DEVELOPMENT AND SUCCESSION OF TEETH IN PERAMELBS. 527 
ing quite incompetent evidence of serial homology between 
individual teeth, on the ground that in some mammals (notably 
in Erinaceus) certain of the milk-teeth function along with 
successional teeth. 
Again, (c) the different depth at which one enamel- 
germ springs from the dental lamina compared with 
another can, in Leche’s view, afford no reliable criterion of 
serial homology. Thus in Erinaceus the dental lamina in 
the situation of several of the antemolar teeth is markedly 
shallowed, and is wholly transformed into an enamel-organ 
very superficially placed. But it is found that the dental 
lamina, at the places referred to, has to produce only one 
tooth, and the anomalous position of the enamel-germ is held 
to be explained merely by the absence of tooth-change in that 
particular segment of the jaw. 
While agreeing in general with the judgment expressed 
above, we are bound to add that the reasoning appended to 
it is not conclusive to us, for the absence of a tooth-change 
will not account for discrepancies of situation in many cases, 
e.g. the striking depth of the upper canine Anlage frequently 
referred to in this paper may be contrasted with the relative 
shallowness of the enamel-germs of the two anterior premolars 
which are without doubt serially homologous with the canine. 
On the other hand, we have found that enamel-organs which 
at first are close under the oral epithelium, and which have 
(as in Leche’s description) no dental lamina or only a very 
short one, superficial to them, do afterwards exhibit quite a 
well-marked dental lamina, and from this by-and-by a residual 
lamina is differentiated. This is what happens in the case of 
the anterior premolars of Perameles, though they never attain 
the great depth of the canine Anlage. 
We are of opinion that the relative depth of the various 
enamel-organs depends upon several factors, partly, perhaps, 
intrinsic to the lamina itself, and dependent upon the degree 
of its formative activity as an epithelial mass; and partly 
extrinsic, and dependent upon its topographical relations to 
various other structures. 
