STRUCTURE OF SHELLS. 233 



This iridescent property may be communicated 

 to shell lac, sealing wax, gmn Arabic, balsam of 

 Tolu, or fusible metal, by taking an accurate cast 

 or impression of the surface of mother of pearl 

 with any one of these substances.* 



Porcellaneous shells have a more uniform and 

 compact texture than those of the former class. 

 The animal matter which unites the carbonate 

 of lime is less in quantity and not so evidently 

 disposed in layers ; but it is more equally blended 

 with the earthy particles, with respect to which 

 it appears to perform the office of a cement, 

 binding them strongly together, although it has 

 of itself but little cohesive strength. The Cyprcea 

 and the Volute are examples of porcellaneous 

 shells. 



In shells of this kind the carbonate of lime 

 assumes more or less of a crystalline arrange- 

 ment; the minute crystals being sometimes in 

 the form of rhombs, and sometimes in that of 

 prisms. In the former case they are composed 

 of three distinct layers, as may be seen by 

 making sections of any of the spiral univalve 

 shells, or simply by breaking them in various 



* When these shells decay and fall to pieces, they separate 

 into numerous thin scales of a pearly lustre. The fine scales thus 

 obtained from the Placuna, or window oyster, are employed by 

 the Chinese in their water-colour drawings to produce the eifect 

 of silver. Some of this powder has been brought to England 

 and used for this purpose. See Gray, Phil. Trans, for 1833. 



