THE FRESH-WATER CLAM AND ITS ALLIES 181 



tion, but it is especially abundant in North America. It 

 occurs in ditches, ponds, and rivers 

 of New England, as well as in the 

 South and West Some species of 

 it inhabit brackish water. Pisidium 

 has both valves unsymmetrical. Its 

 species are still smaller than those 

 of Cyclas, rarely exceeding 10 milli- 

 metres. It likewise is found in 

 ditches, ponds, and streams all over 

 our country. 



The remaining families which 

 we shall consider come from the 

 sea, and all occur on our Eastern 

 coast. 



The Pholadidae 1 and Teredidae 2 

 include certain wood and rock- 

 boring species, which do great 

 damage by boring into wooden 

 vessels and pilings (Fig. 1(3(3). In 

 such locations they are commonly 

 called ship-worms. The boring is 

 done by the movements of the 

 shell, combined with the action 

 of the muscles and foot. The 

 only thing which seems effectively 

 to stop these mollusks from boring 

 into wood is iron rust, so that piles 

 driven full of nails are apt not to FKJ. KM;. TV/WO ncpali*, 



11-1 in a piece of timber. /'. 



be attacked. paUe . SjSt siph ,, lls: T , 



tube; V, valves of shell. 



1 0oAds, Greek name for a boring mussel. From the " Cambridge 



2 repe'w, to bore. Natural History." 



