ON MOLLUSCA. 241 
The relations of the mollusca with the mineral kingdom, 
and consequently with the mass of the earth which it has 
contributed to form, are not without interest ; for without 
attempting here to resolve the physiological question whether 
the conchyliferous mollusca borrow from the inorganic king- 
dom the calcareous matter which composes their shell, or if 
it is formed of all materials, it is however certain that they 
produce no inconsiderable changes in the surface of the earth, 
by accumulating this matter in some places more than in 
others, and consequently, that they alter its physiognomy or 
superficial structure, the study of which constitutes geology, 
or as it is sometimes more properly termed geognosy. 
The modes in which this accumulation is made are alto- 
gether different, according as the mollusca from which the 
shells proceed were fixed or not, lived embedded in mud or 
sand, or were free on the superficies of the rocks or of the 
soil. Thus the oysters in our climates, the pintadini or 
regular avicula in warmer regions, as well as the spon- 
dyles and many other Vivalves, form by their accumulation 
banks more or less extended, strata more or less thick and 
horizontal, in which the shells are still at the present day in 
the position in which they originally lived, and almost with- 
out any admixture of foreign bodies. Although this is less 
evident respecting the cardia, the telline, the lutraria, the 
my, &c., and all the genera of bivalves which live vertically 
sunk in sand or mud, we see nevertheless that these shells 
must also form sorts of strata, because the individuals newly 
born are deposited by their parents above themselves, so that 
the latter, sinking in the sand in proportion as they enlarge 
in bulk, depress their parents, and the individuals below them 
im succession, so as to remove them sufficiently from the 
surface of the soil to prevent their tubes receiving any water, 
the consequence of which is death. Then their shells, vertical 
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