ORDKR TRACHELIPODA. 41 



VEGETABLE FEEDERS, (Phytophagi.) 



Those genera of the Traclielipofls which live on 

 vegetable substances have no projecting siphon, but 

 possess a mouth furnished with jaws ; they are in 

 general land-shells, and consequently, the air which they 

 breathe is conveyed directly to their branchiae. Some 

 of their tribes however, live in fresh water, either in 

 running streams or in stagnant pools : among these, 

 some breathe water and others air. These last are 

 obliged frequently to come to the surface for the purpose 

 of breathing, — others again inhabit salt water, and are 

 unable to exist out of that element. 



The Marble Turban-Shell, {Turbo mavmoratvs.) 



There are as many as thirty or forty species of tlie 

 Turban-shell. The Marble Turban, Turbo marmoratus, 

 is the largest, being as much as four inches across ; it 

 is found in the Indian Ocean. The colour of this shell 

 is of a brownish green, of greater or less intensity. It 

 is ornamented by eight or ten narrow transverse belts, 

 consisting of a series of white or brownish spots. This is 

 one of those shells whose substance, after the outer coat 

 is removed, is of the nature of mother-of-pearl. 



The Turbo littoreus (the Shore Turban), is the well- 

 known periwinkle, with which our rocky coasts abound. 



The Imperial Top-Shell, {Trochus imperialis.) 

 These shells have received their name from their 

 resemblance in form to a boy's top. They are all 

 marine shells, and the apex of their spire is always very 

 sharp-pointed. In some places they are called flat- 



