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 Prototheria, and the exact significance of the tooth- 
succession is not yet decided. The known facts are as 
follows :— 
The majority of the AZe¢atheria 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 
Fa@tTaL MEMBRANES, 
(After Hitt.) 
Yolk-sac Villi. 
Edge of Allantoic Placenta. > 
re \ wh \ 
a, 
Allantois with Villi. yo 
Edge of 
True Allantoic~.. 
Placenta 
“Sr 
Yolk-sac. 
© we! Gee 
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 33 
