CLASSIFICATION OF THE MAMMALIA. G? 



sub-divided by different naturalists. A natural division of 

 this class into two Sub-classes may be made if we take as a 

 basis of classification the manner and condition in which 

 the young are brought forth, and the structure which de- 

 termines the manner and condition in which the young 

 are brought forth. The Sub-classes 011 this basis, are : 



I. VIVIPAROUS, or PLACENTAL MAMMALIA, or those whose 



young have the form of their parents at the time of birth. 

 These are the true or Typical Mammalia ; that is, they best 

 represent the Class. Man, Monkeys, Lions, Horses, Cows, 

 Sheep, Goats, etc., are examples of this Sub-class. 



II. SEMI-VIVIPAROUS, or NON-PL ACENTAL MAMMALIA, or 



OOTICOIDS, or those whose young are born in a very im- 

 mature, that is, in an embryonic condition, as the Kangaroo, 

 Opossum, etc. 



De Blainville, Huxley,, and others, recognize three sub- 

 classes of the Mammalia instead of two. The third sub- 

 class is obtained by dividing the Semi-viviparous Mammals 

 into two groups. These three Sub-classes are based on the 

 nature of the reproductive system : . 



I. MONODELPHIA, the same as the Placental Mammalia, or those 

 whose young at birth have the form of their parents, as Man, 

 Monkeys, Lions, Horses, Cows, etc. 



II. DIDELPHIA, the same, nearly, as the Non-placental Mammalia, 



or those whose young are born when of very small size, and 

 in a very immature condition, as the Kangaroos, Opossums, 

 etc. 



III. ORNITHODELPHIA, or Bird-like Mammalia, or those whose 



structure in some respects is like that of Birds, as the Duck- 

 bill and Echidna. This group was formerly included in that 

 of the Semi -viviparous or Non-placental Mammals. 



Recognizing three Sub-classes as given above, these 

 groups and the Orders into which they may be divided, 

 may now be presented in one view, thus : 



