THE RISE OF THE MAMMALIA 131 



Whereas in the Eutherian Mammals all the permanent 

 incisors, canines, and premolars are preceded by repre- 

 sentatives in the milk dentition, in the Marsupials 

 only two pairs of teeth, viz. the last premolars in 



coed. 



ofid. 



At.- 



bt. 



Fig. 25. The foetus and embryonic membranes of Perameles. am. 

 amnion; coel. extra-embryonic coelom; all. allantoic placenta; 

 all.s. allantoic stalk; y.c. yolk-sac; vase. omph. vascularized 

 coelomic surface applied to yolk-sac ; st. the sinus terminalis in 

 which blood-vessels ramify; HI. omph. the trophoblast. (After 

 Hill, Q. J. M. S. vol. XL.) 



each jaw, have milk predecessors. What actually 

 happens is this during the suckling period the in- 

 cisors, canines, premolars and molars are gradually 

 cut to the surface ; then before the pouch is per- 

 manently left the last premolars are replaced by 



92 



