148 THE HALL OF SHELLS. 
Soft tints, a shining and porcellaneous sur- 
face, and handsome markings are some of the 
characteristics of the shells of the family Olivi- 
de. 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 livig 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 
