MOLLVSCS. 



2t;3 



by having tlie liiiin'c line straiti'lit, and proiliici'il on ritlu'r side into winn-Iii<(_' ears. 

 The valves, which are very ubliijue (Iheir axis being at a i-Dnsideraliie angle with tlu^ 

 hinge line) have a foliaeeous texture, ami are lined on the interior with niother-ot- 

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

 that the surface is covered by minute lines, which pnxluce diffraction sjjectra. .Soon 

 after the fact was discovered that tine lines produced this appearance in the motiier- 

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

 buttons, etc., were made from steel, which hud this same iridescent a])pearance 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, 



^i^^"r<^^ 



74, — Sjmniljihis reijius, tliorny (lyster-. 



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 u])on the 

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



In Acicida there is a single cartilage ])it, and the hinge is furnished with two 

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

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

 prominent species is M. m<ir(/arat!fer<(, the true pearl oyster, which has an extensive 

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

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

 carried on at many jioints, but the finest ])earls are said to come from the islands of 

 Bahrein, Karak, and Corgo in the Persian (iulf. Tln> chief fisheries are those of 



