JKOLLUSCA. Ill 



back on the pillar. The H. stagnalis of Linnaeus, is the 

 type of the genus, of which we possess many British spe- 

 cies. Two of these are truly amphibious, the octona and 

 fbssaria. 



The genus PLANORBIS, instituted by Geoffroy or rather 

 by Petiver, is remarkable for its discoid form, the spire re- 

 volving nearly in a horizontal line, so that all the whorls 

 are obvious on both sides. Cuvier observed that the P. 

 cornea was a sinistral shell, and it remains to be ascertained 

 whether the whorls in the other species have a similar di- 

 rection. We possess several British species of this genus. 



The operculated divisions of fluviatile helices, is more 

 numerous than the preceding, containing at least six genera. 



The genus VALVATA v/as instituted by Muller to include 

 depressed shells with an orbicular mouth, the animal, fur- 

 nished with three tentacula and a plumose appendage, con- 

 sidered as the branchiae. The V. cristata (Helix crist, of 

 Montagu), and piscinalis (the Turbo fontinalis of Montagu, ) 

 are natives of this country. 



The genus, now denominated PALUDINA, instituted by 

 Geoffroy, and afterwards employed by Montfort, is repre- 

 sented by the H. vivipara of Linnaeus. The shell is ovate 

 or oblong, with a regularly elevated rounded spire. The 

 aperture is entire, with the two lips united angularly at the 

 summit. The type of the genus, together with P. tentacit- 

 lata and acula, are natives of Britain. 



In the genus AMPUIXAKIA of Lamark, the shell is glo- 

 bose, the base umbilicated, and the mouth longer than broad. 

 The H. ampullacea is the type of the genus. 



In the genus HELICINA of Lamark, the mouth is semi- 

 lunar, the pillar callous and compressed below. The..//. 



