AN ARRANGEMENT OF THE MAMMALIA. 71 



beyond, carries the arrangement down only to include the fami- 

 lies, yet fortunately these families are not built up upon an 

 enormous collection of subspecific forms having existence only 

 in the minds of those who must create them in order to live 

 and be recognized. 



As in the case of Aves, the Manmialia are completely iso- 

 lated from all other groups ot existing animals, and moreover 

 this is found to be true when we come to take the extinct forms 

 into consideration is so far as they are at present known to us. 

 This very much simplifies the matter of their classification. 



In considering the primary divisions of existing Mammalia, 

 it is to be observed that they fall naturally into three clearly 

 defined groups, or subclasses, which by De Blainville were 

 designated as the Ornithodelphia, the Didelphia, and the Mono- 

 delphia. 



Many zoologists still employ these terms, but the less objec- 

 tionable ones proposed by Huxley in J 880 are coming into more 

 general use. He substituted for these terms the Prototheria, 

 the Metatheria, and the Eutheria respectively, and I employ 

 them in my own classification. 



There are but two families of existing mammals contained 

 in the Prototheria, namely the Ornithorhynchidae (the Austra- 

 lian Duckbills) and the Echidnidae (the Spiny Anteaters). They 

 are the lowest forms of existing mammals known, having a 

 very primitive anatomical structure. 



All the existing marsupial animals of the world have been 

 grouped in the Metatheria. Australia is the great region for 

 these, while in the United States there is but one representative, 

 the Opossum. Structurally, the Metatheria stand between the 

 Prototheria and the Eutheria. Their young are born in a 

 very much undeveloped condition, and are transferred by the 

 mother to the marsupium, where they are attached to her 

 nipples, and subsequently grow, being nourished by the milk 



