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which the third dentition would have taken up if it had replaced the 
second. The distance between the bases of the teeth and the line of 
attachment of the epithelium of the mouth would also be increased, 
and with this the distance between the functional teeth and their suc- 
cessors. There would therefore be required a greater amount of bone 
absorption before the old tooth could give way to the new. The dis- 
advantage of such an expensive process has probably been a factor 
in the restriction of tooth change; this occurs at present mainly in 
those regions, and at a period of development, in which the epithelium 
of the mouth together with its attached dental lamina, is at no great 
distance from the bases of the teeth. From this it is clear that the 
longer tooth displacement and succession are delayed the less likely 
are these processes to occur, since the distance between the functional 
teeth and the dental laminae (potential or actual) is constantly in- 
creasing from youth to maturity. We find as a matter of fact that 
tooth change has ceased in Hatteria long before the full size is reached. 
Conclusion. 
It will, I think, be admitted, that the above theory affords a satis- . 
factory explanation of the anomalies of the dentition of Hatteria, with- 
out unduly exercising the imagination. It is not improbable that 
Hatteria is on the verge of extinction, and it is therefore the more 
important that all possible light should be thrown on its development 
and structure. At the present time several investigators are occupied 
in a study of its embryology, and I have felt justified in elaborating 
my conclusions, in order that it may be shown whether the develop- 
ment of other organs affords confirmatory results. If, as I anticipate, 
this is found to be the case, we shall have an almost unique example 
in which to trace the effects of altered conditions of development. 
We may also obtain additional light on the affinities of Hatteria, and 
in this connection the state of the organs during the winter hibernation 
in the egg should receive special attention. Drnpy has already pointed 
out the Chelonian aspect of the embryo at this stage. 
I would suggest that the lengthening of the incubation period 
occurred at no very remote date. That this is the correct view is 
rendered probable by the completeness of the embryonic dentition, 
and by the clearness with which the ontogenetic processes leading up 
to the unique condition of the teeth of the adult, enable us to trace 
back the origin of these to a simple polyphyodont reptilian dentition. 
It seems to me very probable that since the incubation period was 
lengthened, Hatteria has developed along its own lines, giving rise to 
