1408 



it 



upper jaw always exceeds those in the lower by one. More important is the cir- 

 cumstance that but a single tooth on each side of both the upper and lower jaws ever 

 t 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 al- 

 ready 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 



halld > there are never m re than thf ee pre- 



molar teeth, which in the adult of some forms, 



It 



c, the upper tusk, or canine; pm. the premolars; i j j j it. j 



and m, the molars. ma y be added that ' a ccordmg to recent Tc- 



searches, all the teeth in advance of the last 



premolar appear to represent the milk series of the higher Mammals, which are here 

 permanently retained. 



The fact that the newborn young of the Pouched Mammals, when 



JAWS AND TEETH OF THE RUFOUS 



RAT KANGAROO. 



The letter Vindicates the front or incisor teeth; as in the figure, may be reduced to One. 



Mode of Suck- 

 ling 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 suck- 

 ing by themselves. For this purpose the mammary gland of the female is overlain 

 by certain specially-developed muscles, the perodical 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 Ceta- 

 ceans. That is to say, the larynx, or upper terminal expansion of the windpipe, is 

 prolonged upward 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 arrangement, 

 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 recognized 

 by their great elongation, owing to the weight of the young suspended from them. 

 With the exception of the opossums, which are confined to America, 



and are most numerously represented in the southern half of the conti- 

 tion nent, the living representatives of the order are restricted to Australia, 



New Guinea, and the adjacent islands as far west as Celebes and lyom- 

 bok. 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 Placental Mam- 

 mals, thus showing that these islands have had some connection with those of the 



