158 SHELLS AND SHELL-FISH. PART I. 



of their animals_, than of their shells ; and yet, when 

 we look to the young shells of Aporrhais, and of certain 

 Terebralicp, they might very readily be placed in the 

 same genus : the cuts (^fig. 15.) already given at p. 142. 

 will bring this fact more prominently before the reader. 

 We are not so confident,, however, in regard to the abso- 

 lute affinities of certain species of Potomis, which have 

 the aperture oval, and the basal channel almost oblite- 

 rated : they may, indeed, be the connecting links between 

 Pol. muricata and the genus Clavicantha ; but until La- 

 marck's Pleurotoma echinata and auriculifera are better 

 known, some doubt must hang over them : on the other 

 hand, this is the point of union between the two great 

 tribes of the Zoophaga and the Phytophaga ; so that the 

 very first genus which we should have to notice, if we 

 followed the thread of affinity in this direction, would 

 be that of Melania, in the family of the Turhidce, — be- 

 ing precisely that to which these aberrant species of 

 Potomis^ with an oval aperture, would seem to belong. 



CHAP. VI. 



ON THE PHYTOPHAGOUS TRIBE OF SHELL-FISH. THE PRIMARY 



DIVISIONS OR FAMILIES. THE HELICID,^, OR LAND AND FRESH- 

 WATER SNAILS. 



(148.) The second great tribe of the gastropod shell- 

 fish is that to which, after the illustrious Lamarck, we 

 apply the name oi Phytophaga, since they not only sub- 

 sist upon animal, but also vegetable, substances : they 

 are easily known from the truly carnivorous tribe, by 

 being destitute of the respiratory siphon, and in the 

 mouth not being retractile, or prcboscidiform. In 

 regard to the shells, the differences are equally well 

 defined. From the absence of the respiratory tube, the 

 aperture is round and entire, or at most is only slightly 

 notched in such genera as unite the two groups. The 



