MAMMALIA. 287 



when born, they are able to obtain their natural food — the 

 milk — by their own exertions. In the Non-placental Mam- 

 mals, on the other hand, the young are born at an extremely 

 early period of their development, before there is any necessity 

 that a placenta should be formed for the nourishment of the 

 foetus. In these cases, therefore, the young when born are 

 much more immature and helpless than in the case of the 

 Placental Mammals. So helpless are they, that they are even 

 unable to suck, and have in most cases to be fixed by the 

 mother herself upon the teats, while the milk is forced into 

 their mouths by a muscle which is spread over the mammary 

 gland. Adopting these primary sections as practically suf- 

 ficient in an elementary work, the whole class of the 

 Mammalia may be divided into the following fourteen 

 orders : 



Division A. — Aplaoental Mammals. 

 Order 1. — Monoiremata. 



