64 CLASS GASTEROPODA. 
water. The head, a cylindrical proboscis, terminated by a 
vertically cleft mouth, and armed with little hooks, has a bifur- 
cated tentaculum on each side. 
The common species, Helix Janthina, Lin., has a pretty 
violet shell, and is very abundant in the Mediterranean. When 
the animal is touched, it sheds a thick fluid, of a deep violet 
colour, that dyes the surrounding water. 
NERITA, Lin. 
The columella of the neritz being in a straight line, renders 
the aperture semi-circular or semi-elliptical. This aperture is 
generally large in comparison with the shell, but is always 
furnished with an operculum that completely closes it. The 
spire is almost effaced, and the shell semi-globular. 
NATICcA, Lam., 
Are nerite with an umbilicated shell. The animal of the 
species known has a large foot, simple tentacula, with the eyes 
at their base, and a horny operculum. 
NERITA, Lam. PEULORONTA, Oken., 
The umbilicus is wanting, the shell thick, columella den- 
tated, and operculum stony; the eyes of the animal on pedi- 
cles, by the side of the tentacula, and a moderate foot. 
VELATA, Montf.,, 
Where the side of the columella is covered with a calcareous, 
thick, and convex layer, is distinguished from the above, but 
perhaps without any good reason. (Nerita perversa, Gmel.) 
A large fossil species. Also the 
NERITINA, Lam., 
Where the shell has no umbilicus, and is thin, with a horny 
operculum; the animal is like a true nerita, and most 
