DEVELOPMENT AND SUCOESSION OF TEETH IN PERAMELES. 445 
by the sides of the developing teeth are no rudimentary 
enamel-germs. They are merely portions of a quite indifferent 
“yesidual dental lamina” becoming liberated on differ- 
entiation of the permanent teeth from the parent lamina. The 
swelling of the distal portion of the downgrowth is a mere 
thickening of the free border of the residual lamina, and exhibits 
no differentiation which is really characteristic of the pro- 
duction of actual enamel-organs. 
It is possible that its presence is to be explained in terms of 
a continued formative activity on the part of an unexhausted 
dental lamina, an activity which might conceivably issue in 
the production of enamel-germs of a third series, such as 
Leche has shown exceptionally to occur amongst other 
mammals. We are, however, unaware of any single instance 
in which a differentiation genuinely characteristic of the for- 
mation of an enamel-organ of a third dentition has been 
observed among the Marsupialia. 
We may here touch upon one other strong point in the 
prima facie case we have been attempting to set forth in 
favour of the serial homology of the deciduous premolar with 
the so-called “ prelacteal” teeth. This is brought to light in 
the investigation of those marsupial forms in which the de- 
ciduous premolar is inconstant in its presence or altogether 
absent from the normal dentition. Thus in Dasyurus viver- 
rinus, in spite of the absence in the adult of any representative 
of the last premolar, either deciduous or successional, we find 
the deciduous premolar to be constantly present in the young 
mammary foetus as a small and precociously calcified vestigial 
tooth (figs. 80 and 81). It is indeed considerably larger than 
the vestigial ‘‘ prelacteal”? teeth met with elsewhere, as in 
Perameles, or evenin Dasyurus itself (see di 3, fig. 23). 
But in all other essentials it agrees with the “ prelacteals,” 
e.g. in time of differentiation, relative situation, and in the 
fact of its absorption during foetal life. 
An essentially similar condition we find in Phascologale 
cristicauda, in which also no trace of any milk premolar 
appears in the adult, This form, however, differs from 
