No. 6 (192I) COMMON MOLLUSCS OF SOUTH INDIA I65 



pearl of the finest quality and normally spherical in shape. Very 

 rarely a grain of sand may be similarly entombed. "Muscle- 

 pearls " contain no worm nor other intrusive body ; they originate 

 from some particle of the shell substance dislodged usually by 

 some strain ; this passes into the mantle and is covered up 

 eventually by layers of nacre. They are found usually close to 

 the insertion of certain muscles on the shells; they constitute the 

 bulk of the little pearls called seed-pearls and are usually irregular 

 in shape. 



Solitary individuals of the larger Black-lip oyster (M. margariti- 

 fera) are found with the common species. It is a handsome stoutly 

 built shell, with a shade of green in the very dark ground colour 

 which is decorated with several radial lines of white spots passing 

 outwards from the umbo. The "ears" disappear in the adult and 

 the outline becomes sub-orbicular. The nacre of this species is 

 thick enough to be of value in button making ; but its dark smoky 

 colour greatly impairs its market price. It is fairly numerous in 

 the Persian Gulf. 



The Aviculids proper, the true WiNG-SHELLS, have usually an 

 oval shell with enormously developed " ears " at each end of the 

 hinge. Several species may be termed common on rocky ground, 

 such as the rough bottom off Negapatam and in the Gulf of Mannar 

 and Palk Bay. but they never form beds or banks. Usually they are 

 semi-commensal, living associated with particular kinds of sea-fans 

 and hydroid zoophytes. Two characteristic forms with this habit 

 are the brown Avicida radiata and the brown and white striped 

 A. zebra. The former is a typical wing-shell, the hinder "ear" 

 prolonged into a huge backwardly directed spine. It is found 

 clustering upon the branches of stout sea-fans (Gorgonids) 

 usually characterized by a brownish orange colour. (Fig. 40.) 

 When young the aviculid has the identical rich orange brown 

 colouration of the sea-fan ; its concealment is further aided 

 by the elongated shape of the "ears" — these simulate branches 

 of the host ; shape and colour are so similar that the young 

 shells readily escape observation on casual examination. With 

 growth, these resemblances become obscured, the Wingshells 

 becotning much darker in colour and too stout to escape 

 detection. By such time the shell has thickened sufficiently to 

 become unpalatable to most of the smaller fishes that are its chief 

 enemies when young. 



