28 POPULAR BRITISH CONCHOLOGY. 



Professor Forbes thus ably sums up the various theories 

 which have been promulgated on this very interesting and 

 difficult point. " They may all be classed under five con- 

 clusions : 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 substance of certain parts 

 of the animals. 3rd. That currents of water, set in action 

 by the motions of vibratile cilia, are the agents. 4. That 

 the animal secretes a chemical solvent, an acid, which dis- 

 solves the substance into which it bores. 5. That the com- 

 bined action of a secreted solvent and rasping by the valves 

 effects the perforations.''^ 



The most plausible account in favour of the first of these 

 theories is that given by Mr. Ostler, who describes the young 

 animal first fixing itself by its foot, raising itself almost per- 

 pendicularly, and pressing the prickly part of the shell against 

 the stone in a half-rotatory motion, and thus adapting its 

 motions to its advancing age, the size of the opening, and 

 the progress of the work. Mr. Ostler goes on to say, " The 

 particles rubbed ofi", and which in a short time completely 

 clog the shell, are removed in a very simple manner." When 

 the projected siphon is distended with water, the F kolas 



