In these Mammals the teeth
are by no means always wanting, as the name of the group
would seem to imply; but, when teeth are present, incisors
are either altogether absent, or, at any rate, the median in
cisors are wanting in both jaws. The teeth are always devoid
of enamel, consisting merely of dentine and cement. As they
grow for an indefinite period, they never form roots; and, so
far as our knowledge at present extends, those which first
appear are displaced by a second set only in some of the Armadillos.
The ungual phalanges of the digits support long and strong claws.
There are mammae upon the thorax, and sometimes, in addition,
on the integument of the abdomen; or, in the inguinal region.
The brain varies greatly, its hemispheres being sometimes
quite smooth, with a very small corpus callosum and large anterior
commissure; while, in other cases, the corpus callosum
is much larger, and convolutions appear upon the surface of
are divided into the Phytophaga
vegetable-feeders, and the Entomophuga
, or insect-eating forms.
Leaves are the chief food of the former group, while the latter
delight chiefly in ants, though some take, in addition, worms and carrion.
1. In the Phytophaga
the long bones are without medullary
cavities. The lateral part of the zygomatic arch sends
down a remarkable vertical process. The acromial process of
the scapula coalesces with the coracoid. In the carpus, the
scaphoid and the trapezial bones anchylose and form one. The
ischia become united with the anterior caudal vertebree, and
these anchylose with the proper sacrals to form the long sacrum.
The ankle-joint has the character of a peg and socket, and
the hind-foot is, more or less completely twisted, resting upon
its outer edge, and not upon its sole.
Vascular canals connected with the pulp-cavity traverse
the dentine of the teeth.
are divisible into two groups, one existing,
and the other extinct. The former consists of the Sloths,
; remarkable animals, which are confined to
the great forests of South America, where they lead a purely
arboreal life, suspended by their strong, hooklike, claws to the
branches of the trees.
Their distinctive characters are these: The tail is short,
and the limbs exceedingly long and slender, the anterior being
longer than the posterior pair. In both the fore-and the
hind-limbs the internal and the external digits are rudimentary,
but the hind-foot always has the three middle toes completely
developed; while, in the fore-foot, it sometimes happens
that only two remain. The ungual phalanges are very
long and hooked.
The zygomatic arch is incomplete posteriorly, not being
united by bone with the squamosal. The cervical vertebrae
in this remarkable group sometimes exceed, and sometimes
fall short of, the number (seven) which is so characteristic of
in general; some species of Sloths having
nine, and others only six, vertebrte in the neck.
The pelvis is exceedingly spacious, and the acetabula are
directed backward as well as outward. The femur is devoid
of a ligamentum, teres.
The distal end of the fibula sends inward
a process which fits into a fossa situated upon the outer
surface of the astragalus, giving rise to that kind of peg-and-
socket ankle-joint which is peculiar to these animals.
A good deal of confusion prevails respecting the structure
of the ankle-joint in the Sloths. Cuvier ("Ossemens fossiles," t. viii., p. 143)
writes of the Ai, or three-toed Sloth:
"In the greater number of animals, the principal articulation
of the astragalus connects it with the tibia, by means
of a more or less loose ginglymus, which allows the foot to be
bent on the leg. But here the principal and superior facet of
the astragalus is a conical fossa, into which the pointed extremity
of the fibula penetrates, like a pivot. (See PI. 208, Fig. 2a.
The inner edge of this fossa turns against a very
small facet, which occupies only a third of the lower head of
the tibia. The result of this arrangement is that the foot
turns on the leg, like a weathercock on its support, but that it
cannot be flexed. It further follows that the plane of the sole
of the foot (Cuvier's words are: "II en resulte encore que le plan, le corps du pied,
est presque vertical quand la jambe Test.") is almost vertical when the leg is so, and that the
animal can only place the plantar surface of its foot on the
ground by spreading out the leg so as to make it almost horizontal."
Meckle ("System der vergleichenden Anatomie," 2te Theil., 2te Abtheilung,
p. 457.) has already justly remonstrated against Cuvier's
assertion that only abduction and adduction are possible to
the pes of the Ai, affirming that it is capable of flexion and
extension, though only to a limited extent. A. Wagner follows
Meckel, but Rapp ("Edentaten," p. 46) adopts Cuvier's
statement in its fulness: "Extension and flexion of the foot
cannot take place, but only abduction and adduction. "However,
it is easy to demonstrate on the uninjured dead animal,
or, still better, on the limb from which the muscles have been
removed, while the ligaments have been left intact, that the
pes of the three-toed Sloth is capable of extensive motion in
three directions: first, in abduction and adduction; a movement
in azimuth, when the leg is vertical; secondly, in flexion
and extension; a more extensive movement in altitude, under
the same circumstances; and, thirdly, in rotation upon its
own axis, by means of which the sole can be moved through
90° from a position perpendicular to the axis of the leg to one
parallel with it.
