NEKITESTA. 
43 
very numerous^ sub-similar, very slender, curved 
at the top ; the inner one on each side largest. 
Body and shell spiral. Gill single, formed of 
small leaves placed symmetrically one against 
the other, and forming a transverse mass on the 
back under the mantle. Sides of the foot with- 
out any membranaceous fringe, with beards on 
its upper surface. Foot oblong, shorter than 
the shell, circular, truncated behind, pig, 13 . 
Shell hemispherical, flat beneath ; spire 
lateral ; inner lip flattened, transverse; 
outer arched ; inner surface not pearly. 
Operculum with a process on the 
inner side under the nucleus, forming Neritina 
a kind of hinge with the edge of the 
inner lip of the shell. (Fig. 13.) 
The peculiar structure of the operculum makes 
this family more closely resemble the bivalve shells : 
the processes appear to answer the same purpose 
(that of keeping the two parts in their proper situa- 
tion) as the teeth of the hinges in the bivalves. 
In the exotic genus Navicella^ which, on account 
of its large mouth, has been confounded with the 
Patellce^ the processes occupy the greater part of the 
operculum. 
There is only a single fluviatile genus of this 
family found in Britain. 
1. Neritina Lam. (Neritine.) 
Shell half-ovate, thin; inner lip slightly toothed; 
operculum only slightly calcareous, and fur- 
