2l8 MOLLUSKS: GASTROPODS. 



from their shape ; the Cerithiums from a word which 

 means a Iwrn. The Melanias are fresli-water shells, 

 common in the Western and Southern States. 



Wentletraps, etc. 



The Tower Shell and the Worm-shell of the West 

 Indies, and the true Wentletraps of the tropical and 

 temperate seas, belong in this group. The Royal 

 Staircase, or Wentletrap, Figure 390, was formerly 

 very valuable, and has been sold for a hundred pounds 

 sterling, although now worth only a few dollars. 



Periwinkles. 



Periwinkles live in the sea near the shore. Two 

 species are shown in Figures 392 and 393. They feed 

 on alg£e, — marine plants. 



River Snails. 



These live in fresh water, have the shell covered 

 with a green skin, bring forth their young alive, and 

 the embryo snails, even before birth and when so small 

 that they can scarcely be seen without a microscope, 

 have a perfectly formed shell, a " foot" and operculum, 

 delicate tentacles, and distinct black eyes. 



Violet Snails. 



The Violet Snails live together in large numbers, in 

 the open sea, where they float by means of many air- 

 \-essels, which form a raft, a, Figure 402. The shell 

 is thin, the base deep violet color, and the spire almost 

 white. They yield a violet dye. 



