In the Didelphia,
process" early becomes completely anchylosed with the body
of the second vertebra; and, usually, all the cervical ribs
speedily lose their distinctness, as in Mammals in general.
The coracoid is reduced to a mere process of the scapula
and does not come near the sternum. There is no epicoracoid,
such as exists in the Ornithodelphia
. There is no T-shaped
interclavicle, but the clavicles, which are always present
(except in Perameles
) articulate with the manubrium of
the sternum, in the same way as in ordinary Mammalia
The floors of the acetabula are completely ossified, and consequenly
are imperforate in the dry skeleton. The cochlea is
coiled upon itself.
There is a shallow cloaca, the sphineter muscle being common
to the urinary and genital apertures, but there is no such
urogenital chamber as in the Monotremata
. The ureters open
directly into the bladder.
In the male, the urogenital part of the urethra, and that
which traverses the penis, form one continuous canal, which
opens outward only at the extremity of the penis.
In the female, the vaginal is perfectly distinct from the
urinary passage. The mouths of the Fallopian tubes are fimbriated,
and the ova are not larger than those of the Monodelphia
The mammary glands are provided with long teats.
In all the preceding characters the Didelphia
, and differ from the Ornithodelphia
But they agree with the Ornithodelphia
, and differ from
in possessing either bones or cartilages, attached
to the pubes, in the position of the so-called marsupial
bones of the Ornithodelphia.
Again, the brain, the cerebral hemispheres of which may
or may not have a convoluted surface, is provided with a very
small corpus callosum, and a large anterior commissure. The
hippocampal sulcus is prolonged forward over the corpus callosum.
The crura of the corpus cavernosum of the penis are not
fixed to the ischium.
The embryo does not become connected with the parent
by villi developed from the allantois, and it is born in a very
Certain characters are peculiar to the Didelphia
the testes of the male pass into a scrotum, which is suspended
in front of the penis. In the female, the cremaster muscle is
largely developed, and spreads over the surface of the mammary
gland, which it compresses, so as to drive the milk out
of the projecting teat. There is no fossa ovalis
on the right
side of the septum of the auricles. Very generally, though
not invariably, the Didelphia
possess what is termed a marsupial
pouch, which is a sort of bag, formed by a fold of the integument of the
abdomen, into which muscular fibres of the panniculus carnosus
extend. These support the ventral wall
of the pouch, and are capable of closing its mouth, which may
be directed either forward or backward. The mammary glanda
lie in the dorsal wall of this pouch, into which the teats project.
There is no direct communication between the female generative
organs and the pouch; but the minute young are
transported, in the blind and imperfect state in which they
are born, into the interior of the marsupium,
and each becomes
attached to a nipple, which exactly fills its mouth. To
this it remains attached for a considerable period, the milk
being forced down its throat by the contraction of the cremaster
muscle. The danger of suffocation is averted by the
elongated and conical form of the upper extremity of the
larynx, which is embraced by the soft palate, as in the Cetacea;
and thus respiration goes on freely, while the milk
passes, on each side of the laryngeal cone, into the oesophagus.
It very commonly happens among the Didelphia
two long vaginae are bent upon themselves, their proximal
ends becoming applied together and dilated, and these dilated
portions not unfrequently communicate. Another very general
peculiarity of the Didelphia
is the inflection of the lower
margin of the angle of the mandible inward into a strong horizontal
process. In the genus Tarsipes
, however, this process
There are further anatomical characters which are well
worthy of notice, though they are not so important as the
The integument is always furry, never spiny or scaly, nor
provided with dermal scutes. The pinna of the external ear
is well developed. In the skull the carotid arteries pierce the
basisphenoid to enter the cranial cavity. The tympanic cavity
is in front, bounded by the alisphenoid; and, very generally,
the jugal furnishes part of the articular surface for the mandible.
Many of the cranial sutures, especially in the occipital
region, persist throughout life; and the squamosal, the united
periotic ossifications, and the tympanic bones remain distinct
from one another.
The jaws are always provided with true teeth; and, usually,
these teeth are readily distinguished into incisors, canines,
false molars, and true molars. The canines, however, are
absent in some genera, either in both jaws or in the mandible.
There are usually four true molar teeth, and, as Prof. Flower
has recently discovered, only one grinder succeeds another
vertically. It represents the last premolar. The molars never
possess a complex structure.
