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or covered as by a capsule, which gradually thickens and 
assumes various forms—round, elongated, mallet-shaped, 
and sometimes as regular as though it had been turned in 
a lathe. It is suggested that the mollusk continually re- 
volves the enclosed particle in its efforts to rid itself of the 
irritation, or possibly that its formation is due to natural 
motion, which is accelerated by the intruding body. 
In regard to the formation of pearls the following gen- 
eral statements may bemade: Whatever may be the cause, 
or the process of their production, these interior concre- 
tions may occur in almost any molluscan shells, though 
they are confined to certain groups, and their color and 
lustre depend upon those of the shell interior, adjacent to 
which they are formed. Thus the pink conch of the West 
Indies yields beautiful rose-colored pearl shells consisting 
of three strata: first, the outer yellow or brown conchio- 
line (cuticula or epidermis); second, the prism stratum, 
consisting of layers formed of minute prisms arranged ver- 
tically to the layers and the shell surface ; and third, the 
interior nacreous layer, composed of finely folded leaves 
parallel to the surface of the shell. The last two strata 
consist chiefly of carbonate of lime. These formations 
were illustrated by transverse cuttings and microscopic 
sections. When a wound has been received by the ani- 
mal in any soft part, the tissues become moistened with a 
lime-like material, and especjally with the nacre substance. 
‘ This often happens in the muscles which serve to close the 
shell, and the irregular concretions thus formed are called 
“sand pearls.’? When the growth of the pearl is abnor- 
mally strong, the pressure which it exerts on the outer 
wall of this tissue-pocket becomes so powerful that the 
pocket is absorbed on the side toward the shell, bringing 
the hard pear] directly against the latter. It then becomes 
impossible for the pearl to grow any more at the point of 
contact, for there is no tissue to secrete the lime sub- 
stance; but it grows on the rest of the surface, and the 
