80 PHYTOPHAGA. 



Fam. 1. NERITID.E. 



The sides of the body simple, without any elongated 

 filaments; tentacles awl-shaped, eyes on short 

 pedicels at the outer side of their base (p. 78. f. 1, 

 2.) ; the shell ovate-conical; mouth half-ovate, with 

 a flattened transverse sharp-edged inner lip ; the 

 operculum spiral, half-ovate, and furnished with 

 two internal processes on its front edge, forming a 

 kind of hinge on the sharp edge of the inner lip 

 of the shell. See p. 78. f. 3. 



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 situation) as the 

 teeth of the hinges in the bivalves. 



In the exotic genus Navicetta, which, on account of 

 its large mouth, has been confounded with the Patella^ 

 the processes occupy the greater part of the oper- 

 culum. 



There is only a single fluviatile genus of this family 

 found in Britain. 



1.1. Neritina Lam. (Neritine.) 



Shell half-ovate, thin ; inner lip slightly toothed ; oper- 

 culum only slightly calcareous, and furnished with a 

 sharp flexible outer edge ; foot short, rounded at each 

 end. This genus is separated from the marine 

 Nerita by the pillar being sharp, only slightly 



