74 Pearls. 



organic matter which serves as a uniting medium. 

 In nacreous or pearly shells we find an intimate 

 association of the two textures, often in the form of 

 alternating layers of very thin animal membrane 

 and carbonate of lime. If digested in weak acid 

 the calcareous ingredient of such a shell is slowly 

 removed in solution, while the membranous residue 

 retains with fidelity the original form of the shell. 

 The shell is secreted by the soft external integument 

 of the mollusc known technically as the " mantle," 

 since it forms a kind of cloak, enveloping the vis- 

 cera or internal organs. The edge, and indeed the 

 general surface of this delicate membrane, separates 

 calcareous matter from the food of the mollusc and 

 from the surrounding medium, and thus slowly builds 

 up the texture ot the shell. In some molluscs the 

 shell is formed of only a single piece, or valve, and 

 hence they are termed U^ti-valves. The snail, the 

 whelk, and the nautilus are familiar examples of such 

 uni-valved mollusca. But in other cases the shell is 

 composed of two parts, whence they are termed 

 Bi-valves. It is to this group, as previously stated, 

 that all the ordinary Pearl-bearers belong. 



Marine Pearls. 



The great class of the Lamellibranchiata, or 

 Conchifera, including all the common bivalves, 



