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in certain Cetacea. It will be seen that in Hatteria also the fusion 
affects two dentitions. This is not the place to discuss the Con- 
crescence Theory in detail, but it seems to me that it is of little value 
to the upholders of this theory to show that concrescence may at 
present occasionally occur, unless evidence is brought forward as to 
its efficiency as an evolutionary process. In Hatteria we have a par- 
ticularly clear case in which both cause and effect are to be plainly 
seen. The cause is overcrowding, the effect is an early cessation in 
growth of the teeth concerned. Concrescence here, at least, 
does not pave the way to the formation of tricuspid or 
multicuspid teeth. If an isolated case of this kind can be re- 
garded as an argument, either for or against the theory, the evidence 
derived from the concrescence in Hatteria is rather adverse than 
favourable. 
The Incubation Period and the Teeth. 
From the foregoing it will be realised that the whole course of 
tooth development and succession in Hatteria is of a highly modified 
and specialized character. The chief peculiarities which call for ex- 
planation are as follows: 
1. There is a complete, though somewhat degenerate, dentition de- 
veloped many months before hatching, and never becoming func- 
tional. 
. The functional teeth of the newly-hatched animal are almost cer- 
tainly members of two dentitions. 
3. The dental lamina has peculiar relationships to the epidermis and 
its ingrowths at different stages. 
4, The actual tooth replacement is much reduced, and is confined 
to the earlier periods of growth of the animal. 
5. The mandibular alternating teeth fuse with each other before the 
end of the incubation period. 
6. The posterior teeth in the maxilla, palatine and mandible of the 
adult are practically uniform in size, and have neither prede- 
cessors nor successors. Their number differs greatly in the three 
regions. 
I have already indicated how these peculiarities find their ex- 
pression in the characteristic dentition of the adult animal. I shall 
- now attempt to show how the ontogeny is related to the phylogeny, 
how from a more normal reptilian dentition that of Hatteria may 
have been gradually evolved. The key to the whole of the anomalies 
lies, I believe, in a great lengthening of the incubation 
bo 
