108 



NATURAL HISTORY. 



It is not a rare species, and its colors — white shading into yellow — are 

 not beautiful, but with the negroes it takes the place of gold, silver, and 

 hank hills. Every year large numbers are brought from the South Seas 

 to England, and thence shipped to the west coast of Africa to be used in 

 trade with the natives ; in one year some sixty tons being used. In Africa 

 with a string of cowries one can buy almost anything, — gold dust, ivory, 

 pepper, or a wife. 



The cowries are all carnivorous, and in the tropics they play an impor- 

 tant part as scavengers, as it makes no difference to them whether their 

 prey be alive or dead. In this respect, however, they are excelled by the 

 large and active strombs or wing-shells (Stromlms), which are also tropical 

 forms. One of the largest species occurs in Florida, and its rough shell 

 with its delicate pink interior is one of the most common ornamental 

 shells. While most molluscs crawl with a slow but gradual motion, the 



wig-shells have a 

 hopping or leaping 

 mode of progres- 

 sion which some- 

 times throws it 

 several inches from 

 the bottom. 



Last to be men- 

 tioned of the gas- 

 teropod series are 

 the helmet-shells 

 (Cassis), of which 

 some fifty species 

 are known from 

 the warmer seas. 

 On-' «>!* them which occurs in Madagascar is used for the manufacture 

 of shell-cameos. In this form the outer layer of the shell is white, 

 but beneath it occurs a dark, almost black layer, and it is to this arrange- 

 ment that the whole value of the shell for cameos is due. In the manu- 

 facture of cameos the shell is first cut into pieces of a suitable size 

 by means of an iron wheel fed with emery or diamond dust after the 

 manner so familiar in lapidary work. The artist takes one of these pieces 

 and traces on it the outline of the figure to be reproduced, and then by 

 eans of fine chisels and files carefully works out the design. The thinner 

 makes the white layer, the more the black shows through, and in this 

 way he works out the lights and shades. This operation Requires great 

 at first the workman leaves every projection in excess, reducing 



Fig. 1()'.'. — Helmet-shell (Cassis). 



