108 



PORCELLANEOUS SHELLS. 



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, each of which is formed of very thin plates, 

 marked by oblique lines, which 

 show the direction of the crystal- 

 line fibres, so arranged as to give 

 strength to the shell; and, what 

 is remarkable, on a principle 

 which has been latterly applied 

 to the building of ships. When 

 the form of the crystals is pris- 

 matic, the fibres are short, and 

 the prisms are generally hexago- 

 nal. In one shell, brought from 



Sumatra, the crystalline appearance was so perfect, that 

 some fragments of it were mistaken for a mineral pro- 

 duction. In the earliest state of a shell, called the 

 cyprea — known by its lips being rolled inwards, and 

 both of them being toothed — its substance is very thin, 

 almost colourless, and dull ; the mouth is rather wide, 

 the outer lip not rolled inwards, but having a sharp 

 edge, and neither lip toothed. In the second period 

 of growth, the shell begins to approach the general 

 form that marks the genus ; the lips are curved inwards, 



