490 THE AQUARIAN NATURALIST. 



have been propounded : the borings have been ascribed 

 by an ingenious author to the action of currents of 

 water directed against the parts to be worn away by 

 the ceaseless play of ciliary action; these currents 

 acting not so much by their force as by their constant 

 and long-continued operation, just as the drops from 

 the cave will in time wear a basin in the stone-floor 

 underneath; while, according to Mr. Albany Han- 

 cock, the foot and mantle of these boring mollusks 

 are crowded with siliceous particles, so as to repre- 

 sent rubbing- discs of extraordinary power, which act 

 upon the rock like glass- or sand-paper, wearing it 

 away by prolonged and continual attrition. 



The opinions relative to the manner in which the 

 Pholades excavate, however, may all be classed under 

 five conclusions: 1st, that the boring Mollusca 

 perforate by means of the rotation of the valves of 

 their shells, which serve as augers ; 2nd, that the holes 

 are made by rasping, effected by siliceous particles 

 studding the surface of certain parts of the animals ; 

 3rd, that currents of water, set in action by the 

 motions of vibratile cilia, are the agents ; 4th, that 

 the animal secretes a chemical solvent, an acid 

 which dissolves the substance into which it bores ; 

 5th, that the combined action of a secreted solvent, 

 and rasping by the valves, effects the perforations. 

 " Of all these theories," says Professor Forbes, " the 

 chemical one, so far as a secreted solvent is con- 

 cerned, bears least examination. The substances 

 perforated are wood, limestones, hard and soft, argil- 

 laceous shales, clays, sandstone ; and, in the case of a 

 Pholas in the magnificent collection of Mr. Cuming, 



