242 DAHLAK 



find them in the cavity formed by the angle of contact of the 

 cerebral-form madrepore with the sea-floor. Other species of 

 small cowries seek refuge in the same place: the Cipraea 

 turdus and the Cipraea erosa measuring from an inch to an 

 inch and a half, the minute Cipraea hirundo about half an 

 inch, as well as the volutes represented by the genus Oliva in 

 different species and varieties in a wide range of colours. All 

 of these are used as beads for necklaces and are exported to 

 Europe. 



Top shells belong strictly to the madreporic environment. 

 The natives fish the many big shell species (averaging four 

 to five inches between apex and base) for the mother-of-pearl 

 button industry. This is done in the same way as with pearl- 

 bearing oysters ; that is to say, there is an initial exploration 

 with a ^bucket' such as the octopus fishers use in the 

 Mediterranean. 



Not so common, but perhaps the queen of shells, because 

 of its fantastic variegated colouring is the Charonia tritonis. It 

 is similar in shape and size to our Triton nodiferum, the 

 common knobbed triton which many of our fishermen still 

 use as fog-horns or for calling over great distances. 



The bi-valve lamellibranchians are as plentiful as gastero- 

 pods on the beaches. There are oysters, thorny oysters with 

 spiky pink shells, scallops of various shades, mussels which 

 are similar in shape and colour to those of the Mediterranean, 

 ark shells of the genera Area and Pectunculus, cockles, Venus 

 clams, tellin shells, etc. We observed that these various 

 species, while living on the same coral formation, were 

 each limited to distinct zones in a particular habitat created 

 by specific associations of madrepore. Some of them chose 

 the brain-shaped madrepores as well, establishing themselves 

 above, or more often, below — either along the cavity of the 

 angle of contact between the madrepore and the sea-floor or 



