402 



ORDER III. MARGARITACE,E. 



942. This group contains many genera of much interest ; all 

 of them agreeing in the structure of the shell, which is composed 

 internally of nacre, and externally of prismatic cellular substance. 

 The most important is the Avicula ; which receives its name 

 (meaning little bird) from the wing-like projections near the 

 hinge, which are very long in some species (Fig. 590), One 

 species, the Avicula margaritifera^ produces the most valued 

 Pearls, as well as the greatest quantity of Mother-of- Pearl (the 

 latter being simply the nacreous interior of the shell). The 

 former are separate formations of a similar substance, deposited 

 by the mantle at particular spots, in consequence of some irrita- 

 tion. Various causes will occasion this deposit. If grains of 

 sand find their way between the shell and the mantle, they are 

 generally incrusted by it ; one layer being thrown around 

 another, so that, if the calcareous matter be gradually dissolved 



FIG. 590. AVICULA MACROPTERA. FIG. 591. AVICULA IMARGARITIFERA. 



away by a weak acid, a series of concentric spheres of membrane 

 remain. Many pearls, however, inclose no such nucleus ; yet 

 still are produced by mechanical irritation of the mantle. It has 

 been observed that, if the shell be penetrated by boring- worms, 

 pearly matter is deposited at the spot perforated ; and an arti- 

 ficial expedient was thus suggested, which has been put in 

 practice with a certain degree of success. This was, to obtain 



