2 CLASSIFICATION OF THE MAMMALIA. [CHAP. 



The classification and the names of the subdivisions used 

 throughout this work correspond, in the main, with those 

 given by Professor Huxley in his " Introduction to the 

 Classification of Animals," 1869. 



The modifications of this classification which appear to be 

 necessitated by more recent acquisitions of knowledge will 

 be found fully detailed in the author's article, Mammalia, 

 in the ninth edition of the Encyclopedia Britannica, vol. xv. 

 (1883), from which the following introductory observations 

 are extracted : 



One of the most certain and fundamental points in the 

 classification of the Mammalia is that all the animals now 

 composing the class can be grouped primarily in three 

 natural divisions, which, presenting very marked differen- 

 tiating characters and having no existing or yet certainly 

 demonstrated extinct intermediate or transitional forms, may 

 be considered as sub-classes of equal value, taxonomically 

 speaking, though very different in the numbers and import- 

 ance of the animals at present composing them. 



These three groups are often called by the names 

 originally proposed for them by De Blainville : (i) Or- 

 nithodelphia, (2) Didelphia, (3) Monodelphia^ the first being 

 equivalent to the order Monotremata, the second to the 

 Marsupialia, and the third including all the remaining 

 members of the class. Although actual palseontological 

 proof is wanting, there is much reason to believe that each 

 of these, as now existing, are survivors of distinct branches, 

 to which the earliest forms of Mammals have successively 

 given rise, and for which hypothetical branches or stages 

 Huxley has proposed the names of Prototheria, Metatheria, 

 and Eutheria?- 



I. The characters of the Ornithodelphia or Prototheria 

 1 Proceedings of the Zoological Society, 1880, p. 649. 



