GENERAL LIFE OF MAMMALS. 735 
There is some uncertainty as to the primitive form of the placenta, 
but the fact that it is discoidal in Pevameles seems to confirm Balfour’s 
view that this form must be placed lowest. 
The formation of the allantoic placenta in Perameles is in essentials 
the same as in Eutherian Mammals, but in details there are some 
striking differences. The most noteworthy of these is, perhaps, that the 
cells of the uterine epithelium, instead of disappearing at an early stage, 
as in Eutherian Mammals, proliferate greatly, lose their cell outlines, 
and by the increase of the nuclei form what is known as a syncytium. 
Later this syncytial layer becomes highly vascular, and forms the 
maternal portion of the placenta, whereas, as already seen, in Eutheria 
it is the uterine mucosa which forms the maternal part of the placenta. 
Into the vascular syncytium the allantoic capillaries grow down, until 
ultimately maternal and foetal vessels are separated merely by their 
endothelial walls and a mere trace of syncytial protoplasm. The 
connection between the yolk-sac wall and the uterus is effected in a 
similar manner. 
GENERAL LirE oF MamMMALs 
Most Mammals live on dry land. The bats, however, 
have the power of flight, and various forms are able to 
take long swooping leaps from tree to tree. Thus there are 
“flying phalangers,” such as Fefaurus, among Marsupials ; 
“flying squirrels,” such as Pteromys, among Rodents; 
“flying lemurs” (Gadeopithecus), allied to Insectivores. Not 
a few are aquatic,—all the Cetaceans, the two Sirenians, and 
the Pinniped Carnivores, such as seals and walruses ; while 
water-voles, beavers, otters, polar bear, and many others 
are also at home in the water. Burrowers are well repre- 
sented’ by moles and rabbits; arboreal forms by squirrels 
and monkeys. ; 
As to diet, man, many monkeys, the pigs, and many others, 
may be called omnivorous ;: kangaroos, hoofed animals, and 
most rodents are herbivorous; the Echidna, the ant-eaters, 
hedgehogs and shrews, and most bats, are insectivorous ; 
most of the Carnivora are carnivorous; dolphins and seals 
feed chiefly on fishes; but in most cases the diet varies not 
a little with the available food-supply. 
The struggle for existence among Mammals is sometimes 
keen among fellows of the same kind; thus the brown rat 
(Mus decumanus) tends to drive away the black rat (AZ 
rattus); but stress, due to over-population, is sometimes 
mitigated by migration, as in the case of the lemmings. The 
struggle seems to be keener between foes of different kinds, 
