In the Monodelphia
, the os
very soon becomes anchylosed with the second
cervical vertebra, of which it appears merely as the odontoid
process; and the cervical ribs early become inseparably united
with their vertebrae. The coracoid is reduced to a mere process
of the scapula, and there is no epicoracoid similar to that
of the Ornithodelphia.
Clavicles may be present or absent. When completely
developed they articulate directly, or by the intermediation of
more or less modified remains of the sternal end of the coracoid,
with the sternum, and not with any interclavicle. The
acetabula are imperforate. The pelvis is devoid of marsupial
bones; though, in some Carnivora
, there are small cartilages
in the inner tendons of the external oblique muscle, which
have a corresponding form and relations.
The anterior commissure and the corpus callosum, no less
than the cerebral hemispheres themselves, vary greatly, the
brains of some Edentata
very closely approaching those of the Didelphia
in respect of the corpus callosum and anterior commissure;
while, as regards the hemispheres themselves, they
may either be so small as to allow the cerebellum to be completely
exposed on the dorsal aspect, or so large as completely
to cover it and project beyond it. The external surface
of the hemispheres, again, may be either perfectly smooth
or extremely convoluted.
The cochlea is coiled spirally. The reproductive and
urinary apertures, as a general rule, open quite separately from
the rectum. The ureters always open into the bladder. The
testes may remain in the abdomen throughout life, or may
pass into a scrotal pouch. But, when this scrotum forms a
distinct sac, it lies at the sides of, or beliind, the penis, and
not in front of it. The cystic urethra is always continuous
with that part of the urethra which traverses the penis.
The ova are small, and the mouths of the Fallopian tubes
are fimbriated. The vagina is a single tulie, which may, however,
be partially divided by a longitudinal partition. The
cremaster has no relation to the mammary glands, which are
provided with distinct teats.
The allantois is always well developed, and gives rise to
a placenta; and the young are born of large size, and active.
The great majority of the Monodelphia
, as thus defined,
are divisible according to the characters of their placenta into non-deciduata
In the non-deciduata the foetal villi of the placenta are, at
birth, simply withdrawn from the uterine fosste, into which
they are received, and no part of the maternal substance is
thrown off in the form of decidua, or maternal part of the placenta.
In the deciduata, on the other hand, the superficial
layer of the mucous membrane of the uterus undergoes a
special modification, and unites, to a greater or less extent,
with the villi developed from the chorion of the foetus; and, at
birth, this decidual and maternal part of the placenta is thrown
off along with the foetus, the mucous membrane of the uterus
of the parent being regenerated during, and after, each pregnancy.
There are, however, two orders of existing monodelphous Mammalia
, the nature of the placentation of which is not yet
fully made out. One of these is the Sirenia,
of which is unknown. The other is the ill-defined and heterogeneous
assemblage called Edentata.
Some of the members of this group
certainly possess deciduate placentae, while, in
others, it appears questionable whether the decidua is, or is
not, developed. And, as this group, the Edentata
, is decidedly
the lowest of the whole division, I shall take it first in order,
while the Sirenia
are arranged, provisionally, among the Non-deciduata.