GENERAL INTRODUCTORY 5 



ever, such as the Top Shells (Turbinidse and 

 Trochidse), the Pearl Oyster (Meleagvina), and many 

 freshwater Mussels (Unio, Anodonta, etc.), as well as 

 Nautilus, this last form of shelly secretion is not 

 differentiated from the nacreous layer, and is very 

 abundant. In the case of the Pearl Oyster and 

 freshwater Mussels, foreign bodies introduced acci- 

 dentally or intentionally between the mantle of the 

 animal and the shell become coated with pearl. In 

 this way " blister pearls," and occasionally detached 

 pearls, are formed ; but the true pearl of commerce, 

 as will be shown later (infra, p. 79), is developed 

 within the tissues of the animal. 



The successive additions along the growing edge 

 of the shell generally leave ridges or marks parallel 

 with it, that are known as " lines of growth." The 

 deposition of shell does not go on continuously ; 

 every now and again there comes a period of rest, 

 and these rest periods are frequently indicated by 

 the occurrence of a stronger ridge or mark. The 

 different details of sculpturing on the surface of the 

 shell — striae, ribs, spines, etc. — are all the products 

 of corresponding irregularities on the margin of the 

 mantle, and were, when first formed, situated on the 

 growing edge. Certain molluscs, especially among 

 the Gastropoda, further possess the power of dis- 

 solving and removing portions of their shells, either 



appear to differ in microscopic structure from the shell im- 

 mediately beneath them. 



