lecture xL 
the genus Pinna by an animal allied to the Snail 
and Slug j whereas in the rest of the Bivalves, the 
inhabitant is more or less allied in shape to an 
Oyster or a Muscle. 
Of the Linnsean genera of Bivalve Shells, one 
of the most important is that of Mytilus, since it 
contains the valuable species called the Mother-of- 
Pearl Shell; which is the Mytilus Margaritiferus 
of Linnasus. This shell, which grows to a very con- 
siderable size, is of a flattened and rounded shape, 
with the back or hinge-part strait. Its colour on 
the outside is brown, variously spotted and clouded 
according to circumstances, and on the inside, as 
every one knows, of the most brilliant, iridescent, 
silvery lustre. It is a shell of very considerable 
thickness, and when properly cut and polished is 
the beautiful substance usually known by the 
name of Mother of pearl, and of which so many 
ornamental articles are formed; and from the car- 
tilaginous or tendinous hinge at the back-part of 
the shell, in a petrified state, is produced that very 
rare and beautiful extraneous fossil called the 
AndrodamaSj (the Helmintholithus Androdamas of 
Linnaeus,) which when cut and polished, in the 
disposition of its fibres, and in its colours, bears 
