168 JOURNAL, R.A.S. (CEYLON). [VOL. XVI. 



Placuna placenta, commonly known as the " window- 

 shell " of the Chinese (because its semi-transparent valves 

 are frequently used in China for illuminating dwelling- 

 houses), is the " pearl oyster " of the Red Sea, the Persian 

 Gulf, Karatchi, the Bay of Tamblegam near Trincomalee, 

 and the Eastern Archipelago generally as far as Hongkong. 

 Although it produces only " seed " pearls, an immense 

 number of people are annually employed in its collection. 

 The pearls themselves are seldom of much value, though there 

 is generally a demand for them in the markets of the East. 

 They are, however, of little importance in the present 

 inquiry ; and so we need not further consider them here. 



True Pearl Oysters. 



All of the species of true pearl oysters belong, as I have 

 just said, to the family of Aviculidw — or " wing-shells " — and 

 with this family it will now be necessary for us to form 

 some acquaintance. In the first place, it is one of most 

 respectable antiquity. No less than three hundred species 

 have been recorded in the fossil state, some even from the 

 lower Silurian strata. As a large number of these fossil 

 forms have been discovered in northern latitudes, it is proba- 

 ble that at one time in the earth's history the family was 

 very widely distributed, though it is now almost entirely 

 restricted to tropical and temperate seas. At present only 

 twenty-five recent species are known. Of this number, five, 

 which belong to the genus Meleagrina, are now fished for 

 pearls. These constitute the pearl oysters of Madagascar, 

 Ceylon, Japan, the South Sea Islands, Panama, California, 

 and the northern and western coasts of Australia. Zoologi- 

 cally speaking, they do not belong to the true oysters 

 (Ostreidw), but are much more nearly allied to the mussels, 

 being furnished with a hyssus, or " cable," by which they 

 are in the habit of mooring themselves to rocks and other 

 substances at the bottom of the sea. 



The classification of the different species (and varieties) of 

 Meleagrina has not yet been systematically worked out ; 



