TEREBRATING POWERS OF THE PBOLAS. 257 



may have renewed the serrated edge and rough sur- 

 face of the valves, and thus enabled the creature to 

 renew its wondrous operations. 



We now come to a consideration of the foot, 

 which, as many writers aver, forms the ' sole tere- 

 brating agent.' 



Although this sweeping statement is incorrect, I 

 will freely admit that the foot constitutes an agent 

 second only in importance to the shell of the animal. 

 A casual examination of any Pholas perforation will 

 show that the foot could not have been the only 

 instrument by which the cavity was formed, from 

 the peculiar rings that line the lower portion of its 

 interior. These rough appearances, I feel con- 

 vinced, could be formed by no other means than the 

 rotatory motion of the shelly valves. 



The valves, however, could not rotate and press 

 against the surface of the rock, were it not for the 

 aid which the foot affords to the animal, by its being 

 placed firmly at the base of the hole, and thus made 

 to act as a powerful fulcrum. 



This supposition fully accounts for the lowest 

 extremity of the rocky chamber being always 

 smooth, and hollowed out into a cup-like form by 

 the action of the fleshy foot above alluded to. 



The foot for a long time was a complete puzzle 

 to me : I was unable to satisfy my mind as to 

 how it acquired its seeming extraordinary power. 



The phenomenon was fully explained when I became 



17 



