260 AUTHOR'S REMARKS ON THE 



an extremely fine point to fit the opening of the 

 tunnel. But as this is not the state of ma-tters, the 

 conclusion forces itself upon us, that that portion of 

 the orifice situated above the shell of the animal 

 must be enlarged by the constant extension and re- 

 traction of the siphons, aided by currents of water 

 acting on the interior surface of the cavity. 



This same theory will also serve to explain how 

 it is that all Pholades situated at the same depth in 

 the rock, are not all of a uniform size. I have fre- 

 quently seen a piece of rock exhibit the peculiarity 

 of two burrows of vastly different proportions as 

 regards breadth, being precisely the same depth from 

 the surface of the stone. This appears to me 

 equally wonderful and puzzling at first sight, as the 

 ' boring' question. 



What age is attained by any species of the rock- 

 borers before they arrive at full growth, there are 

 no means of knowing. This point, like several 

 others in the history of these animals, still remains a 

 mystery, nor is it likely soon to be cleared up. The 

 largest specimen of P. crispata that I have seen is 

 at present in my possession. Each valve measures 

 three and a half inches in length, by two inches in 

 breadth. Some foreign specimens of this species, 

 and especially of P. dactylus, are, however, frequently 

 found of much larger dimensions. 



On no occasion have I ever examined any Pholas 

 excavation that had lost its conical shape, a fact that 



