104 rilOLADID.^.. 



so much thickened, forming a sort of cushion capable of 

 considerable protrusion at the will of the animal. The 

 foot is small, and passing through a much constricted ori- 

 fice, gives origin to a byssus, which anchors the shell close 

 to the base of the excavation, and thus holds the rubbing 

 apparatus in immediate contact with the part to be exca- 

 vated." 



Such are the opinions expressed on this subject, worthy 

 of note for their own weight, or on account of the reputation 

 of their authors, of whose chemical acquirements, at least, 

 several of them are not favourable specimens. They may 

 all be classed under five conclusions : 1 st. 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 eff*ected 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. 4th. That the animal secretes a che- 

 mical solvent, an acid which dissolves the substance into 

 which it bores. 5th. That the combined action of a se- 

 creted solvent, and rasping by the valves, effects the per- 

 forations. 



Of all these theories, the chemical one, so far as a se- 

 creted solvent is concerned, bears least examination in the 

 case of the PJioladidce. The substances perforated are 

 wood, limestones hard and soft, argillaceous shales, clays, 

 sandstone, and, in the ease of a Pholas in the magnificent 

 collection of Mr. Cuming, wax. The notion of a secreted 

 solvent, that would act indifferently on all these sub- 

 stances, is, at present at least, purely hypothetical ; and, 

 since all attempted tests have failed to detect an acid, gra- 

 tuitously so ; for we can hardly suppose that any of those 

 who have taken this view of the cause would maintain that 



