30 POPULAR BRITISH CONCHOLOGY. 



count for the wearing away of the rock by the rubbing of 

 flinty atoms in the foot of the animal, has this important dif- 

 ficulty to encounter ; namely, that after the most anxious 

 microscopic researches no such siliceous particles have 

 been found in the PJwlas, although they do exist in some 

 other families of boring molluscs. 



The third theory may be good as far as it goes, but is 

 inadequate by itself to account for the whole result. No 

 doubt, currents of sea-water between the shell and the 

 walls of its habitation must materially aid, not only in 

 clearing away the particles when dislodged, but also to 

 some extent in dislodging them. 



As to the fourth, and perhaps on the whole the most 

 popular notion, — that of a chemical solvent secreted by the 

 animal, — it would probably puzzle the acutest chemist to 

 find or invent a solvent which would act equally on wood, 

 clay, sandstone, chalk, and wax. 



The fifth theory, or that of the combined action of rasp- 

 ing and solution, combines the difficulties of both. 



Now, it may seem presumptuous on my part, after so 

 many learned opinions have been cited, and the question 

 left still undecided in the latest and most careful treatises, 

 to venture on anything like a judgment of my own. ^' Who 



