THE FREtiH-WATER CLAM AND 77'N ALLIED 185 



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

 ters or beds to rocks near low tide (Fig. 17:2). 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, ou 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- 

 lishing has been carried on since the earliest preserved 

 historic records. The pearl banks of Ceylon are known 



1 A small bird (acts). 



