224 THE WORLD OF THE SEA. 
to produce nacre, in round or egg-shaped globules. M. Lamiral 
saw a pearl as large as a bantam’s egg, perfectly round, and as 
white as milk, taken from a Gigantea tridacna. 
The Marine Pinna produces rose-coloured pearls; the Haliotis 
fris, green ones. Other bivalves form blue or grey, or yellow, and 
very rarely black pearls. There is a mollusk found in the sand of 
the river Mourne, at Omagh, in the north of Ireland, in which 
small pearls are found, and some of them are of a very fine water. 
It is a singular reflection that the gem so admired and coveted by 
man, should be the product of disease in a helpless mollusk. 
