MAMMALIA. 



497 



features which might indicate an independent evolution of 

 the allantoic placenta within the group. 



The epipubic bones have the same relations as those of 

 the Frototheria, and the exact significance of the tooth- 

 succession is not yet decided. The known facts are as 

 follows : — 



The majority of the Metatheria retain the one set of teeth 

 throughout life, with the single exception of the third upper 

 and lower tooth on each side behind the canine, hence 



Fig. 342. — Diagram of Embryo of Perameles with 

 FcETAL Membranes. 



(After Hill.) 



Yolk-sac Villi. 

 Edge of Allantoic Placenta. / \ 



Allantois with Villi 



Amnion. 



Yolk-sac. 



Prokalymma. 

 Note the allantoic villi. 



termed the third premolar. This tooth usually resembles 

 the teeth behind it rather than those in front, and at some 

 time (earlier or later according to the species) it falls out 

 and is replaced by a permanent tooth. 



The next fact to note is the later discovery of a series of 

 tooth-germs in the front of the jaw, which never cut the 

 gum but are absorbed after reaching a certain stage. The 

 deciduous premolar is said to rise in connection with these, 

 and the most reasonable view seems to be to regard these 

 M Zl 



