THE ECONOMIC USES OF SHELLS. 463 



from such dependencies as the Laccadive and Maldive Islands I have been 

 unable to ascertain any figures. Unfortunately the exports of cowries are not 

 distinguished separately from other shells so that no conclusions as to the extent 

 of the trade can be drawn. 



Turning to the question of the many ways in which shells are used for orna- 

 ments it is somewhat difficult to know where to commence, for the subject 

 could easily be enlarged upon so as to fill a fair sized book. 



The two main branches of the subject are undoubtedly pearls and mother- 

 o' -pearl, though there is besides in India at any rate the extensive Chunk or 

 Conch shell fishery. 



Taking Pearls first, it may be noted that from the most ancient times to the 

 present day India has been looked upon as the chief market of the wojld for 

 the best specimens ; but it must be admitted that this reputation is not alto- 

 gether deserved as it is really from Ceylon and from the Persian Gulf that the 

 majority, and certainly the best, of them come, though they largely find their 

 way to other countries through the Bombay market. 



Pearls are of course produced by quite a large number of bivalve shells, but 

 the really valuable ones are found in the so-called " Pearl-oysters " of various 

 species belonging to the genus Margaritifera, which is a section of the genus 

 Pteria (=Avicula). Great confusion has existed as to the various species of 

 this genus, but I suppose we may take a paper by Mr. H. L. Jameson published 

 ia the Zoological Society's Proceedings (1901) in order, as he describes, " to 

 prevent further confusion of the common commercial form by zoological and 

 economic writers," as the most recent authoritative opinion on the subject. 

 From this paper it is evident that there is great variation in any one so-called 

 species and that the sub-division into species is more or less arbitrary. 



Quoting Rev. A. H. Cooke in the Cambridge Natural History, " Pearls are the 

 result of a disease in the animal .... When the Avicula is large, well formed, 

 and with ample space for individual development, pearls scarcely occur at all 

 but when the shells are crowded together, and become humped and distorted, as 

 well as affording cover for all kinds of marine worms and parasitic creatures, 

 then pearls are sure to be found." It was formerly supposed that pearls were 

 formed around some foreign matter, such as a grain of sand, that had become 

 imbedded in the mantle of the animal and so been a source of irritation, but 

 the usual cause is now generally recognised to be either a minute parasite or 

 an ovum, or group of ova, that has escaped from an overgrown ovary and be- 

 come imbedded in the mantle. Although originating in the mantle they 

 frequently work their way out and lie loose between it and the shell, or be- 

 come attached by subsequent nacreous deposit to the mother-o'-pearl surface 

 of the latter. The hollow warty pearl, known as " blister pearl," is supposed 

 to be produced by a deposit of nacreous matter at the point of invasion of a 

 boring parasite. Now although the various species of " Pearl-oysters " all 

 produce pearls of sorts, the larger species, such as M. margaritifera (Lin.), are 

 fished chiefly for their marketable value for the " Mother-o'-pearl," and thei-e 

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