SHELLS AND MOLLUSCOUS ANIMALS. 201 



prickly pidduck, or prick- stone (Pholas dactylus), a 

 valuable animal to the fishermen of our coasts, as 

 it is extensively used for bait. In some countries 

 it is eaten as food. Indeed, all the mollusks of 



the bivalve family might be eaten ; as there are 

 none which are known to be poisonous, though 

 many are somewhat indigestible, and a few are of 

 unpleasant flavour. A general description of the 

 shell of this species will suit, more or less, several 

 others of the genus Pholas. It is so fragile, that 

 it may be easily broken, and a thicker shell would 

 only encumber an animal shut up in the rocky 

 seclusion. It is white, thin, and in its early stage 

 transparent, about one or two inches in length, 

 and the two large valves, instead of fitting into 

 each other, are open at both ends. Just inside of 

 each of these valves is a little spoon-shaped piece. 

 Besides this, the shell has several lesser pieces, or 



