GENERAL CHARACTERS. 237 
incisor teeth in the upper jaw; and in such cases the number of pairs of these 
teeth in the upper jaw always exceeds those in the lower by one. More 
important is the circumstance that but a single tooth on each side of both 
the upper and lower jaw ever has a milk 
predecessor. This tooth, marked pm in the 
accompanying figure, corresponds to the 
fourth or last premolar of the dog (Vol. I. 
p- 10); and consequently all the four teeth 
behind it are molars. Now, as we have 
already seen, it is but very rarely that there 
are more than three of these molars in 
Placental Mammals, whereas in the present 
order there are nearly always at least four. JAWS AND TEETH OF THE RUFOUS 
On the other hand, there are never more than RAT-KANGAROO. 
three premolar teeth, which in the adult of The letters 7 indicate the front or incisor teeth ; 
some forms, as in the figure, may be reduced eae Fer eaye ene een ene Dre 
to one. It may be added that, according to 
recent researches, all the teeth in advance of the last premolar appear to represent 
the milk-series of the higher Mammals, which are here permanently retained. 
Mode of ' The fact that the new-born young of the Pouched Mammals, when 
Suckling Young: first transferred to the teats of the mother, are little more than mere 
animated lumps of flesh, renders it imperative that some special arrangement 
should be made for their nutrition, as they are quite incapable of sucking by 
themselves. For this purpose the mammary gland of the female is overlain by 
certain specially-developed muscles, the periodical contraction of which injects a 
supply of milk into the stomachs of the helpless young. In order to prevent the 
young marsupials from being choked during this injecting process, their throats 
are provided with an arrangement similar to that obtaining permanently in the 
Cetaceans. That is to say, the larynx, or upper terminal expansion of the wind- 
pipe, is prolonged upwards so as to extend into the hinder aperture of the nostrils 
at the back of the palate; and consequently there is a closed tube from the nostrils 
to the lungs, on either side of which the milk can flow without danger of choking 
the young animal. When there is no longer any necessity for this special arrange- 
ment, the larynx is shortened, and respiration and swallowing are carried on after 
the usual manner. It may be added, that in the Pouched Mammals the teats are 
confined to the region of the abdomen, and that the number of teats is frequently 
greater than that of the young. Such teats as have been in use may always be 
recognised by their great elongation, owing to the weight of the young suspended 
from them. 
Geographical With the exception of the opossums, which are confined to 
Distribution. America, and are most numerously represented in the southern half 
of that continent, the living representatives of the order are restricted to 
Australia, New Guinea, and the adjacent islands as far west as Celebes and 
Lombok. Exclusive of the Egg-laying Mammals, the Marsupials form almost the 
whole Mammalian fauna of Australia, where the chief other types are certain 
Rodents and Bats. In the more western islands they are, however, mingled with 

