52 NERITID^. 



Only three families of this Order inhabit the fresh 

 waters of this country. They are, — 



I. Neritid^. 



II. PaLUDINIDyE. 



III. Valvatid^. 



All these freshwater Snails have two tentacles, and the 

 same number of eyes, which are placed at the base of the 

 tentacles. Their shells are furnished with an epidermis 

 and operculum. 



Family I. NEEITID^. 



Body oval, having a short spiral turn at the end : eyes 

 placed outside the tentacles at their base : gill inside the 

 mantle : sexes separate. 



Shell semiglobose, with an excentric spire and a semicircu- 

 lar mouth : operculum having an excentric and short spire, 

 and furnished imderneath with an apophysis or projecting pro- 

 cess which locks into the columellar or piUar lip. 



Although the members of this family are very 

 numerous and widely dispersed beyond the limits of our 

 seas, we have only a solitary representative, forming the 

 single species of one genus. 



NERITI'NA*, Lamarck. PI. III. f. 1, 2, 3, 4. 



Body furnished with a strong and prominent snout or 

 muzzle : tentacles long : eyes placed on footstalks : foot broad. 



Shell triangular-oblong : ojperculum calcareous and sohd. 



The mollusks of this genus inhabit waters which have 

 a stony or gravelly bottom. Their habits are sluggish ; 

 raising their shell but little dming their march, and then 

 only showing their tentacles, eyes, and the front of their 



* Diminutive of Nerita, a genus of marine shells. 



