BI VALl -£ MOLLUSC A. 



325 



is as exactly and neatl)' cut as if it bad been perforated by tlie 

 sharpest tool, and that a corroding dissolvent could not act with this 

 regularity, but would attack the harder and more tender parts un- 

 equally. This objection, which M. Quatrefages opposes to the idea 

 of a chemical solvent, appears to us to admit of no reply. But, 



Fig. 129. — Pholas dactylus having hollowed out a shelter in a block of gneiss. 



while opposing unassailable reasons against the two theories, M. 

 Quatrefages does not leave us without a reasonable explanation of a 

 very puzzling phenomenon. " Let us not forget," he says, " that 

 the interior of the gallery is constantly saturated with water ; conse- 

 quently all the points of the walls which are not protected by the 

 tube are subjected to constant maceration. In this state a mechanical 

 action, even though incons'derable, would suffice to clear away the bed 

 of fibre thus softened, and, if this action is in any degree continuous. 



