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 Amcula ; 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 Amcula 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 MACROPTJERA. FIG. 591. AVICULA MARGARITIFERA. 



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 



