CLASS I. 

 MOLLUSCA GASTEROPODA. 



ORDER TRACHELIPODA. 



Animals provided with gills for breathing fresh water. 



Family I.— NERITACEA. 



The species of this family are inhabitants of both fresh water 

 and the ocean. Their form is semiglobular, or oval ; destitute 

 of a columella; with aperture marginated, and transverse; always 

 provided with an operculum. 



Genus L_NER1TINA.— Lamarck. 



Shell thin, external surface generally smooth, and frequently 

 covered with a strong, horny epidermis ; spire mostly very short, 

 sometimes nearly concealed, and at others obsolete; aperture 

 semicircular; outer hp plain, sharp, and destitute of teeth or 

 crenulations internally, but within the lower region of the aper- 

 ture, it is provided with a somewhat elongated, transverse pro- 

 minence, which seems the fulcrum for the articulation of the 

 operculum ; inner lip flattened, and reflected on the columella, 

 and placed obliquely to the axis of the shell; edge generally 

 short, and dentated or crenulated; as the animal increases in 

 dimensions, that part of the columellar lip is absorbed, which 

 makes it appear as having no columella ; operculum testaceous, 

 semicircular, closing the aperture entirely, covered with a horny 

 epidermis, and provided internally at the lower end with a tooth- 

 like appendage, which fits into a hollow between the prominence 

 and lip. 



1. Neritina fluviatilis, pi. I, f. 1, 2, 3. 

 Neritinajluviatilis, Turton, Man., p. 13, f. 124; Alder, Mag. 

 Zool. and Bot., II, p. 117; Thompson, Ann. and Mag. Nat. 

 B 



