PERFORATED ROCKS. 345 



mountain shelves of limestone. Holes made by the Pholas 

 and Saxicava were noticed as being smaller at the aperture, 

 and increasing inward as the shell grows, and assuming its 

 shape ; while those produced by snails were larger at the 

 aperture, and so irregular in form and direction as fre- 

 quently to trencli one upon the other. Dr. Buckland attri- 

 buted these dissimilar perforations to the agency of an acid 

 secretion ; in the Pholas to his solvent powers softening the 

 rock, and aided by the rasping action of the shell ; in the 

 common snail to the secretion of the same acid in very 

 small quantities by the foot of the animal, during his daily 

 retreat to such habitats. Professor Owen suggested that 

 rock-boring mollusks owed their excavations to the action 

 of extremely minute cilia, which move incessantly and inde- 

 pendently of the will of the animal j producing currents in 

 the water necessary to their existence when lodged in rocks, 

 which currents increased in power as the shell proceeded 

 inward. Mr. J. Philips,, on the contrary, alluded to the 

 beautiful regularity of holes made by Pliolades, in proof of 

 their being rather formed by the shell than by currents of 

 water. Sir H. De la Beche observed, that free carbonic 

 acid, applied to limestone, will convert it into a bicar- 

 bonate, soluble in water j that the acid exhaled by the ani- 



