192 



POPULAR CONCHOLOGY. 



Family 5. — Neritida* 



The shells constituting this family are chiefly 

 distinguished by the smallness and flatness of 

 the spire, which appears placed on one side; 

 the mouth is generally semicircular, being half 

 closed by a flat partition, which forms the left lip. 

 Some are marine, and others fluviatile ; they 

 have all an operculum of a peculiar form, hav- 

 ing a little projection which slips under the 

 partition of the left lip, and thus securely 

 closes the mouth. There are four genera. 



59. Navicella. — Shell oblong, convex 

 above, w T ith the apex straight and 

 bent down to the edge, not spiral ; 

 inner lip thin, and assuming the 

 form of a plate across one end of 

 the aperture ; shell altogether much 

 resembling a Patella, but having 

 an operculum which is shelly. 

 Animal, head very large, having two tenta- 

 cula, the eyes placed on the summit of two 



* Les Neritacees, Lamarck. 



