158 THE HALL OF SHELLS. 



have raised figures of yellow on a pink 

 ground. 



" As lias been seen, tlie shells used for these 

 jewels and called cameo shells are generally 

 the large Cassides. The carving of the cameos 

 is a fine art, carried to an exquisite perfection 

 by the Roman cameo cutters." 



" What gives the different colors to shells ? " 

 asked Tom. 



" That is another of the mysteries iu which 

 Nature has been chary of her confidences," an- 

 swered Miss Bremely. "It seems to be in 

 some way the result of a secreting work done 

 by the border of the mantle. Light also is 

 their painter. We observe that shells near the 

 shore are richer and brighter in coloring than 

 those which are shut out from the sunlight in 

 deep ocean beds. Tropic seas yield us the 

 most brilliantly colored shells, as tropic groves 

 produce birds of gayest plumage. 



" George W. Tryon tells us in his Conchol- 

 ogy that ' bivalves which habitually lie upon 

 one side have the upper valve colored and 

 pictured, while the under valve is white and 

 colorless.' He also quotes authorities who 

 seem to have discovered in certain cases an 

 adaptation in the color of shells to the color of 

 objects upon which they rest. 



