OTHER BIVALVE MOLLUSKS 117 



by builders. The bottoms of wooden ships are cov- 

 ered with copper, and the timbers used in building 

 wharves are either soaked in poison before using or 

 sheathed with metal. A still better plan is to build 

 the piers entirely of concrete, or to cover the wooden 

 piles with a thick coating of cement. Unless some 

 such precautions are used the woodwork of a wharf 

 may be ruined in the brief space of one or two years. 

 Much good, on the other hand, is done by the borers, 

 for they tend to break up the timbers of old wrecks, 

 and to cause obstructing ledges of rock to crumble 

 and disappear. 



Xylotria stutchburryi^ Jeff., the Little Teredo, has 

 a very small shell, of which the valves are white and 

 triangular, while the pens are minute and club- 

 shaped. 



This closes our description of the two-valved mol- 

 lusks. Very much concerning the details has been 

 left unwritten, and for every observer there remain 

 plenty of interesting facts, which are only waiting 

 for some patient discoverer. 