No didelphous mammal has three incisor teeth upon each
side above and below; and none but Phascolomys
equal number of incisors in each jaw, the number of the
upper being usually in excess of that of the lower jaw.
The number of the dorso-lumbar vertebrae is almost always
nineteen; and, of these, six are usually dorsal. The atlas is
generally incompletely ossified in the ventral median line.
The manus usually possesses five digits, but in Perameles
the outer digits become rudimentary.
The fibula is always complete at its distal end. In some
cases it becomes anchylosed with the tibia, while in the
), the Phalangers (Phalangistidae
and the Opossums (Didelphiae
), it is not only free, but is capable
of a rotatory movement upon the tibia, similar to the
movement of pronation and supination of the radius upon the
ulna in Man. The rotation of the fibula toward the ventral
side of the tibia is effected by a muscle which, in great measure,
occupies the place of the interosseous ligament, and is
analogous to the pronator quadratus
in the fore-limb. This
muscle is antagonized by the extensors of the digits, so far as
they arise from the fibula.
The digits of the pes vary remarkably in their form and
relative development among the Marsupialia
; the different
subdivisions of the order being very well distinguished by the
modifications of the hind-foot.
Thus in the especially carnivorous Marsupials-the Didelphidae
of America, and the Dasyuridae
, of the Australian
province-the second and third digits of the pes are not
united together by the integument. In the Didelphidae
hallux is nailless, but large and opposable, so as to convert
the pes into a prehensile organ like that of many Primates
in the Dasyuridae
, on the other hand, the hallux is rudimentary
or absent. In all the other marsupials, the second and
third digits of the pes are syndactyle, or united together by integument.
In the Wombat, the fourth toe is bound together
with the other two, and the small hallux is devoid of a nail.
In the Phalangers, only the second and third toes are syndactyle,
and they are slender, compared with the other digits,
while the hallux is well developed and opposable. In the Peramelidae
(Bandicoots) and Macropodidae
metatarsus is much elongated, and the second and third digits
united and slender, while the fourth toe is very large. The
hallux is reduced to its metatarsal bone in the Peramelidae,
and the fifth digit is small or rudimentary. In the Kangaroos,
the hallux disappears altogether, but the fifth digit remains
well developed, though not so large as the fourth.
There is a great range of variation in the characters of the
brain. The carnivorous Marsupials (Didelphys, Dasyurus,
) exhibit the lowest type of cerebral structure, the
olfactory lobes being very large and completely exposed,
while the cerebral hemispheres are comparatively small and
quite smooth. In the Kangaroos, on the other hand, the
cerebral hemispheres present numerous convolutions and are
much larger in proportion to the olfactory lobes, which they
The stomach may be simple, as in most Marsupialia,
provided with a cardiac gland (Phascolarctos, Phascolomys
In the Kangaroos, it becomes immensely elongated, with
longitudinal muscular bands and transverse sacculations, so that
it resembles the human colon. The caecum, which is large in
the Kangaroos, but absent in the Dasyuridae
, is provided, in
the Wombat, with a vermiform appendix like that of Man.
The liver always possesses a gall-bladder. There are two venae cavae superiores,
and they receive the venae azygos
their respective sides. The tricuspid valve in the heart is
membranous. There is no inferior mesenteric artery, and the
external and internal iliacs arise separately from the aorta.
There are no vesiculae seminales, and the glans penis is
bifurcated in many species. The marsupial pouch is absent in
some Opossums and Dasyuridae.
When it is present, its
mouth is usually directed forward, but in Thylacinus
it looks backward. In Thylacinus
"marsupial bones" remain cartilaginous. The condition of
the foetus is known only in the Kangaroos, and further observations
on the embryology of the Didelphia
are much needed.
The foetus is said to possess a large umbilical sac, the vessels
of which extend on to the plaited chorion; and a small allantois;
and to be devoid of a thymus gland.
are at present confined to the Australian
and the Austro-Columbian provinces, some few species stretching
beyond the borders of the latter into the northern parts of
North America. The Didelphidae
alone are found in Austro-
Columbia, all the other groups being Australian.
Gigantic, Kangaroo-like, or Phalangistic, forms (Nototherium,
), have been found in post-tertiary
deposits and caves in Australia. In Europe, Didelphidae
occur in Eocene strata; Didelphidae, Dasyuridae,
(Phascolotherium, Amphitherium, Plagiaulax
middle Mesozoic rocks; and Macropodidae