THE FRESH-WATER CLAM AND ITS ALLIES 185 



familiar blue-black, wedge-shaped shells attached in clus- 

 ters or beds to rocks near low tide (Fig. 172). They are 

 edible. In France they are reared for the market on 

 woven nets, which are submerged at each tide. 



FIG. 172. A bed of Modiola, the horse-mussel, on a mud-bank. Photo, by 



W. H. C. P. 



The Aviculidae l are a group closely allied to the last, but 

 not represented on our shores. It is important because 

 mollusks of this family produce the best pearls. Pearl- 

 fishing has been carried on since the earliest preserved 

 historic records. The pearl banks of Ceylon are known 

 1 A small bird (avis). 



