PELECYPODS 431 



pearl is based on the brilliancy of the nacre, the size, and the form. 

 Luster, or " orient," is to the pearl what brilliancy is to the dia- 

 mond. The round form has the greatest value, next the pear- 

 shape, and lastly the oval. Pearls of inferior value are produced 

 by other species than Meleagrina margaritifera, the true pearl-oyster. 

 Many bivalves secrete similar bodies, but as they have the nature 

 of the shell, those produced by mollusks having lusterless inte- 

 riors are of no value. Beautiful pink pearls are found in the 

 gasteropod Strombus gigas, the conch-shell of the West Indies. 

 The shells of the pearl-oyster are themselves very valuable, being 

 the " mother-of-pearl " used in the manufacture of buttons, knife- 

 handles, inlay-work, and so on. They have three grades of com- 

 mercial value ; of the first are the " silver-lipped," from the South 

 Seas, of the second the " black-lipped," from Manila and Ceylon, 

 and of the third the " bullock-shells," from Panama. 



GENUS Avicula 







A. atlantica. A Floridian species which is often found attached by 

 its byssus to large algse which have been washed upon the beach. The 

 shell is reddish-brown, with many white radial lines. Its obliquely oval 

 shape, the long wing-like extension of the dorsal margin, the inequivalve 

 shells, and the byssal sinus placed just below the forward extension of 

 the hinge, are characteristics which will enable one to recognize the spe- 

 cies at once. The structure of the shell is unusual in being so largely 

 composed of organic matter that it possesses elasticity near its thin- 

 ner edges. 



GENUS Pinna 



P. muricata and P. seminuda. Two species of this family which 

 are very abundant in Floridian waters and are often to be found asso- 

 ciated upon the muddy or sandy shores of bays. Like Modiola plicatula, 

 they attach themselves by means of their byssus to shells or stones 

 which lie under the surface. With the pointed end of the shell directed 

 downward, they remain almost wholly concealed in the sand. Only 

 about one quarter of an inch of the broad end of the shell projects 

 above the sand. The edges of the valves are as sharp as knife-blades, 

 and great caution must be used while walking barefooted along a beach 

 inhabited by Pinna. The shells of both species are of a triangular 

 wedge-shape and are composed of a translucent crystalline, matter. 

 P. muricata is beset with triangular erect scales which project out 

 like spines. Their edges are also sharp like pieces of broken glass. 

 P. seminuda also has these raised scales, but they are far more numer- 



