148 THE HALL OF SHELLS. 



Soft tints, a sMning and porcellaneous sur- 

 face, and handsome markings are some of the 

 characteristics of the shells of the family Olivi- 

 dse. The animals of this family are exclusive- 

 ly tropical, and upon sandy flats of warm seas 

 they revel, moving about with considerable 

 quickness, burrowing under the wet sand when 

 the tides are low, and leaving no trace of their 

 hiding places. 



The number of both fossil and living spe- 

 cies is large, and it is impossible to tell all the 

 shades of coloring and the various patterns of 

 banding and nebulous painting to be found 

 upon them, 



A curious characteristic of this group of 

 mollusks is its dual picturing. Underlying 

 the external porcellaneous and decorated sur- 

 face of the shell is another layer decorated 

 with an entirely different pattern, the two lay- 

 ers making their different growths at the same 

 time. This under layer is, however, never ex- 

 posed except by the outer one becoming worn 

 off or when acids are used in removing it. 



The external colorings and markings of 

 these shells have also been obliterated by the 

 application of heat. So the Pacific islanders, 

 who delight in these shells for ornamenting 

 their bracelets and belts, but prefer them pure 



