MOLLUSCA. lil 
back on the pillar. The H. stagnalis of Linneeus, is the 
type of the genus, of which we possess many British spe- 
cies. Two of these are truly amphibious, the octona and 
-fossaria. 
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 was instituted by Miiller to include 
depressed shells with an orbicular mouth, the animal, fur- 
nished with three tentacula and a plumose appendage, con- 
sidered as the branchiz. The V. cristata (Helix crist. of 
Montagu), and piscinalis (the Turbo fontinalis of Montagu, ) 
are natives of this country. 
The genus, now denominated PaLup1Na, instituted by 
Geoffroy, and afterwards employed by Montfort, is repre- 
sented by the @. vivipara of Linneus. 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. tentacu- 
lata and acuta, are natives of Britain. 
In the genus AmMpuLuLaARiA 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 Hexicrna of Lamark, the- mouth is semi- 
lunar, the pillar callous and compressed below. The #. 
