188 



SHELLS AND SHELL-FISH. 



for this fold on the pillar^ it could not be distinguished 

 from a Limncsus. The fifth and last type is Ancylus, 

 composed of the freshwater limpets (^fig. S2.), not even 

 mentioned by Cuvier, but con- 

 stituting one of the most re- 

 markable divisions of the whole 

 group. 



(173.) The five genera, 

 placed opposite to those of the 

 genera of AcliatincB, and the 

 sub-families of the HelicidcB, 

 will be found to represent each 

 other thus : — 



Analogies of the Limnacin^. 



Genera 



of the 



Jdmnacince. 



Planorbis. 



Ancylus. 



Potomophila. 



Physa. 



Limrusus. 



Analogies. 



f Shell depressed, disk.like;7 

 X aperture round. j 



C Shell fragile, much smaller 7 

 \ than the animal. 3 



C Shell thin; aperture large;) 

 i pillar plaited. 3 



f Outer lip thin ; last whorl 7 

 contracted. 3 



C Outer lip reflected; basal 7 

 \ whorl very large. 3 



The general analogies of the whole group may be 

 first slightly touched upon. The Limnacince occupy 

 that station in the circle of the Helicida, which corre- 

 sponds to that of the Cephalopoda in the great tribe of 

 Testacea. Hence they are all aquatic animals ; and if 

 this analogy is correct, it follows that Planorbis is the 

 typical group, because, in their flat discoid shells, they 

 perfectly agree with the equally depressed and convo- 

 luted Nautili. We have seen, also, that in Cyclotus 

 a strong approach is made to this very same structure. 

 The little shells of several of the slugs (^LimacincE) , if 

 no regard be paid to the animals, might very well be 



