NERITINA. 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, p- ^3 

 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 Nerithm 

 a kind of hinge with the edge of the fluvmtdis. 

 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 lij) slightly toothed; 

 operculum only slightly calcareous, and fur- 



