246 THE HISTORY OF CREATION, 
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 foetal portions of the placenta, does not become de- 
veloped. This takes place exclusively in the seven higher 
orders of Placental animals, and we may, therefore, according 
Cloacal “iets aie Animals| Placental Animals 
Three poral ‘| MonoTREMA | MARSUPIALIA PLACENTALIA 
Mammals. OuarroneE: tieeeael Miueneceaall 
PHIA 
1. Cloaca formation Constant Embryonal Embryonal 
2. Nipples of the pec- Wanting Existing Existing 
toral glands, or milk 
warts 
3. Fore collar bones, United Not united Not united 
or clayicles, grown to- 
gether 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 
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 Jnde- 
ciduata. 
But in the various orders of Placental animals the placenta 
differs not only in important internal differences of struc- 
ture, which are connected with the absence or the presence 
