GENERAL INTRODUCTORY 5 
ever, such as the Top Shells (Turbinide and 
Trochidz), the Pearl Oyster (Meleagrina), and many 
freshwater Mussels (Unio, Anodonta, etc.), as well as 
Nautilus, this last form of shelly secretion is not 
differentiated from the nacreous layer, and is very 
abundant. In the case of the Pearl Oyster and 
freshwater Mussels, foreign bodies introduced acci- 
dentally or intentionally between the. mantle of the 
animal and the shell become coated with pearl. In 
this way “blister pearls,” and occasionally detached 
pearls, are formed; but the true pearl of commerce, 
as will be shown later (infra, p. 79), is developed 
within the tissues of the animal. 
The successive additions along the growing edge 
of the shell generally leave ridges or marks parallel 
with it, that are known as “lines of growth.” The 
deposition of shell does not go on continuously ; 
every now and again there comes a period of rest, 
and these rest periods are frequently indicated by 
the occurrence of a stronger ridge or mark. The 
different details of sculpturing on the surface of the 
shell—striz, ribs, spines, etc.—are all the products 
of corresponding irregularities on the margin of the 
mantle, and were, when first formed, ‘situated on the 
growing edge. Certain molluscs, especially among 
the Gastropoda, further possess the power of dis- 
solving and removing portions of their shells, either 
appear to differ in microscopic structure from the shell im- 
mediately beneath them, = 
