202 SHELLS AND MOLLUSCOUS ANIMALS. 



valves near the hinge, which, however, sometimes 

 fall off, and are so easily broken, as that they 

 cannot be relied on as characteristic. The shell is 

 marked with ridges set with prickles, which are 

 thicker and larger at one end than the other. 

 The animal has a long siphon, and if we place a 

 stone-piercer in a vessel of water, it will elongate 

 this wonderfully. 



The Pholades are not the only family of mollusks 

 which pierce their way into stone and wood. Not 

 less remarkable are the boring powers of some of 

 the genus Mya, and the Rock-boring Venus, and 

 others which dwell in cavities exactly fitted to 

 their shells. The well-known Ship-worm ( Teredo 

 norvagicus, or navalis, as it is often called) , is very 



destructive to submerged wood, insinuating itself 

 into the bottom of ships, even when the oak is 

 perfectly sound, and by its ravages destroying 

 many a noble vessel, a valuable floodgate, or sub- 

 stantial pier. The teredo works with astonishing 

 rapidity, and will completely riddle a hard and sound 

 piece of wood, in the space of five or six weeks. 



