute to produce that flexibility o f
this organ which allows the head o f the animal to incline forwards and rest upon
Its bosom.” Dr, Buckland, from whose Paper on the “ Adaptation o f the Structure
o f tlie Sloths to their peculiar Mode o f Life,”* the preceding judicious
physiological remark is quoted, adduces the authority o f Mr. Burchell in proof
that the Sloth can in a remarkable manner and with great facility twist its head
quite round, and look in the face o f a person standing directly behind it, while
at the same time the body and limbs remain unmoved. A sing le glance at the
length and slenderness o f the cervical region o f the spine, and of the feeble condition
o f the transverse and spinous processes in the vertebræ composing that part o f
the skeleton o f the Sloth, is enough to show its adaptation to increase the rotatory
motion and flexibility o f the neck.
In describing the skeleton o f a spe cie s of Armadillo {Dasypus 0-cinctus, Lin n .)t
I was led in lik e manner to point out the subserviency o f the peculiarities o f the
cervical vertebræ to the habits and mode o f life o f that animal ; observing that the
“ anchylosis o f the cervical vertebræ obtains in the Cetacea, as well as in the genus
Dasypus, and that as in the aquatic order this firm connexion o f the cervical vertebræ
a ssists materially in enabling the head to overcome the resistance o f the
dense fluid through whicli they perpetually move, so in the Armadillos a like
advantage may he derived from tliis structure during the act o f displacing the
denser material in which they excavate their retreats. ”J
Having in view these well-marked examples o f the subserviency of the structure
o f the bones o f the neck to the habits o f existing spe cie s o f the order B ru ta ,
I proceeded to investigate the structure o f the corresponding part of the skeleton
in the Scelidotherium, hoping thereby to gain a new and useful element in the
determination o f the problem at present under discussion, as to the affinities and
habits o f the extinct Megatlierioid quadrupeds.
The fossil, in its original state, yielded a view o f so mucli o f tlie anterior part
o f the bodies o f the cervical vertebræ as proved tliat they were neitlier so numerous
as in tlie Sloth, nor anchylosed together as in the Armadillos: after a long
and careful chiselling at the hard matrix in which they were imbedded, the trans-
* Linn. Trans, vol. xvii. (1833) p. 17. t Zool. Proceedings, 1832, p. 134.
% The anterior prolongation of the sternum in front of tlie neck and the corresponding anterior position of
the clavicles and scapulæ occasions a transference of such a proportion of the moving powers of the head from
the cervical vertebræ to these bones in the mole, as renders any modifications of these vertebræ, like those in the
Armadillo, uncalled for.
vervSe and spinous processes were exposed to view, as they are represented in
P la te s X X . and X X IV . The description o f these processes has already been
given.
On comparing the cervical vertebræ o f the Scelidotherium with those o f the
ex isting B ru la , the close st resemblance to them was found in the skeleton o f the
Orycterope. Now this quadruped, though not so rapid a hurrower, or so strictly
a subterranean spe cie s as the Armadillos, participates, nevertheless, to a certain
extent, in their fossorial habits, and is c lo se ly allied to them in general structure :
it differs from them, indeed, mainly in a modification o f the dental system, in
the absence o f dermal armour, and of anchylosis o f the cervical vertebræ. But
the advantages which, as a burrower, it would have derived from the latter structure,
are compensated for by the shortness o f the cervical vertebræ, and by the
great development and imbricated or interlocking co-adaptation o f the transverse
and anterior spinous processes o f the cervical vertebræ. Th e analogous quadruped
in the South American Continent—the great ant-eater (myrmecophaga juhata)
which use s its powerful compressed fossorial claws for breaking through the hard
walls o f the habitations o f its insect prey, but which does not excavate a subterraneous
retreat for itself, presents the cervical vertebræ o f a more elongated form,
and M'ithout that development o f the spinous and transverse processes which tend
to fix the neck and increase the size o f the muscles which move the head ; and, if
we could conceive that its fore-feet were employed to scratch up vegetable roots,
instead of disinterring termites, there would be no reason to expec t any modification
o f the cervical vertebræ as a direct consequence o f such a difference in the
application o f its fossorial extremities : when, therefore, we find that the cervical
vertebræ do actually differ in two myrmecophagous spe cie s, to the extent observable
in the Cape and South American ant-eaters, we arrive legitimately at the
conclusion that such difference relates to fossorial habits o f the one spe cie s, iu
which habits the other does not participate.
Now, therefore, if this conclusion be ju st in regard to the Orycterope, it must
bear with more force upon the question o f the habits o f the Scelidotherium as the
mechanism for strengthening the connection o f cervical vertebræ, and for augmenting
the surface o f attachment o f the muscles which worked the head and
neck, is more strongly wrought out in that extinct species.
Th e great size and strength o f the spinous process of the dentata, and the
mode in which it is interlocked with the spinous and oblique processes o f the third
cervical, together with the imbricated disposition o f the transverse processes of
this and the succeeding vertebræ, and the remarkable heigh t o f the dorsal spines,
all combine to indicate in a very striking manner, if not to demonstrate, that the
conical head o f the present spe cie s, which is comparatively small and slender, and