MOLLUSCA : TIIEIK SHELLS. 49 



consists of a, inultitude of layers of carbonate of lime, 

 alternating with animal membrane, and that the pres- 

 ence of the grooved lines on the most highly polished 

 surface, is duo to the wearing away of the edges of the 

 animal laminae, whilst those of the hard calcareous 



larainaa stand out There is one shell, however, 



the well-known Haliot'is splendent, which afibrds lis the 

 opportunity of examining the plaits without any dis- 

 turbance of the arrangement, and thus presents a clear 

 demonstration of the real structure of nacre. This shell 

 is for the most part made up of a series of plates of 

 animal matter, resembling tortoise-shell in its aspect, 

 alternating with thin layers of nacre ; and if a piece of 

 it be submitted to the action of dilute acid, the calcare- 

 ous portion of the nacreous layers being dissolved away, 

 the plates of animal matter fall apart, each one carry- 

 ing with it the membranous residuum of the layer of 

 nacre that was applied to its inner surface. It will be 

 found that the nacre-membrane covering some of these 

 horny plates, will remain in an undistui'bed condition ; 

 and their surfaces then exhibit their iridescent lustre, 

 althouo-h all the calcareous matter has been removed 

 from their structure. On looking at the surface with 

 reflected light under a magnifying power of seventy- 

 five diameters, it is seen to present a series of folds or 

 plaits, more or less regular ; and the iridescent hues 

 Avhicli these exhibit, are often of the most gorgeous de- 

 scription. If the membrane be extended, however, 

 with a pair of needles, these plaits are unfolded, and it 

 covers a much larger surface than before ; but its irides- 

 cence is then completely destroyed. This experiment, 

 then, demonstrates that the peculiar lineation of the 

 surface of nacre (on which its iridescence undoubtedly 

 3 



