LIFE HABITS AND DENTITION 



65 



Mammals received from Gill the name Eutheria. This term 

 unfortunately was used in a more restricted sense by Huxley 

 to signify merely the Placentals and their immediate ancestors. 

 The Marsupials and their ancestors (from which also the hy- 

 pothetical placental ancestors arose) Huxley considered a sub- 

 class in themselves and termed the group the Metatheria. This 

 subclass is now considered an infraclass of the subclass Theria 

 (Parker and Haswell) the other infraclass being the Eutheria 

 (Huxley). The relations of these various groups are pre- 

 sented in diagrammatic form below, the terms being used in the 

 significance which Huxley gave them to signify successive 

 stages of evolutionary development (after Bensley). 



Monotremata 



Marsupialia 



Placeiitalia 



Eutheria 

 Huxley) 



Metatheria 

 " (Huxley) 



Prototheria 



A placental Mammal differs from a Marsupial in its more 

 advanced type of placentation and in remaining in the uterus 

 until it has attained a higher degree of development. From 

 the chorion of the fetus in the placental Mammal villi pene- 

 trate into the maternal mucosa to form a "placenta." No pla- 

 centa is formed in the case of the Marsupial* and the young are 

 born in a very immature state. After birth they are placed 

 by the mother in the marsupium or pouchf where they attach 



*The Marsupials differ quite considerably in the form of their placentation, the 

 Bandicoot (Perameles) and the Dasyure (Dasyurus) presenting a type most nearly 

 like that of true Placentals. 



fThe pouch is present in most but not in all Marsupials. 



