THE FREHH-WATER CLAM 231 



ing into wooden vessels and pilings (Fig. 218). In such loca- 

 tions they are coimnonly 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 moUusks from boring into wood is iron rust, so that 

 piles driven full of nails are apt not to be attacked. 



In the razor-shells (Solenidae,i Fig. 219), the animal, like 

 the shell, is elongated and subcyhndrical. By means of its 



Fig. 219. — Ensis dimius. Young. Nat. size. Photo, by W. li. C. P. 



foot, which can be protruded between the valves, it excavates 

 with great rapidity a hole in the sand or mud into which it 

 draws itself. These mollusks are sometimes used for food, 

 but it is not easy to get them in quantity. 



The hen-clams (Mactridse ^) include species that are more 

 elliptical and larger than the common hard-shelled clam or 

 " quohog," and have a proportionately hghter shell (Fig. 220). 

 They are not often used as food because they become very 

 tough when cooked. 



The soft-shelled clams (Myidae ^) include the common clam 

 of New England, Afya arenaria, abundant along our whole 

 Eastern coast and much used as food. These clams occur 

 in great numbers in every mud-flat. Their great siphon is 

 extended at high tide to the surface of the ground, so as to 



' sol^n, a channel or tube. ^ niaklra, a l^aking-trough. 



' mya, Greek name for a kind of mussel. 



