188 



SHELLS AND SHELL-FISH. 



PART I. 



for this fold on the pillar, it could not he distinguished 

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

 composed of the freshwater limpets (fig. 32.), not even 

 32 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 Achatina., and the 

 sub-families of the Helicidte, 

 will be found to represent each 

 other thus : 



Analogies of the LIMNACIN^J. 



Genera 



of the 



Limnacints. 



Planorbis. 



Ancylus. 



Potomophila. 



Physa. 



Limrueus. 



Analogies. 



Sub-families Genera 

 of the of the 



Helicidce. Achatinee. 



The general analogies of the whole group may be 

 first slightly touched upon. The Limnacina; 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 (Limacinai), if 

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



