CATECHISM OF CONCHOLOGY. 67 



nished internally with a long, flat and curved 

 tooth ; probably intended to strengthen the head, 

 on which this curious boring instrument is placed. 

 Q. Are not this kind of shell-fish very inju- 

 rious to shipping ? 



A. They are so, unless proper precautions are 

 taken by tarring the sides of the vessel, or 

 sheathing them with copper. But, as the au- 

 thor of the Conchologist's Companion observes, 

 " The evil which they produce is readily ob- 

 viated by a little care and contrivance ; but the 

 good which they are appointed to effect is 

 incalculably great." 



Q. What good can they possibly do ? 

 A. They consume decaying substances, such 

 as planks of wood, and the wrecks of vessels, 

 c. and thus enable the waves to break them 

 into small pieces. Were it not for their incessant 

 labours we have every reason to believe that 



those mighty rivers 



" To whose dread expanse 



Continuous depth and wond'rous length of course 

 Our floods are rills," 



would in time become impeded by the vegetable 

 masses, and innumerable trunks and branches of 

 large forest-trees, which are continually carried 

 into them ; a considerable proportion of which, 



