STRUCTURE OF SHELLS. 209 



balsam of Toln, or fusible metal, by taking an ac- 

 curate 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 dis- 

 posed 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 as- 

 sumes more or less of a crystalline arrangement ; 

 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 imi valve shells, or simply by breaking 

 them in various directions. Each layer is composed 

 of very thin plates, marked by oblique lines, which 

 show the direction of the crystalline fibres. The 

 direction of the layers and fibres is also rendered 

 manifest by the planes of cleavage, when they 

 are broken into fragments. The plates of the 

 outer and inner layers are always directed from the 

 apex of the cone to its base, so as to follow the 



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

 numerous thin scales of a pearly lustre. The fine scales thus ob- 

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

 Chinese in their water-colour drawings to produce the effect of 

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

 for this purpose. (Gray; Phil. Trans, for 1833, p. 794.) 



VOL. I. P 



