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. 20). 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 



