Placental animals are more variously differentiated and perfected, and this, moreover, in a far higher degree, than Marsupials, and they have, on this account, long since been arranged into a number of orders, differing principally in the formation of the jaws and feet. But what is even of more importance than these, is the different development of the placenta, and the manner of its connection with the maternal uterus. For in the three lower orders of Placental animals, in Hoofed animals, Whales, and Toothless animals, the peculiar spongy membrane, which is called the _deciduous membrane_, or _decidua_, and which connects the maternal and the fœtal portions of the placenta, does _not_ become developed. This takes place exclusively in the seven higher orders of Placental animals, and we may, therefore, according to Huxley, class them in the main group of _Deciduata_, or animals with _decidua_. They are contrasted with the three first-mentioned legions of indeciduous animals, or _Indeciduata_. ---------------------------+-------------------+-------------------+------------------- | _Cloacal Animals_ | _Pouched Animals_ |_Placental Animals_ _Three Sub-Classes_ | MONOTREMA | MARSUPIALIA | PLACENTALIA _of_ | _or_ | _or_ | _or_ _Mammals._ | ORNITHODELPHIA | DIDELPHIA | MONODELPHIA ---------------------------+-------------------+-------------------+------------------- 1. Cloaca formation | Constant | Embryonal | Embryonal | | | 2. Nipples of the pectoral | Wanting | Existing | Existing glands, or milk | | | warts | | | | | | 3. Fore collar bones, | United | Not united | Not united or clavicles, grown | | | together in the middle, | | | with the breast bone, | | | and forming a forked | | | bone | | | | | | 4. Marsupial bones | Existing | Existing | Wanting | | | 5. _Corpus callosum_ of | Feebly | Feebly | Strongly developed the brain | developed | developed | | | | 6. Placenta | Wanting | Wanting | Existing