2. The Platyrrhini
are essentially quadrupedal and plantigrade,
though some, like the Spider Monkeys (Ateles
assume the erect posture. They all possess tails, and
in some genera (e. g., Ateles
) this organ becomes very flexible
and muscular, and the under surface of its extremity is devoid
of hair and highly sensitive. The tail, thus modified, is a
powerful prehensile organ, and serves as a fifth hand. The
partition between the nostrils is broad and separates them
widely, so that the nose is remarkably wide and flat, whence
the name of the group. The ears are rounded and bare.
There are no cheek-pouches, nor ischial callosities, in any
Platyrrhine Monkey. In most, the fore-limbs are shorter than
the hind-limbs, but the reverse is the case in the Spider Monkeys
The pollex differs less from the other digits than it
does in the Catarrhini.
It is more nearly parallel with, and
in the same plane as, the other digits of the manus; and,
though capable of extensive adduction and abduction, can
hardly be said to be truly opposable. The hallux is large, and
susceptible of extensive movements in abduction and adduction.
The number of the dorso-lumbar vertebrae varies from
seventeen to twenty-two, the greatest number being possessed
which has 22 (14 + 8 or 15+7). In
those forms which have prehensile tails the terminal caudal
vertebrae are flattened from above downward. The articular
surface of the head of the humerus looks more backward than
inward; and, not unfrequently, there is a foramen above the
inner condyle. The carpus contains nine bones. The pollex
is generally complete, but, in Ateles
, it is reduced to a small
metacarpal (to which, usually, a single minute nodular phalanx
is articulated), and is completely hidden beneath the integument.
The pelvis is, generally, elongated, and the anterior
ramus of the pubis lies at right angles with the long axis of
the narrow ilium. The tuberosities of the ischia are everted,
but not rugose. In Ateles
, the pelvis is broader, and the pubis
forms a more open angle with the ilium. The calcaneal process
is always very short, and compressed from side to side.
The brain-case is rounded and devoid of strong crests.
There is no distinct mastoid process, and the styloid is not
ossified. The coronal suture is generally V-shaped, the apex
of the frontal bone extending far back on the vertex of the
skull. The alisphenoid and the parietal bones unite upon
the side-walls of the skull. The external auditoiy meatus is
not ossified, the tympanic bone retaining its foetal, hoop-like
form. The frontal bones approach one another on the floor of
the skull, but rarely unite over the junction of the presphenoid
with the ethmoid.
On the inner surface of the periotic bone
there is a fossa overarched by the anterior vertical semicircular
canal, in which the flocculus rests. In Ateles
the greater part
of the tentorium is ossified. In other respects, the skull presents
extraordinary variations among the Platyrrhini;
two extremes being presented by the Howling Monkeys (Mycetes)
and the Squirrel Monkeys (Chrysothrix).
former, the face is very large and prominent, with a low facial
angle. The roof of the brain-case is depressed; the plane of
the occipital foramen is almost perpendicular to the basi-cranial
axis; and that of the tentorium is very much inclined.
The occipital condyles are, consequently, situated at the posterior
end of the basis cranii,
and the basi-cranial axis is as
long as the cerebral cavity. In Chrysothrix
, on the contrary,
the face is relatively small, with a high facial angle; the braincase
is moderately arched; the plane of the tentorium is horizontal,
like that of the occipital foramen, which lies but little
behind the middle of the base of the skull. The basi-cranial
axis is much shorter than the cerebral cavity. The premaxillo-maxillary
suture disappears early in Cebus.
The formula of the adult dentition is i.
3.3/3.3=36. The crowns of the molar teeth usually have two
transverse ridges, ending in four cusps. In the upper molars
an oblique ridge crosses the crown from
the antero-external to the postero-internal cusp. The permanent
canines usually make their appearance before the last
The stomach is simple, the caecum large, and devoid of
any vermiform appendix; the liver is usually five-lobed; and
the kidney has a single papilla.
The ventricles of the larynx are not usually developed into
air-sacs. In Ateles
, however, a median air-sac is developed
from the posterior wall of the windpipe between the cricoid
cartilage and the first ring of the trachea. A very remarkable
modification of the hyoid and larynx takes place in Mycetes
The cornua of the hyoid are rudimentary, but its body is converted
into a large thin-walled bony drum, the cavity of which
communicates, beneath the large epiglottis, with that of the
larynx. The thyroid cartilage is very large, and the cartilages
of Wrisberg and Santorini are replaced by a fibrous mass,
which is united posteriorly with its fellow of the opposite side.
In addition to the hyoidean air-sac the ventricles of the larynx
are dilated and prolonged upward, coming into contact above
the larynx; two pharyngo-laryngeal pouches may be added
to these. Mycetes
is famous for the distance to which its
howling voice can be heard in the South American forests.
Although the pollex is rudimentary and apparently functionless
, all its characteristic muscles (abductor, adductor,
) are present, except the
the pedal interossei
are flexores breves,
and lie on the plantar surfaces of the metatarsal bones, as in
the Marmosets; but both the adductor hallucis
and the transversus pedis
are well developed.
The brain varies remarkably in different Platyrrhini.
the cerebral hemispheres project beyond the
cerebellum to a greater relative extent than in any other Mammal,
namely, by one-fifth of their total length. On the other
hand, in Mycetes,
the cerebral hemispheres hardly hide the
cerebellum, when the brain is viewed from above.
, the outer surface of the brain is almost as much
convoluted as in the Catarrhine Apes. Ateles
has the external
perpendicular fissure almost obliterated by the annectent gyri,
and, so far, exhibits a higher type of brain than the Catarrhini;
in Pithecia, Chrysothrix,
external sulci gradually disappear, until the brain is almost as
smooth as in the Marmosets. On the inner faces of the hemispheres,
however, the internal perpendicular, the calloso-marginal,
the calcarine, and the collateral sulci remain, while, in
the interior of the hemispheres, the posterior cornu and the hippocampus minor
are always present.
of the cerebellum is large and projects beyond
the level of the posterior margins of its hemispheres; the flocculus
is large and lodged in a fossa of the periotic ossification,
as in the Marmosets. The upper ends of the pyramids are
separated by corpora trapezoidea
from the pons Varolii.
The penis is usually terminated by a large, button-shaped
glans. The cavity of the tunica vaginalis
is not shut off from
the abdomen, and the testes lie at the sides of, rather than behind,
the penis. The female Ateles
has a long clitoris, which
depends from the vagina.
occur only in the Austro-Columbian province,
and are known in the fossil state only in certain caves
of that region.