62 TYPES OF ANIMAL LIFE 



and reptiles on the one side, and ordinary beasts upon 

 the other. It is impossible here to describe these pecu- 

 liarities ; it must suffice to affirm their existence, and the 

 reader who cares to pursue the subject further will find a 

 description of them in every modern manual of compara- 

 tive anatomy. These characters justify the separation of 

 the platypus and echidna from all other beasts, and we 

 must recognise that thev form a sub-class by themselves, 

 which, on account of the resemblance which in many 

 respects it presents to birds, is called Ornithodelpliia^ and 

 the platypus and echidna are termed " ornithodelphous 

 mammals." 



But the marsupials, apart from these two forms, are 

 now also universally recognised as by themselves con- 

 stituting another sub-class, which, on account of its 

 uterine structure, is termed Didelphia, and all marsupials 

 are, therefore, spoken of as " didelphous mammals." 



All the rest of the class of beasts — i.e. of the class mam- 

 malia — constitute the third sub-class, which in the number 

 of its species of course enormously exceeds that which 

 contains the marsupials. This third sub-class, to which 

 all those orders belong which were known before 

 Australia was discovered, is distinguished as the sub- 

 class Monodelphia, and all the creatures in it (the bat, the 

 mole, the ape, the squirrel, the dog, the deer, the sloth, 

 the ant-eater, the hedgehog, cVrc), are known as " mono- 

 delphous mammals." 



Our readers may now be able to appreciate how great 

 was the hidden interest of that American beast known as 

 the opossum. Little did those who first observed it suspect 

 that it was an example of a group of animals so profoundly 

 difierent from all mammals previously known. It was, 

 in fact, impossible to appreciate its importance correctly 

 till the beasts of Australia had been discovered and cjuld 



