15 



assigning the proper value by weight to a valuable article of 

 small weight, form and colour also considered." 



The pearls of commerce are, of course, for the most part 

 those which are formed within the soft tissues of the animal, 

 and not the irregular pearly excrescences {oddnmutta) which 

 are found as outgrowths of the nacreous layer of the shell, 

 frequently at the point of insertion of the adductor muscle. 

 The nacreous layer of the Grulf of Manaar pearl oyster shell 

 is very thin and of hardly any commercial value, the shells, 

 after the extraction of the pearls by the process of decom- 

 position, being used mainly in the manufacture of chundm. 



As regards the cause of the formation of pearls, concern- 

 ing which many theories have been hazarded, the most 

 prevalent idea being that they are a " morbid secretion " 

 produced as the result of disease, I may quote from the 

 excellent " Guide to the Shell and Starfish Galleries in the 

 British Museum (Natural History,)" ^ which tells us that 

 some small foreign body, which has accidentally penetrated 

 under the mantle and irritates the animal, is covered with 

 successive concentric layers of nacre, thus attaining some- 

 times, but rarely, the size of a small filbert. The nacre is 

 generally of the well-known pearly- white colour, very rarely 

 dark, and occasionally almost black.^ The effort of the 

 animal to get rid of the irritation caused by a foreign sub- 

 stance between its valves, by covering it over with nacre, and 

 thus converting it into a pearl, is strikingly illustrated by 

 two specimens in which, in the one case, an entire fish, and, 

 in the other, a small crab has been so enclosed. According 

 to Streeter (o/j. cit.) the nucleus of the pearl may be either 

 a grain of sand, the frustule of a diatom, a minute parasite, 

 or one of the ova of the oysters, thin layers of carbonate of 

 lime being deposited around the object concentrically, like 

 the successive skins of an onion, until it is encysted. 



Writing in 1 859 '^ as to wliat may be termed the worm 

 theory of pearl formation, Dr. Kelaart stated that " as this 

 report may fall into the hands of scientific men, I shall 

 merely mention here that Monsieur Humbert, a Swiss 

 Zoologist, has, by his own observations at the last pearl 



' Printed by order of the Trustees, 1888. 



* Among the pearls from the samples lifted at Tuticorin in November 

 1888 there is one dumb-bell shaped specimen of which one half is white, 

 the other dark brown. 



* Report on the Natural History of the Pearl Oyster of Ceylon, 1858-59. 



