GEXERAL CHARACTERS. 



2 37 



JAWS AND TEETH OF THE ROFODS 

 RAT-KANGAROO. 



The letters i indicate the front or incisor teeth ; 

 c, the upper tusk, or canine ; pm, the premolars ; 

 and m, the molars. 



incisor teeth in the upper jaw : and in such eases 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. Xow, as we have 



alreadjr 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. 



On the other hand, there are never more than 



three premolar teeth, which in the adult of 



some forms, as in the figure, may be reduced 



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. g rs t 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, Xew Guinea, and the adjacent islands as far west as Celebes and 

 Lombok. Exclusive of the Egg-laying Mammals, the Marsupials fonn almost the 

 whole Mammalian fauna of Australia, where the chief other types are certain 

 Rodents and Bats. In the more western islands thev are, however, mingled with 



