174 ON THE NATURAL HISTORY OF 



pay the cost of the Ceylon Railway. But I shall not be performing my 

 duty as naturalist if I do not also place before the Ceylon Government the 

 dark side of this bright future. The mortality among oysters from natural 

 and violent causes is very great, as has been observed in small banks in 

 Trincomalie harbour. Scarcely half of them reach the third year. The 

 ratio of mortality decreases after that, and among older oysters it is pro- 

 bably not more than five or six per cent, per annum, until they arrive at 

 the last year of their existence, when they die off very rapidly ; so that it 

 cannot be expected that more than a fourth or fifth of the young oysters 

 discovered this year on the banks of the Arripo, will live to maturity or to 

 the age fit for fishing. Even this is a good prospect for the future, but 

 there are other causes which may operate in diminishing this number. 

 Oysters, particularly the young, are likely to be smothered by sand. 

 Although the divers report that there is scarcely any ground current on 

 the bank, there is reason to believe that at some seasons the force of water 

 is increased, and that oyster banks may be partially covered over bj' sand 

 or sea mud. I have no doubt in my own mind that the sudden disappear- 

 ance of young oysters from banks is owing, sometimes, to these sand or 

 mud deposits. I have seen the fatal effects of even a few inches of sand on 

 my artificial beds in shallow water ; and that such deposits will kill even 

 old oysters is evident, as I have ascertained that they cannot live for more 

 than a day or two when thus covered. That such sudden deaths do occur 

 on the Pearl Banks of Arripo is more than probable, from the fact that the 

 divers frequently bring up large quantities of dead oyster shells of all sizes 

 from parts of the sea where no living ones were found the year before ; 

 and sometimes, when I have asked for sand or mud, they brought up 

 oyster-shells with it. The natural inference follows, that living oysters 

 were at one time buried under the sand, and that, subsequently, the saud 

 being washed away, the shells were left exposed. This I have observed in 

 the harbour of Trincomalie, where some clusters of oysters that I translated 

 had disappeared, but in a few months their shells were discovered on the 

 spot where they had lived. 



Recent observations corroborate the statement I had previously made, 

 that Pearl oysters exert their locomotive powers frequently, and that in the 

 act of doing so they may be washed away to some distance, while their 

 progress is only arrested by meeting with impediments, such as a stony 

 surface or shell, and that there, after a time, they refix themselves by 

 forming new beards. One of the reasons of the oyster casting off its beard 

 is, doubtless, that it may form a longer one more suited to its enlarged 

 body. If we consider that the beard (fibres so called), when once formed, 

 does not grow longer or thicker, the fact is of importance to the naturalist 

 who first maintained that oysters change their position by casting off their 

 old beards, or cables, and forming new ones. In old oysters a young beard 

 is not found. 



A deficiency of hard substances, on which the oyster can reattach itself, 

 must sometimes occur, from either a natural scarcity of coral, stones, or 



