260 MANUAL OF THE MOLLUSCA. 



Valvata, Miillor. Valve-sliell. 



Types, V. piscinalis (PI. IX., Fig. 28). V. cristata (PI. IX., 

 Pig. 29). 



Shell turbinated, or discoidal, umbilicated ; wliorls round or 

 keeled; aperture not modified by tbe 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. 



Pamily YIII. — Neritid^. 



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- 

 hinidce. Teeth 7 ; uncini very numerous. 



Nerita, L. Nerite. 



Etymology, Nerites, a sea-snail, from nere%%. 

 Type, N. ustulata (PI. IX., Fig. 35). 



* Fig, 111. Nerita polita, L. (from Quoyand Gairaard), Newlrelaad. 



