DESCRIPTION OF THE EXPEDITION. 145 
visit. The adults also remain for some weeks on the bank browsing on the “ grass,” 
and here again they are open to attack. After laying their eggs they disappear and are 
said to go off to sea; but this is doubtful, although we did meet with isolated animals 
far from any land or reef. At present new areas are being opened up and a fair 
supply of shell is still obtained for commerce. Sooner or later the shelled form will have 
to be preserved and its young looked after in its early stages, or even grown up to 
profitable size, which fortunately is an easy matter. 
We were asked to look at another organism of possible economic value, the black- 
lipped oyster (Margaritifera margaritifera), the shells of which are used largely for 
buttons and inlay work. They are found scattered over the whole of the lagoon with 
isolated specimens on the reef. In the latter position they often get so grown over by 
corals and other growths that they become part of the reef itself, just capable of 
opening their shells sufficiently to allow a current of water to be created by their gills 
to bring them food. In the Cymodocea they do not flourish, the only specimens found 
there lying loose upon their sides and having their shells almost clean. As their 
normal position of attachment is with the free edges of their shells upright, they 
certainly owed their presence there to currents, which had carried them off certain 
barer areas where they seemed to flourish. These areas were mostly patches, where 
' the Cymodocea seemed unable to root itself properly on account of the ground being 
— a Yo 
relatively hard, owing to abundant stones in the mud; these gave points of attachment 
for the oysters, which were upright, firmly attached, and healthy, in spite of being 
overgrown by every kind of sedentary organism to be found on reefs. Most of the shells 
were bored into by alge and sponges; some showed quite arborescent growths through 
_ the nacre, but in few was it really damaged. A few pearls of small value were found 
— in some, but this is a point of little importance, as their value in this type of fishery is 
only about a fifth of that of the shells. So far as we could see, fine sand appeared 
to make but little difference to the oysters. The real reason why they were not much 
more abundant appeared to be that their spat could not find fixed objects for attachment. 
_ It seemed to us to be due to the greater ease of attachment that we found the oysters 
in areas free of grass. Indeed, we are inclined to suggest that artificial areas for 
attachment might be erected and certain pools marked off and kept expressly for 
spawning. The chief enemies of the spat are rays, but this danger could be probably 
overcome by taking large spawning shells into one of the barachois of South Island and 
keeping them there surrounded by hurdles, on which the spat might affix itself. For 
_ this purpose the barachois might have to be barred up, as is done for fish-preservation 
© in Diego Garcia, where we also found specimens of the black-lip shell. Subsequently 
the hurdles would of course be transferred to the main lagoon of the atoll. 
We left Farquhar at dawn on Oct. 2 for Providence reef, about 40 miles to the 
north (fig. 39). On our way we took serial temperatures down to 400 fathoms, and 
_ sounded in order to fill some blanks on the chart, getting depths of 867, 890, 876, and 
613 fathoms on a bottom formed mainly of broken coral and shell. The same evening 
we anchored to the south-west of the bank, and spent an uncomfortable night on 
account of the swell which came sweeping round its end. Plankton-nets were placed 
ref Be 
