CHAP. II] THE MAMMALIA 29 



3. Epi-pubic bones are present and well developed. 



4. A single sphincter muscle surrounds both anus and uro- 

 genital aperture. 



5. The corpus callosura is absent. 



6. The placenta, when present, is functional for only a short 

 period. 



The Metatheria (or Marsupialia) are divisible into two Orders, distin- 

 guished principally by the characters of their dentition, particularly of the 

 incisor teeth. There is thus to be distinguished the Order Polyprotodontia, 

 whose members have numerous incisor teeth (more than three on each side). 

 The Opossums belong to this Order. The second Order (Diprotodontia) 

 possess not more and usually fewer than three incisor teeth on each side. 

 The Kangaroos are examples of this Order. The Metatheria bridge the gap 

 between the Prototheria and the Eutheria. The latter may be held to share 

 an ancestor with the Metatheria, and this ancestor in turn was derived 

 probably from a Prototherian form. 



SECTION B. EUTHERIA. 



1. Theria having no marsupium. The young are nourished 

 in utero for a considerable period, through the agency of a 

 placenta. 



Malar bone 

 Lachrymal bone 



Glenoid 

 fossa 



... '; ■ . Tympanic bone 

 Ansphenoid ' r 



Fig. 14. Cranium of an Eutherian Mammal (Dog) : for comparison with 

 Fig. 13 ; note the smaller extent (backwards) of the alisphenoicl (shaded), the 

 malar and lachrymal bones. 



2. In the skull (Fig. 14), the alisphenoid does not contribute 

 to the wall of the tympanic cavity ; nor does the jugal (malar) 

 (except in Hyracoidea and some Rodentia) contribute to the floor 

 of the glenoid cavity. 



3. No epi-pubic bones are present. 



