INTRODUCTORY CHAPTER. 



17 



fecundity alone which prevents them from being totally 

 annihilated. A silky thread is produced by the Pinna, which 

 may be spun, and is occasionally made into gloves, &c., 

 but more as a curiosity than for any useful purpose ; and 

 from several species of Mollusca may be procured a strong 

 colouring matter of a purple hue, which was formerly used 

 as a dye, but has now been supplanted by other animal, 

 mineral, and vegetable productions, which are easier of 

 access. Secondly, as regards the uses of shells ; in many 

 countries, particularly in China and Japan, the valves of 

 some of the genus Placuna, which are extremely thin and 

 semi-transparent, are used instead of glass, for windows. 

 Lime may be burned from a great many varieties, and is 

 so procured in America and elsewhere, particularly in 

 Holland, where many vessels are employed in fishing for 

 shells, which are afterwards brought to Haarlem, where 

 the lime-kilns are situated. In the Hebrides, Antilles, 

 and many other islands, no other lime is known than that 

 procured from shells. A species of Strombus at Mar- 

 tinique, the Concholepas in certain places on the coast of 

 Peru, and a species of fossil oyster in Senegal, are burned 

 for lime. Some shells are used as money in different 

 countries, particularly the Cyprcea ; and in conclusion, 

 pearls and mother of pearl, both the produce of this class 

 of animals, are well known valuable and beautiful objects. 



To counterbalance in some measure the benefits derived 

 by mankind from Mollusca, there are some which are 

 injurious, but to no very serious extent: snails and slugs 

 do much damage in our gardens by their voracity ; and 

 many of the marine species, which make their residence 

 in wood and stone beneath the surface of the sea, occa- 

 sionally do us much damage, by boring into dikes, piers, 

 and even ships. 



The duration of life in the Mollusca is little known ; 



c 



