250 VARIOUS THEORIES ON THE SUBJECT. 



The third theory,. which endeavours to account for 

 the wearing away of the rock by means of silicious 

 particles situated in the foot and other parts of the 

 animal, has been for some time proved to be erro- 

 neous, from the fact, that the combined skill of some 

 of our best anatomists and microscopists has 

 failed to discover the slightest presence of any 

 particles of silex in the Pholadida?, although these 

 are believed to exist in other families of the boring 

 acephala. 



The fourth theory, that of ciliary currents as an 

 accessory agent in boring, is worthy of greater con- 

 sideration, chiefly from the evidence we possess of 

 the immense power which the incessant action of 

 currents of water possess in wearing away hard sub- 

 stances. 



We come now to what may be considered the 

 most important of the theories above enumerated, 

 viz., the mechanical action of the valves of the Pholas 

 in rasping away the rock, &c. This hypothesis is 

 one which most naturally suggests itself to the mind 

 of any impartial person, on examining, for instance, 

 the rasp-like exterior of the shell of Pholas crispata. 1 

 But as I shall endeavour to show, although the shell 

 forms the principal, it does not by any means con- 

 stitute the sole agent in completing the perforating 

 process. 



1 Specimens of this species, I may mention, have always formed the subject of my 

 experiments, and therefore are alone alluded to in the following remarks. 



