MOLLUSCA. 197 



Pinna muricata lias been called by the English 

 (c the great silk mussel ;" and P. fldbellum furnishes 

 a similar silky byssus. These three species all 

 inhabit the Mediterranean. 



The genus Pinna is also remarkable by the fact 

 that these mollusca, especially P. nobilis, produce 

 pearls. These are generally small, and of an amber 

 colour or reddish, sometimes grey or of a lead 



FIG. 24. Pinna nobilis, L., showing its byssus, called by Eeaumur 

 the " Silkworm of the sea." 



colour; others are black, and shaped like a pear. 

 They are frequently large enough to be of con- 

 siderable value. 



The shells of these mollusca, which are not 

 handsome enough to be employed in ornamental 

 work, etc., can still be made useful in a variety of 

 ways. They are composed of carbonate of lime, 

 with a very little phosphate of lime and other salts, 

 and organic matter. On soils which require lime, 

 pulverized shells may be found of service, especially 



