MOLLUSC A: THEIR SHELLS 59 



tion of dilute acid, the calcareous portion of the nacreous 

 layers being dissolved away, the plates of animal matter 

 fall apart, each one carrying with it the membranous re- 

 siduum of the layer of nacre that was applied to its inner 

 surface. It will be found that the nacre-membrane, cover- 

 ing some of these horny plates, will remain in an undis- 

 turbed condition; and their surfaces then exhibit their 

 iridescent lustre, although 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 which these exhibit are often of the most gorgeous 

 description. 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 iridescence is 

 then completely destroyed. This experiment, then, dem- 

 onstrates that the peculiar lineation of the surface of nacre 

 (on which its iridescence undoubtedly depends, as origi- 

 nally shown by Sir D. Brewster) is due, not to the out- 

 cropping of alternate layers of membranous and calcareous 

 matter, but to the disposition of a single membranous layer 

 in folds or plaits, which lie more or less obliquely to the 

 general surface." ' 



Those beautiful objects so much prized for personal 

 adornment pearls, are concretions accidentally formed 

 within the shells of such mollusks, and are wholly com- 

 posed of the inner layer. Drs. Kelaart and Mobius have 

 recently published some highly interesting observations on 

 the causes both of the iridescence and of the pearly lustre ; 



1 Carpenter, "The Microscope," p. 594. 



