CHAP. VI. THE PHYTOPHAGA GENERALLY. 159 



whole are regular spiral shells, — a character by which 

 they are separated from the Scutihranchia, or limpets. 

 The great majority are external shells, and all but one 

 small group — the slugs and their representatives, — have 

 their habitation sufficiently large to contain the entire 

 body. Their modes of breathing and propagation are 

 various, and greatly diversified even in genera close to 

 each other ; so that nothing can yet be determined of 

 a general nature on these points. Although the greater 

 part of the Helicidce, or snails, live upon land, and 

 therefore breathe differently, a portion of the same 

 family live in fresh water ; and even some of the slugs 

 appear to be found only on the sea shore : the rest of 

 the tribe, excepting the Melaniance, are all marine. 



(149.) The secondary divisions or families are first 

 distinguished by the formation of their animals, and 

 secondly, by the nature of the shells : this latter mode, 

 however, will lead to great confusion, if not regulated 

 by the former ; since there are many genera in different 

 families, Avhose shells are nearly of the same form, 

 although inhabited by very different mollusks. As this 

 will become evident when we enter into their details, 

 we shall at once characterise the chief divisions. The 

 first, or the most typical, are the Helicidcs, which in- 

 cludes the testaceous land snails, the naked slugs, and 

 the pulmonary fluviatile shells.* The second, or Tro- 

 chidce, are entirely marine : the shell is almost always 

 perlaceous ; the body of the animal is furnished with 

 lateral filaments, and its mouth with lips, as in the 

 last ; and the shells of both are turbinated and spiral. 

 The third, or Haliotidce, are known by their flat ear- 

 shaped shells, having only the rudiment of a spire, and 

 without any pillar ; hence they may be even called 

 spiral limpets. The fourth is the Naticidfe, or nerits, 

 where the spire also is very small, but the pillar is 

 always thick ; the exposed part, or inner lip, often very 

 broad; and the animal slug-shaped. The fifth, or 



* This group corresponds with a few trifling exceptions, to the Pulmo- 

 naria of the Eegne Animal.. 



