72 MOLLUSCA. 
relation between the different parts, but this influence is not 
sufficiently obvious. The character thus furnished is of an 
uncertain kind. It is influenced by the age of the indivi- 
dual, and therefore can only be employed with caution in 
specific distinctions. 
The last character of the bivalves which we shall notice 
is the power which some of them possess of piercing stones 
and wood for the purpose of forming to themselves a retreat. 
These are termed borers. It was supposed by many that 
the animal secreted a liquor with which it dissolved the bo- 
dies into which it penetrated, but the sagacious Reaumur 
soon ascertained that the boring was performed by means of 
a rotatory movement of the larger valves. M. Fleurieu- 
Bellevue states, that the calcareous stone in which the ru- 
pellaria lithophaga is found, is often discoloured in the im- 
mediate neighbourhood of its recess. This may arise from 
other secretions of the animal, or even from the stagnant 
sea water in the hole, and not from the action of the phos- 
phoric acid, or any other solvent supposed to be employed 
by the animal. These would act" equally on the shell as on 
the calcareous rock. But the borers are not confined to 
calcareous rocks, they also lodge in slate-clay, and other ar- 
gillaceous strata. Thisis very often the case with the Pho- 
lades. But this character can never be extensively employ- 
ed, as the same species which, at one time, may be found 
imbedded in stone, will be observed at another seated among 
the roots of sea-weed, or buried among gravel. 
From the preceding remarks it will appear obvious, that 
there are many characters furnished by the shell, which give 
us indications of corresponding peculiarities in the structure 
of the animal, and on that account ought to be employed in 
every natural arrangement. These characters have this cir+ 
