52 NERITID^. 



Only three families of this Order inhabit the fresh 

 waters of this country. They are^ — 



I. Neritid^. 



II. PALUDINIDiE. 



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 underneath mth an apophysis or projecting pro- 

 cess which locks into the columellar or pillar 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. 



NERITrNA*, 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 : ojoerculum calcareous and sohd. 



The mollusks of this genus inhabit waters which have 

 a stony or graveUy bottom. Their habits are sluggish ; 

 raising their shell but little during their march, and then 

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



* Diminutive of Nerita, a genus of marine shells. 



