THE BORING POWER OF PHOLADID^. 31 



shall decide when doctors disagree?" but I must confess 

 that it does not seem difficult to imagine the Pholas making 

 its way into its stony bed by the simple process of licking 

 a hole. M. Keaumur observed, that when he removed the 

 living animal from its place, and put it on soft clay, it 

 quickly before his very eyes buried itself in that substance. 

 Although the stones in which some of the species burrow 

 are certainly harder than the clay described, yet it must be 

 remembered that the clays in cabinet specimens are much 

 harder than when under water; and that this sucking and 

 licking, and rubbing with the foot, which is certainly of 

 stouter material than the rest of the animal, would be power- 

 fully aided by the action of salt water. We see, in the caves 

 below cliffs, how water can excavate hard rocks when acting 

 in circular currents, and we can easily imagine how it would 

 act in enlarging a cavity already begun. In fact, we see, in 

 the case of deserted holes of boring bivalves, instances of the 

 enlarging power of small currents. If Reaumur^'s specimens 

 could so easily perforate the soft clay during the progress 

 of a mere experiment, how shall it be difficult to under- 

 stand perforation, by the same simple means, of a harder 

 substance during the gradual growth of the animal ? While 

 thinking on tliis matter, it occurred to me to try a very 



