BULLETIN OF THE UNITED STATES FISH COMMISSION. 321 



Vol. VI, ]\o. 21. ^Vashington, D. C. Dec. 30, 1886, 



99.— PEAKI.S AND PEABIL FISSEBIES.* 



By Sng^lneer M. WEBER. 



Many mussels cover the inside of their shell with a layer consisting 

 of animal membranes and carbonated lime. Thereby a pecaliar luster 

 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 doubtless a natural process tak- 

 ing place in certain mussels. The formation of pearls, on the other 

 hand, is ascribed to accidents, 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 any holes, so that the mol- 

 lusks can fully develop, pearls are but rarely found; while the forma- 

 tion of pearls is very frequent when the shells are irregular. Some- 

 times hundreds of pearls are found in the last-mentioned shells; but 

 frequently scarcely one of them possesses any commercial value. 



Eeal pearls are found only in bivalves ; but a useful product is found 

 in some univalves. The products of the following varieties 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 opal white pearls of a sub 

 dued luster. 



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



(6) Flacima placenta. — Translucent, with lead-colored pearls. 



(7) Ostrea edulis. — The common oyster. 



(8) Modiola vulgaris. — The horse -mussel. 



(9) Ttirbinella scolymus. — The chank-shell; pale-red pearls. 



(10) Turbo olearius marmoratus. 



'"Om Perlerog Perlefiskerierne." From the Norsk Fiskeritidende, Bergen, Norway, 

 October, 1886. Translated from the Danish by Herman Jacobson. 



An excellent reference in this connection is to the chapter on Pearls and the Pearl 

 Fisheries, in P. L. Simraoads's Commercial Products of the Sea. 

 Bull. U". S. F. 0.86 21 



