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 whole of the animals composing the class are arranged 

 primarily in three natural divisions, which, presenting very 

 marked differentiating characters, and having no existing 

 intermediate or transitional forms, may be considered as 

 sjid-dasses of equivalent value, taxonomically speaking, though 

 very different in the numbers and importance of the animals 

 composing them. 



These three groups, to adopt the names originally pro- 

 posed for them by De Blainville, are : — i. Monodelphia ; 

 2. DiDELPHiA ; and 3. Ornithodelphia. 



I. The Monodelphia, sometimes called F/acentalia, 

 comprise the great bulk of the class. The main characteristic 

 of the animals composing it is, that their young are nourished 

 for a considerable period within the uterus of the mother 

 by means of an organ called the placejita^ a villous and 

 vascular development from the outer surface of the foetal 

 envelopes, which, being in contact with corresponding 

 vascular developments from the inner wall of the uterus, 

 permits an interchange of materials between the circu- 

 lating fluids of mother and young, thus brought into the 

 closest proximity. This organ varies much in shape and 

 structure in the different minor divisions of the sub- 

 class. 



It is very difficult to subdivide the Monodelphia into any 

 groups larger than orders, or to arrange these orders in 

 anything like a linear series, as most of them have affinities 

 in many directions. 



One group may be placed apart as having no distinct 



