960 MANUAL OF THE MOLLUSCA. 
VALVATA, Miller. Valve-shell. 
Types, V. piscinalis (Pl. [X., Fig. 28). V. cristata (Pl. IX., 
Fig. 29). 
Shell turbinated, or discoidal, umbilicated; whorls round or 
keeled; aperture not modified by the last whorl; peristome 
entire ; operculum horny, multi-spiral. 
Animal with a produced muzzle; tentacles long and slender, 
eyes at their outer bases; foot bi-lobed in front; branchial 
plume long, pectinated, partially exserted on the right side, 
when the animal is walking. Lingual teeth broad; uncini 3, 
lanceolate; all hooked and denticulated. 
Distribution, 18 species. Britain and North America. 
Fossil, 19 species. Wealden—. Britain, Belgium, &c. 
Famity VIII.—NERITIDZ. 
Shell thick, semi-globose; spire very small; cavity simple, 
from the absorption of the internal portions of the whorls ; aper- 
ture semi-lunate ; columellar side expanded and flattened ; outer 
Fig. 111. * 
lip acute; operculum shelly, sub-spiral, articulated. 
At each end of the columella there is an oblong muscular im- 
pression, connected on the outer side by a ridge, on which the 
operculum rests; within this ridge the inner layers of the shell 
are absorbed. 
Animal with a broad, short muzzle, and long slender tentacles; 
eyes on prominent pedicels, at the outer bases of the tentacles ; 
foot oblong, triangular. Lingual dentition similar to the tur- 
binide.* Teeth 7; uncini yery numerous. 
Nerita, Ll. Nerite. 
Liymology, Nerites, a sea-snail, from nereis. 
Type, N. ustulata (Pl. [X., Fig. 35). 
* Fig. 111. Nerita polita, L. (from Quoy and Gaimard), New Irelajd, 
