234 SHELLS OF MOLLUSC A. PAKT in. 



nearly allied to the Pinna that this combination of the 

 organic and mineral elements is seen in this distinct 

 form ; it is beautifully displayed in the nacreous shells. 



In many shells the internal layer has a nacreous or 

 iridescent lustre, shown by Sir David Brewster to de- 

 pend upon the striation of its surface, by a series of 

 nearly parallel grooved lines. When Dr. Carpenter had 

 dissolved the calcareous matter from a thin piece of 

 nacreous substance, taken from the shell of the Haliotis 



Fig. 169. Section of nacreous lining of the shell of Avicula naargaritacea 

 (pearl oyster). 



splendens, or Ear Shell, there remained an iridescent 

 membrane, which presented to the microscope a series 

 of folds or plaits somewhat regular, and splendidly iri- 

 descent, but when the plaits were unfolded and the 

 membrane stretched, the iridescence vanished. So the 

 varied hues of mother-of-pearl are owing to the folds of 

 an organic membrane. 



The shells of the Gastropoda, or crawling mollusks, 

 have a structure peculiar to themselves, but by no 

 means so much varied as that of the bivalve class. The 

 Strombus gigas, or Queen Conch, the Cassis, or Helmet 



