THE MOLLUSCA 191 



they had but one valve, forming a little pearly boss, about 

 the tenth of an inch in diameter. 



The shell which is outward is the flat one, and if they 

 have fixed to a large stone or the surface of a rock the 

 whole of the convex shell becomes firmly cemented down, 

 so that its convexity cannot be seen outwardly. 



By many fishermen those so fixed are supposed to be of 

 a different species, and are by them called " Rock 

 Oysters." 



Those that are dredged up loose from the bottom that 

 is, those that we see in commerce are such as affixed to 

 small objects at the bottom, and in which the convex 

 shell, not being applied to a flat surface, has developed its 

 convex form. 



The object to which the veliger fixed itself can frequently 

 be seen in the adult oyster, close to the hinge on the con- 

 vex side. It is usually a small stone or a fragment of shell. 



In oyster-fishing districts it is customary to strew the 

 bottom with broken shell to try and induce a " fall of 

 spat." This is termed " cultching " the bottom. 



This " fixing " on the part of the young oyster has 

 enabled the formation of the large and important industry 

 of " oyster culture." 



In many districts, especially on the coasts of Brittany, 

 where there is suitable locality, such as a fairly land-locked 

 bay, or, in default of such, in enclosed areas, ordinary 

 roofing tiles are coated with a friable cement made of 

 clay and sand with a little lime or cement. These pre- 

 pared tiles are arranged in stacks, with their concave sides 

 downwards, in such way as to form nests of little tunnels, 

 and then breeding oysters are strewed around. The 

 veliger s affix chiefly to the under side of these tiles, in 

 numbers varying from units to hundreds on each. 



As soon as the breeding time is over these tiles are taken 

 into sheds, where women and boys scrape off the cement, 



