34 THE NOTOG^IC REALM. [CHAP. 



discharging their secretion by means of nipples. From the higher 

 mammals (Eutheria) they are distinguished by the imperfectly 

 developed condition of the newly-born young, and the absence 

 of any prenatal connection between the vascular system of the 

 foetus and the maternal parent by means of the organ known 

 as the placenta. Very generally the young are carried about for 

 some time after birth in a pouch situated on the abdomen of the 

 parent, where they at first remain immovably fixed to the nipples, 

 the milk being injected into their throats by the action of a special 

 muscle. In the carnivorous and insectivorous forms the number 

 of incisor teeth in the upper jaw usually exceeds the three pairs 

 which form the general maximum limit in the higher mammals. A 

 further peculiarity of the order is to be found in the replacement 

 of the teeth. Instead of the whole or nearly the whole of the first, 

 or milk-set of teeth in advance of the true molars or hinder cheek- 

 teeth being replaced by a second set of permanent teeth, only one 

 tooth is thus (and that by no means invariably) replaced. The 

 tooth thus replaced was long regarded as corresponding to the last 

 or fourth milk-molar of the higher mammals, while the apparently 

 replacing tooth was identified with the last or fourth premolar of 

 the same. From recent researches, however, it would seem that 

 in reality this is not a case of true replacement, and that the tooth 

 which makes its appearance late in life is a retarded premolar, 

 representing the fourth in that series, while the replacing tooth is 

 the fifth. 



Marsupials may be divided into two main sections or sub- 

 orders, readily distinguished from one another by their dentition, 

 both of which are represented in Notogaea. In the first of these, 

 or Diprotodont sub-order, which is the more specialised of the two, 

 the incisor teeth are separated by a gap from those of the cheek- 

 series, and do not usually exceed three in number on each side of 

 the upper jaw 1 , and in the lower jaw are generally reduced to a single 

 pair, while the tusks, or canines (<:), are either small or wanting. In 

 their habits the members of this section are more or less exclusively 

 herbivorous. On the other hand, in the Polyprotodont mar- 

 supials, all of which are mainly carnivorous or insectivorous in 



1 The only exception to this occurs in the South-American forms. 



