18 
thickening layers, as they are formed, pass directly into 
the nacre layers on the inside of the shell, and thicken 
the shell itself. Through these over-layers the pearl is 
connected with the shell as though by a succession of cov- 
ering clothes. At first it clings to the shell at one point 
only, afterward enlarging the area of its adhesion. In this 
manner twin or united pearls are formed. Whatever be 
the method of their formation, it would seem that pearls 
can be formed only at the expense of the shell, for every 
substance necessary to their growth is drawn from sources 
which normally secrete the shell. Hence the presence of 
the pearl can usually be detected on the outside of the 
shell. Normal appearing shells rarely contain pearls, 
while on the other hand those that are deformed often con- 
tain pearls of great beauty. There are three indications 
on which pearl-fishers rely for detecting from the outer as- 
pect of the shell the presence of pearls. These are: first, 
the thread, that is, the recess of elevation extending from 
the vertex to the edge; second, the kidney-shape of the 
shell, that is, an indentation on the ventral side ; and third, 
the contortion of both shells toward the middle plane of 
the animal. 
The families with iridescent interior layers are the fol- 
lowing: Among cephalopods, the Nautilus and Ammonite 
- groups, the latter wholly fossil, and among gasteropods, 
the Turbo and Haliotis families. In all these forms, the 
removal of the outer layers of the shell reveals the splen- 
did pearly surface beneath. Such shells, thus ‘‘ cleaned ”’ 
with dilute acid, are familiar as ornamental objects, while 
those of the common oyster and clam are dead white or 
dark purple, according to their proximity to the part of 
the mantle which secretes the white or the dark material 
of the shell. The true pearly or nacreous (iridescent) in- 
terior belongs to only a few families of mollusks, and in 
these alone can pearls proper be formed at all; while in 
