212 SHELLS AND THEIR ANIMALS. 



History we read that the suffrages of the 

 Athenians were on certain occasions marked 

 upon a shell. Pearls, the effect of disease in 

 certain Mollusca, form a portion of the revenue 

 of many kingdoms, and are among the most 

 costly ornaments of the noble and the wealthy. 

 In former times the dye extracted from the Pur- 

 pura was a most valuable article of commerce, 

 but it has now been superseded by that which 

 the Cochineal furnishes in so much greater 

 abundance. 



