THE DATE SHELLS. 121 



tangle lifting with the waves, he sees the remains of former cliffs, 

 that have long since crumbled away piecemeal and fallen into the 

 sea. 



It is true that the continual dashing of the billows will, in time, 

 destroy the hardest rocks, and that even a granite cliff can not 

 withstand the action of water. But the process of destruction — 

 if we may use such a word — is greatly accelerated by the multi- 

 tudinous holes bored by the Pholas and other burrowers, and the 

 rock is rapidly undermined by the joint action of the mollusks 

 and the waves. The upper portion of the cliff is thus left with- 

 out support, and when a few heavy rains have loosened the earth, 

 down comes a mixed mass of rook, soil, and herbage, and a fresh 

 face is thus made to the cliff. The loose earth is soon washed 

 away by the waves, the submerged portion of the fallen rock is 

 eagerly seized upon by the burrowers, and in process of time the 

 whole mass crumbles away, is broken up by the storms, and the 

 fragments are gradually rubbed to atoms by the action of the 

 waves. Thus it is that the cliffs recede on one side, while the 

 soil advances on another, and so the whole face of the world is 

 gradually changed. 



Perhaps the Date Shells are even more powerful as burrow- 

 ers than the mollusks which have just been mentioned^ One spe- 

 cies, the Fork-tailed Date Shell {Lithodomus caudigera), is able to 

 bore into substances which the pholas can not penetrate. It is 

 truly a wonderful little shell. Some of the hardest stones and 

 stoutest shells are found pierced by hundreds of these curious be- 

 ings, which seem to have one prevailing instinct, namely, to bore 

 their way through every thing. Onward, ever onward, seems to 

 be the law of their existence, and most thoroughly do they carry 

 it out. They care little for obstacles, and if one of their own 

 kind happens to cross their path, they quietly proceed with their 

 " work, and drive their tunnel completely through the body of their 

 companion. 



The precise method employed in excavation is at present un- 

 known, for the shape of the shell, and the exactitude with which 

 it fits the burrow, prove that the mollusk does not form its tunnel 

 by means of the protuberances on the surface of the shell, and no 

 other method of boring has at present been discovered. 



There is another notable burrower among the bivalve marine 



