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 alge 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- 