The anatomical arrangements upon which the execution
of these movements depend are the following: The astragalus
presents two facets to the bones of the leg, one of which
(when the pes is in the position usual in other quadrupeds)
looks inward and upward, while the other looks outward and
upward. The former, convex from before backward, as well
as from side to side, is by no means a mere rim, though it is
not so wide as the other. It is the proper proximal surface
of the astragalus, and articulates with the tibia. The other
surface is excavated by a deep conical pit. Into this is received
a correspondingly conical process of the distal end of
the fibula, which is directed from above and without, downward
and inward-not vertically, therefore, but very obliquely.
Hence, even if the pivot fitted its socket quite accurately,
there would still be abundant opportunity for flexion and extension,
though the movement of the pes would be obliquely
inward, as well as upward, in the former case; and obliquely
outward, as well as downward, in the latter. But the socket
fits the pivot loosely, and hence, as experiment demonstrates,
the movement of the pes in flexion and extension is but very
The true movement of abduction and adduction is so much
less extensive than the movement in flexion and extension,
because it is checked by the short and strong internal and external
lateral ligaments of the ankle-joint.
With respect to the rotation of the foot on its own axis-
it is to be observed, in the first place, that the calcaneum, cuboides,
the three cuneiformia,
the three complete
and the three rudimentary metatarsals, and the three basal
phalanges of digits ii., iii.,
are anchylosed together
into one bony mass; while, as in the manus, there is hardly
any motion between the basal and the middle phalanges.
Practically, in fact, the only bones of the pes which are movable
upon one another are: 1. The distal phalanges, which
have a movement of extension and flexion through 180° upon
the middle phalanges. 2. The tarso-phalangeal synostosis
above described is freely movable on the astragalus; and the
joint is disposed in such a manner as to allow the sole of the
foot to be rotated from the plantigrade position in which it is
perpendicular to the axis of the leg, to the scansorial position,
in which it lies parallel with the axis of the leg. It may be
doubted, however, whether the former position can be given
to the sole by the living animal. The tibialis anticus
and the extensor hallucis longus
are extremely strong muscles, and
have no effcient antagonists; so that their tonic contraction
must pull the navicular metatarsal tuberosity, into which they
are inserted, as far upward as it will go, causing the tarso-phalangeal
synostosis to rotate upon the astragalus, and thus
obliging the sole of the foot to look inward.
In the two-toed Sloth, or Unau (Cholaepus
), the general
structure of the ankle-joint is the same, but the fossa of the astragalus looks
almost directly outward, and the pivot of the
fibula is more nearly horizontal, when the leg is vertical. The
tibial facet of the astragalus looks directly upward. Hence,
the movement of the pes is more exclusively one of flexion
and extension than in the Ai. No anchylosis of the tarsal,
metatarsal, and phalangeal bones occurs, but the rotation of
the distal moiety of the tarsus upon the astragalus as much
more complete and permanent than in the Ai. The calcaneum
is twisted round under the astragalus, in such a manner
that its proper external face becomes inferior, while the articular
surface for the cuboid is not only below, but is partially
internal to, the navicular facet of the astragalus. As a result
of this position of the cuboid, the outer metatarsals, which it
supports, are placed directly beneath the inner ones, and the
pes rests absolutely upon its outer edge, the plane of the sole
The Sloths, it thus appears, are naturally club-footed; but
neither in the Ai, nor in the Unau, does this depend in any
way on the structure of the ankle-joint. On the contrary, it
results, in the Unau, from the manner in which the calcaneum
articulate with the astragalus; and, in the Ai,
from the action of the muscles on the tarso-phalangeal synostosis.
Neither in the Ai, nor the Unau, is there any thing to
interfere with free flexion and extension of the pes.
The teeth are five in number on each side above, and four
below, and become sharpened by mutual attrition into a chisellike
form. The stomach is remarkably complex.
are, for the most part, like the Sloths,
South American forms, but they are entirely extinct; and
while, in most respects, they resemble the Sloths, in others
they present an approximation to Ant-eaters.
The jugal arch may be complete or incomplete. The articular
surfaces of the dorsal vertebrae are sometimes complicated
in a manner similar to that observed in the Ant-eaters. The
tail is very long and strong. The limbs are short and subequal,
while the fore-foot has the ulnar digit imperfect, as in
the Ant-eaters. The fibula has no inward process, and the
astragalus is consequently devoid of any fossa upon its outer
surface. But another kind of peg-and-socket ankle-joint is
produced by the interlocking of the surfaces of the tibia and
of the astragalus.
The great extinct animals. Megatherium, Mylodon, Megalonyx,
etc., the remains of which have been found almost wholly
in later tertiary deposits of America, belong to this group.