ORDER PECTINIBRANCHIATA. 59 
is round, and furnished with an operculum; the animal, which 
has slender tentacula, with the eyes at their anterior base, re- 
spires by means of gills. In a species found in France, 
Valv. cristata, Miill., Drap. 1. 32,33; Gruet-Huysen. Act. 
Nat. Cur. X. pl. xxxviii., the gills, formed like a feather, pro- 
ject from under the mantle, and float externally, vibrating with 
the breathing of the animal ; on the right side of the body is 
a filament, which resembles a third tentaculum ; the foot is 
divided anteriorly into two hooked lobes; the penis of the 
male is slender, and reflected into the branchial cavity ; the 
shell, which is hardly three lines broad, is greyish, flat, and 
umbilicated. Found in stagnant water. Add Valvata pla- 
norbis, Drap., &c. 
It is here that we must place the completely aquatic shells, 
or those respiring by gills, which belonged to the old genus 
HELIX: 7. e. those in which the penultimate whorl forms, as 
in the helices, lymnez, &c., a depression, which gives the 
aperture more or less the form of a crescent. 
The first three genera are still closely allied to turbo. 
PALUDINA, Lam. 
This genus has been lately separated from the cyclostoma, 
because there is no ridge round the aperture of the shell; 
because there is a small angle to that aperture, as well as to 
the operculum; and finally, because the animal, being pro- 
vided with gills, inhabits the water like all other genera. of 
this family. It has a very short snout, and two pointed ten- 
tacula ; eyes at the external base of the latter, but on no par- 
ticular pedicle ; and a small membranous wing on each side of 
the fore-foot of the body. The anterior edge of the foot is 
double, and the wing of the right side forms a little canal, 
which introduces water into the respiratory cavity, the inci- 
pient indication of the siphon in the following family. 
The common species, Helix vivipara, L.,Drap.,whose smooth 
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