DEVELOPMENT AND SUCCESSION OF TEETH IN PERAMELES. 535 
gous with a third Eutherian dentition (fourth dentition of 
Leche). 
The hypothetical product of the residual dental lamina may 
perhaps be represented as tooth-series to the mth power, 
rather than as any definite series. Kiikenthal (24) aptly 
remarks—and in this he is following the lead of Baume (26)— 
that the two mammalian dentitions are sisters whose mother 
is the undifferentiated dental lamina. This is true on the 
whole, but according to our theory it requires the further 
comment that the “Ersatzleiste” in Marsupials is analo- 
gous to the mother after the second parturition (the 
product of the first pregnancy having been early 
aborted), and ought not to be mistaken, as heretofore, for 
the younger daughter. The theoretical possibility of the 
occurrence of a third pregnancy need not be excluded. Leche 
has shown that occasionally in mammals such an event does 
actually occur; but so far as our experience goes the formal 
possibility or promise of a third dentition has been absolutely 
unfulfilled in Marsupials. In this connection we may ask the 
reader to compare Leche’s figures (e.g. his figs. 106, 113, 115, 
116, 118, 121, 124, and 137) of residual laminar downgrowths 
in various marsupials, which he labels as “ knospenformig,” 
or even actually as ““Schmelzkeime,” with his figures (e. g. 
figs. 28—380, 74, 84, 94, and 95) of residual downgrowths of 
the lamina by the lingual sides of the true permanent or resi- 
dual teeth of certain other mammals, and also with the two 
figures (our figs. 76 and 77) we have reproduced from his figs. 
140 and 142, showing the similar appendage to p3 in Phasco- 
larctus. It will be seen that no structural differences serve 
to distinguish the downgrowths in these different cases. 
In view of the foregoing discussion the term “ knospen- 
formig ” will be seen to be thoroughly out of place as applied 
to the slightly thickened laminar downgrowths figured in 
cross-section in Leche’s figs. 187 and 140. 
It has been shown in connection with our Stages 11 and 111 
that the first rudiments of the enamel-organs of the individual 
teeth arise as localised enlargements of the primary lamina 
