b IXTRODrCTIOX TO COXCHOLOGY. 



folded or plaited surfaces^ although divested of calcareous 

 matter^ exhibit iridescent hues of the most gorgeous 

 description. But if the membrane is spread out with a 

 needle^ and the plates unfolded to a considerable extent, the 

 iridescence is no more seen; a fact which clearly demon- 

 strates that the beautiful effects presented by the nacreous 

 portion of shells, commonly called mother-of-23earl, is pro- 

 duced by the disposition of single membranaceous layers in 

 folds or plaits, lying more or less obliquely to the general 

 surface. Pairy prisms are they, refracting rays of light that 

 fall upon them, and presenting a constant succession of rich 

 and varying colours that 



" Fhing several from each siu'face, form 

 A trembling radiance of revolving hues. 

 As the site varies iu the gazer's hand." 



The most resplendent nacre is that with which the large 

 Aviciila of the Pacific adorns the interior of her shell ; and 

 the pearl, called by Orientals a globe of light, is the product 

 of its superabundant flow. 



Young conchologists, who cannot pursue the investigations 

 of scientific men, may yet derive much pleasure from an easy 

 chemical experiment, in the operation of which shells be- 

 come distinctly exhibited, without anv material alteration 



