50 THE ZOOLOGIST. 



PEARLS AND PEARL FISHERIES. 

 By M. Weber.* 



Many Mussels cover the inside of their shell with a layer 

 consisting of animal membranes and carbonated lime. Thereby 

 a peculiar lustre is produced on the inside of the shell, which is 

 called " mother-of-pearl." A smaller portion of this secretion 

 often forms excrescences shaped like drops or kidneys, which 

 either are imbedded more or less firmly in the inside of the shell, 

 or lie loose in the soft parts of the animal, especially in its 

 so-called "beard." These are what are generally known as 

 " pearls." 



The formation of mother-of-pearl is no doubt a natural process 

 taking place in certain Mussels. The formation of pearls, on the 

 other hand, is ascribed to accident, and probably is caused by a 

 sickness of the Mussel, or by some wound inflicted on it. This 

 view has been reached by noticing the circumstance that, when 

 the shells are large, and the inside smooth, clean, and without 

 holes, so that the mollusks can fully develop, pearls are but rarely 

 found ; while the formation of pearls is very frequent when the 

 shells are irregular. Sometimes hundreds of pearls are found in 

 the last-mentioned shells ; but frequently scarcely one of them 

 possesses any commercial value. 



Real pearls are found only in bivalves ; but a useful product 

 is found in some univalves. The products of the following 

 species are known in commerce : — 



(1) Avicula margaritifera. — Which produces the most valuable pearls, 

 but whose shell is worthless. 



(2) Meleagrina margaritifera. — Principally valued on account of the 

 mother-of-pearl. The shells are often 6 to 18 inches long. Its pearls are 

 also of great value. 



(3) Strombus gigas. — The conch-shell of the West Indies. 



(4) Tridacna gigas. — The Giant Clam, with open white pearls of a 

 subdued iustre. 



(5) Pinna squamosa. — With black and red pearls. 



(6) Placuna placenta. — Translucent, with lead-coloured pearls. 



* " Om Perler og Perlefiskerierue." From the ' Norsk Fiskeritidende,' 

 Bergen. Translated from the Danish by Herman Jacobson (Bull. U.S. Fish 

 Commission, 1887, pp. 321—328). 



