PEOLAS. 101 



other mollusks bore. He mentions that the boring is 

 effected by currents produced by vibratile cilia, aided in 

 some cases by rasping. He remarks that the valves of 

 Lithodomus are not adapted for mechanical boring; that 

 the crypts of Saxicava are not circular ; that the valves of 

 Teredo probably do not correspond to the bore ; that Pho- 

 las conoides is often found in hard timber, though its valves 

 are not adapted for any boring or filing. He further ob- 

 serves " there is a cartilage between the two small spinous 

 processes of the hinge in the PJiolas candidus : in other spe- 

 cies of Pholas which have no rudiment of it, and allied 

 genera which have a particular character of articulation, 

 I consider the motion of the valves as but a secondary 

 cause in the perforation of the substances in which the ani- 

 mals are found." * 



Mr. G. B. Sovverby objected to the notion that the cavi- 

 ties of Pholas were produced by rotatory motions of the 

 shell, since they are fitted to the latter, and since some of 

 the shell bones are externally smooth. He also objected to 

 the notion of a solvent. 



Mr. W. Thompson, in his paper on Teredo cited under 

 the account of that genus, expresses his belief that the 

 smoothly rounded termination of the cell made by that 

 animal, is due to " the action of a solvent supplied by the 

 proboscis, which thus acts as a pioneer in mining the jjas- 

 sage that is afterwards increased to its final dimensions by 

 the boring action of the primary valves.*" 



Dr. Drummond, in his "Letters to a Young Naturahst," 

 suggested the possibility of the animal of the boring mol- 

 lusks decomposing the sea salt, as its wants may require, 

 and applying the liberated muriatic acid to the solution 

 of the calcareous rock. 



* Loc. cit. p. {{9. 



