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. This is 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- 



