188 



SHELLS AND SHELL-FISH. 



PART I. 



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

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

 composed of the freshwater limpets (^fig. 32.), 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 AchatincB, and the 

 sub-families of the Helicidce, 

 will be found to represent each 

 other thus : — 



Genera 

 of the 



I.imnacince. 



Planorbis. 



Ancylus. 



Pototnophila. 



Physa. 



Limrueus. 



Analogies of the Limnacin^. 



Analogies. 



Sub-families Genera 

 of the of the 



Helicidce. Achatina. 



r Shell depressed disk-like ; | Lucernin^. Cyclostoma. 

 l_ aperture round. 3 



f Shell fragile, much smaller 7 Lt^.^^iNiE. Helicina. 

 (_ than the animal. y 



} Shell thin ; aperture large ; ) Lj^n^cin^. ClausUm. 

 i pillar plaited. 3 



(•Outer lip thin; last whorl 7 ^^„^^,^^ 

 i contracted. 3 



Outer lip reflected ; basal 



Achatina. 



{ 



whorl very large. 



i« 



ELiciN.£. Bulimus. 



The general analogies of the whole group may be 

 first slightly touched upon. The LimnacincE occupy 

 that station in the circle of the Helicidce, 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 Planoi-his 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 Ci/rlotus 

 a strong approach is made to this very same structure. 

 The little shells of several of the slugs (Limacincp), if 

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



