Whilst there exists little divergence of opinion as to the orders into which the Mammalia may be divided, different authorities adopt different views as to the great primary divisions of the class. Here it will be sufficient to mention two modes of subdividing the Mammalia, of which the first will be accepted as sufficient for practical purposes.
I. The Mammalia may be divided into two great primary divisions, according as the structure known as the "placenta" is present or absent. The "placenta" or "afterbirth " is a highly vascular organ which is developed upon the exterior of the envelopes of the foetus, and which is so closely connected with the inner wall of the uterus as to allow of an interchange of material between the blood of the embryo and that of the mother.* The "Placental" Mammals are thus enabled to carry their young for a much longer period than are the "Implacental" Mammals, and hence the young animal in the former is born in a much more perfectly developed condition than in the latter. The sub-class "Implacentalia," in which there is no placenta, comprises only the orders of the Monotremata and Marsupialia. The sub-class Placentalia, in which a placenta is present, comprises all the other orders of Mammalia.
* No traces of vascular prominences comparable to the Mammalian placenta occur in any animals below the rank of Mammals, except in some Sharks and some Ascidians. In the Sharks, however, the vascular eminences are developed from the umbilical vesicle, and not, as in Mammals, from the allantois.
II. The Mammalia may be divided as follows into three primary divisions according to the structure of the reproductive organs. This arrangement was proposed by De Blainville, and is accepted by Huxley, Rolleston, Flower, and other distinguished authorities:
a. Ornithodelphia, characterised by the fact that the uterine enlargements of the oviducts do not coalesce, even in their inferior portion, to form a common uterine cavity, but open separately as in the Birds and Reptiles. Furthermore, the two uteri open, not into a distinct vagina, but into a cloacal cavity, into which the rectum and ureters also discharge themselves; so that the condition of parts is very much the same as it is in Birds.
This division includes only the Duck-mole (Ornithorhynchus) and the Porcupine Ant-eaters (Echidna), forming collectively the single order of the Monotremata.
b. Didelphia, characterised by the fact that the uterine dilatations of the oviducts continue distinct throughout life, opening into two distinct vaginae, which in turn open into a urogenital canal, which is distinct from the rectum, though embraced by the same sphincter muscle.
This sub-class contains the Marsupialia, such as the Kangaroos, Opossums, Wombats, etc, most of which are almost entirely confined to Australia. They have many other characters in common which will be spoken of hereafter.
III. Monodelphia, characterised by the fact that the uterine enlargements of the oviducts coalesce to a greater or less extent to form a single uterine cavity, which, however, generally shows its true composition by being divided superiorly into two cornua. The uterus opens again into a single vagina, which is always distinct from the rectum. This sub-class corresponds with the division of the "Placental" Mammals, and includes all the Ma7nmalia except the Monotremes and Marsupials.