MOTHER-OF-PEARL AND BLISTER PEARLS 125 



these layers lies the most important one, the nacreous layer, consisting of mother- 

 of-pearl. This layer is secreted by the external surface of the mantle of the oyster 

 and may continue to increase in thickness during the animal's existence. Its 

 structure is composed of a series of exceedingly thin plates ( laminae ) , overlapping 

 one another and arranged parallel to the shell's surface. Since the mother-of-pearl 

 surface shows both the faces and zigzagged edges of the laminae, it displays, 

 through the phenomenon of light interference, the iridescence designated as nacre. 



Pfund (1917) found that the thickness of the laminae for many different 

 samples of mother-of-pearl from a fresh- water mussel {Obovaria ellipsis) (Lea) 

 lies between the limits of 0.4[x and 0.6\l. 



Clement and Riviere (1922) studied the deposition of calcium carbonate in an 

 albuminous medium and were able to obtain deposition in the form of a diffrac- 

 tion grating of a physical structure, resembling mother-of-pearl, which possesses 

 the same property of iridescence. 



Species Utilized 



The principal genera of mollusks utilized commercially for the manufacture of 

 mother-of-pearl objects are Margaritifera Jameson, or Meleagrirm, Gasteropoda, 

 Trochus, and Haliotis. Certain species of the genera Pinna, Malleus, Trigonia, and 

 Nautilus are also taken, but these mollusks are of lesser commercial importance. 



Margaritifera maxima Jameson, the large white mother-of-pearl shell of 

 Australian, Papuan, and Malayan waters, commercially called "silver-lip", "gold- 

 hp", etc., M. margaritifera, the "Bombay", "Tahiti", "Gambier", "Auckland", 

 "Panama" shell, etc., M. vulgaris Jameson, the Ceylon pearl oyster, and Margari- 

 tifera radiata, the Shark Bay shell, are the species of Margaritifera or pearl oyster 

 of the greatest commercial importance. Haliotis splendens, H iris, and H. mida are 

 the species of abalone which are utilized to the greatest extent. The species Turbo 

 olearius, or marmoratus, and Trochus dilatatus (niloticus) of the genus Gas- 

 teropoda are of more commercial value than other species of that genus. The 

 species of Trochus shell fished in Australian waters is Trochus (Rochia) niloticus 

 Linne. It has a larger base whorl than the Trochus maximus, which is common in 

 the Philippine waters. 



Geographical Distribution 



Margaritifera vulgaris Jameson. This small species of shell grows only to about 

 the size of a silver dollar, is generally dirty white or yellowish in color, and 

 weighs from 8 to 50 shells to the pound. It is fished principally for pearls. 



These shells are used only in the manufacture of cheap shirt buttons. When 

 market conditions are bad or freight high, it is not profitable to ship them. In 

 this case they are dumped in enormous piles on the beach and allowed to 

 deteriorate. Often, however, after years of exposure, they are reclaimed. As many 

 as several tons of these shells, buried for years in Shark Bay, have been thus 

 dug out and shipped to Europe. They are not used industrially in America. 



Lingah ''-Persian Gulf. Great quantities of pearls are obtained near Bah- 

 rein. 



Western Australia— Shark Bay. The fishing here has been carried on for about 

 80 years. Although formerly very productive, it has been "over-fished." It is now 



** In the trade all the Margaritifera vulgaris are called "Lingahs." 



