Industrial and Manufacturing Uses of Shells. 273 



more nearly to the nature of nacreous than of porcelanous 

 material. 



The word cameo, derived from an Arab word, signifying 

 bas-relief, was originally restricted to hard stones, such as 

 onyx, sardonyx, etc., engraved in relief; but the name has 

 since been extended to gems cut on shell, lava, and other 

 substances. 



The good workman always carefully puts his work on 

 the shell in such a manner that the direction of the laminae 

 of the central coat is longitudinal. In cameos the central 

 layer forms the body of the relief, the inner layer being the 

 ground, and the outer the third or superficial colour, which 

 is sometimes used to give a varied appearance to the sur- 

 face of the figure. The cameo-cutter selects from the 

 shells which have the three layers : — i. Those which have 

 the layers strongly adherent together, for if they separate 

 his labour is lost ; 2. Those in which the middle layer is 

 thick ; 3. Those in which there is a good distinction of 

 colour between the layers ; and 4. Those in which the 

 . inner layer is of the colour suited to his purpose. 



The kinds now employed, and which experience has 

 taught him are best for his purpose, are — i. The bull's 

 mouth (Cassis rufa), which has a red inner coat, or what is 

 called a sardonyx ground. The shell is red with several 

 series of thick knobs, the outer lip deep yellowish red. 

 2. The black helmet (C Madagascariensis), which has a 

 blackish inner coat, or what is called an onyx ground, and 

 shows up white upon a dark claret colour. The shell is 

 often nearly a foot long. 3. The horned helmet {C. 

 comutd), white with an orange yellow ground ; and 4. The 

 queen conch (Strotnbus gigas), with a pink ground. This 

 shell is about 10 inches long, aperture rose-coloured, lip ex- 

 tremely broad, rounded above. S.piigilis, another species, 



