CHAP. VI. 



THE LIMNACIN^. 



187 



great divisions of the Helicid^, which include all the 

 land-shells and slugs, we may enter upon the fifth and 

 last, which are aquatic. 



(172.) TheLiMNAciN^is that sub-family which con- 

 tains the whole of those spiral fluviatile shells which, 

 like all the Helicidce, respire by a lateral perforation. 

 It is difficult for the student to distinguish these river 

 shells from those of the fluviatile genera in the next 

 family ; but they are in general much thinner, and in 

 no instance have the Limnacinw even the vestige of an 

 operculum. Like the Cyclostomce, these animals have 

 but two tentacula ; but they are, in general, very short 

 and broad, having the eyes at their base : their shells are 

 all very thin, devoid of any bright colour, and the aper- 

 ture is always simple, — that is, without any thickened 

 margin. They appear to arrange themselves 

 into the following genera : — 1 . Planorbis, 

 where the shell is disk-shaped, and with- 

 out any pillar, as in our common P. cor- 

 neus. (fig. 30.) 2. Limnceus, having a 

 very large and wide aperture on the right 

 side, and the basal whorl so large as to 

 contain the whole animal. 3. Physa, where 

 the aperture is much smaller, the base contracted, and 

 the mantle so large that its two lobes fold over the 

 shell. In the fourth sub-genus Potomophila, the shell 

 very much resembles the last, but the pillar is marked 



with a distinct plait or fold in 

 the middle ; the only species we 

 are yet acquainted with, istheCo- 

 novulus buHmo'ides * of Lamarck. 

 {fig. 31.) In this, as in innu- 

 merable instances, analogy has 

 been mistaken for affinity. It 

 is clear there is a resemblance between this and the 

 sub-genera Auricula and Conovidus ; but we think 

 there is no doubt that this is a fluviatile shell, and, but 



* Ency. Meth. pi. 459. fig. 7- 



