iro LECTURE Xf. 



the genus Pinna by an animal allied to the Snail 

 and Slug ; whereas in the rest of the Bivalves, the 

 inhabitant is more or less allied in shape to an 

 Oyster or a Muscle. 



Of the Linnaean 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 Linnseus. 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 

 Androdamas, (the Helmintholithus Androdamas of 

 Linnaeus,) which when cut and polished, in the 

 disposition of its fibres, and in its colours, bears 



