4 THE LIFE OF THE MOLLUSCA 
edge of the mantle. Its function is to protect the 
underlying layers from the action of acid in the 
water, or from that of the weather on land. It 
varies greatly in appearance, being sometimes smooth 
and shiny, at others, rough and coarse; frequently it 
is fibrous (Plate XXVIII., Fig. 2a). In many forms 
it readily rubs off; in others it is firmly united to 
the true shell beneath. The second, or principal 
layer, usually forms the greater thickness of the shell 
proper, or “ostracum,” and is secreted by cells 
farther from the mantle margin ; it may be coloured, 
and is often made up of prisms of calcite, as in 
Pinna, though it frequently has a porcellaneous 
structure. The cells more remote from the mantle 
edge deposit the innermost layer of the ostracum, thus 
thickening and strengthening the shell. This layer 
(“nacreous layer”) is of arragonite, and frequently 
formed with overlapping plates, thus giving rise to . 
the iridescent appearance known as Mother-of-Pearl. 
The remaining surface of the mantle also secretes 
shelly matter on occasion, either for the purpose of 
_ further strengthening the shell, of repairing an injury 
remote from the edge, or of filling up unoccupied 
spaces (Plate VII., Fig. 13); and in most Mollusca 
this deposit differs in structure from that of the 
‘other layers.* In the pearl-producing shells, how-- 
* The term “hypostracum ” has been applied to the shelly 
layers immediately under the points of attachment of the 
muscles that secure the animal to the shell, but they do not 
