ON MOLLUSCA. 243 
augmentation of land are to be seen in various parts of our 
coast. ‘These are the depositions which, in the course of time, 
by the long continued action of the pressure of superior strata, 
as well as by the tendency of inorganic matter, thus pressed 
and broken, to crystallize, grow more and more solid, and are 
converted into calcareous rocks, which end by no longer pre- 
senting any traces of their ancient organic disposition. 
Previously to turning from the consideration of this divi- 
sion of the animal kingdom, as a whole, to a short review of 
its several sections, it may be useful to advert to the general 
principles by which these animals have been classified. 
These are of the same kind which are applied to the other 
types of the animal kingdom. The facility of collecting and 
of preserving the shells or envelopes of these animals, the 
beauty of form and colour which frequently distinguishes 
them, and especially the consideration that they exist alone 
in the composition of certain strata of the earth, has some- 
‘times given rise to a contrary opinion: but this was truly 
erroneous. Thus the principle, par excellence, that the 
entire organization should be our guide, and that it is the 
external organs which should represent it, and furnish dis- 
tinctive characters, is equally admissible in this as in every 
other part of zoology. But as the ensemble of the organiza- 
tion of the moilusca is sometimes pretty rigorously translated, 
so to speak, by the shell, and as the latter is evidently one of 
the most prominent of the external parts, and of which there 
is the most need in the accessory application to geology, a 
deception has resulted in the application of the principle, and 
a belief that it was possible to arrive at a methodical classifi- 
cation of the mollusca, by a simple consideration of the shell; 
this unquestionably appears to be a mistake. 
The consideration of the local habitat should be allowed 
no importance in the classification of the mollusca, still less 
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