1922] 



MADRAS PEARL FISHERIES 



117 



right valve is drawn partly within the left, the margins of the two 

 valves being brought together to a width of nearly half an inch all 

 round. This gives additional protection against unwelcome 

 intruders. When young and in vigorous growth the ventral margin 

 of the valves is prolonged into finger-like projections set at fairly 

 regular intervals (fig. 2). Their growth is discontinuous, so that as 











Fio. 2. Two young pearl-oysters showing free and vigorous growth and a great deve- 

 lopment of the marginal dictations. In the second figure a strong ray of dark 

 colour passes obliquely across the shell from the umbo. Natural size. 



the later ones arise and project further beyond the earlier, we get 

 eventually a series of radial lines of overlapping digitations run- 

 ning from the umbo — the prominent region forming the beak of the 

 shell — outwards and downwards to the lower or ventral edge. The 

 older gradually get worn away by attrition and in oysters over three 

 years old, especially when crowded and suffering from shortage of 

 food, renewal becomes slow and may even cease, eventuating in a 

 smooth-surfaced, even-edged condition indicative of senility. In 

 such oysters, the hinge groove is deep and wide and the "ears" 

 almost obliterated. Coral and sponge growth upon the valves is a 

 frequent feature and as is natural, many are attacked by the 

 burrowing sponge Cliona, which so riddles them with intricate 

 tunnelling as to impart the appearance of being "worm-eaten," 

 the technical name given such shell by the dealers in mother-of- 

 pearl. Five years appear to be the ordinary span of life of the 

 Indian oyster; the records of inspection show that if oysters be not 

 fished within this age, the bed may disappear by the following 



