154 MADRAS FISHERIES BULLETIN VOL. XIV, 



It forms no byssus, lying more or less prone on the muddy 

 bottom of estiiarine creeks and landlocked bays (Palk Bay, 

 Tuticorin Harbour, and the Buckingham canal) and in the sheltered 

 bays along the Bombay coast from Goa to Sind. 



The foot is long and trumpet-shaped ; it is employed largely 

 in freeing the interior from mud that finds its way in and would 

 otherwise quickly choke up the fine network of the gills. The 

 shell is peculiarly soft ; both in appearance and softness it 

 resembles mica, particularly whilst immature. Small shells are 

 usually colourless, clear, and translucent ; the Chinese ages ago 

 noted this, and with their usual ingenuity put it to the useful 

 purpose of glazing windows and verandah roofs. For this purpose 

 the young shells are cut into squares about 2^ inches across, and 

 then secured between narrow vertical strips of wood. The 

 Portuguese borrowed the idea and introduced it into their Indian 

 possessions in the l6th century. To-day the windows of the older 

 houses in Goa and Diu are still glazed in this manner — the 

 Governor's old palace at Marmugoa has fine examples of such 

 windows. 



In some localities — not all — the window-pane oyster produces 

 a large abundance of seed pearls, soft, often misshapen but still 

 valuable for use in indigenous medicine in India and China, and 

 also for the production of an expensive lime for use in betel- 

 chewing. Four such places in the Indian region are Chittagong, 

 Tamblegam Bay in Ceylon, the Gulf of Kutch and the creeks of 

 the Indus. In all these localities Placuna forms beds of great 

 extent — they are numbered by the million — and the revenue derived 

 from their fishery is often of considerable importance. Usually 

 these fisheries belong to the State — a royal prerogative of ancient 

 standing. 



The anatomy and habits of this mollusc are dealt with in 

 detail in volume II of the " Marine Zoology of Okhamandal." * 



The Ark-shells {Arcidae) are a large family including many 

 Indian species, varying widely in appearance and habit. All 

 agree however in having the hinge set with a large number of tiny 

 plate-shaped teeth disposed perpendicular to the edge. The shells 

 are often angular and the edge of the mantle set with minute 

 composite eyes. The mantle is open and no siphons are present. 



* Hornell, J., London (Williams and Norgate), 1916. 



