MOLLUSCS. 



263 



by having the hinge line straight, and produced on either side into wing-like ears. 

 The valves, which are very oblique (their axis being at a considerable angle with the 

 hinge line) have a foliaceous texture, and are lined on the interior with mother-of- 

 pearl, giving them an iridescent appearance. These rainbow hues are due to the fact 

 that the surface is covered by minute lines, which produce diffraction spectra. Soon 

 after the fact was discovered that fine lines produced this appearance in the mother- 

 of-pearl, some ingenious person applied the same method in the arts, and at one time 

 buttons, etc., were made from steel, which had this same iridescent appearance pro- 

 duced by engraving microscopic lines upon the surface. Lately this phenomenon of 

 diffraction has been turned to a scientific use, and to-day glass or speculum metal, 



Fig. 274. — Spondylus reglus, tUorny oyster. 



ruled with very fine lines, is used to produce the spectrum studied in spectroscopic 

 analysis. 



The hinge of the shell is without teeth, or with these elements obscure, while the 

 ligament is partly internal. The shells gape in front, but are closed behind. The 

 small foot spins a byssus ; the mantle margins are free throughout their extent, and two 

 adductor muscles are present. Of these the posterior is large, the anterior small and 

 placed under the beaks of the shell, producing an almost imperceptible scar upon the 

 inner surface. All the species are from the warmer waters of the globe. 



In Avicula there is a single cartilage pit, and the hinge is furnished with' two 

 teeth; the right valve has a notch near the anterior ear for the passage of the byssus. 

 Meleagrina lacks the hinge teeth, and the ears of the hinge line are small. The most 

 prominent species is M. margaratifera, the true pearl • oystei-, which has an extensive 

 distribution, being found in Madagascar, the Persian Gulf, Ceylon, Australia, Philip- 

 pine Islands, the South Sea Islands, Panama, West Indies, etc. The pearl fishery is 

 carried on at many points, but the finest pearls are said to come from the islands of 

 Bahrein, Karak, and Corgo in the Persian Gulf. The chief fisheries are those of 



