PEARL SHELLS. 239 
FAMILY AVICULADZ. 
(Pearl Shells.) 
The members of this family effect a trans- 
ition from those genera which have but a single 
muscle, to those which have two; for though their 
shells display two, or even more muscular impres- 
sions, one is always found to predominate greatly 
over the rest, which are but small. 
The shell has its exterior portion commonly 
black, or of the colour of horn, and composed of 
prismatic cellular substance; the interior layers 
are composed of nacre, and are therefore brilliantly 
pearly. Their form is very irregular, the valves 
being unequal, and always developed more towards 
one side than the other. The hinge is without 
teeth, the ligament thin, simple, or notched. 
The animais have a mantle which does not 
adhere to the shell, entirely open all round, pro- 
longed into irregular lobes, without siphons; there 
is a small foot which is grooved, and capable of 
spinning a strong byssus. 
The most important genus of the family con- 
tains the Wing-shells, one species of which is the 
Pearl-oyster, from the interior shell of which we 
obtain the substance known as mother-of-pearl, 
which is largely imported into this country, and 
manufactured into shirt-buttons, knife-handles, 
and various trinkets; it is also much used for 
ornamental inlaying. But that which renders 
this shell of much more value is the production of 
pearls, the beauty of which has always rendered 
them fit companions of the choicest precious stones, 
for the purpose of personal adornment. 
