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NERITACEA.— Four Genera. 



Shell semifluviatile or marine, semiglobular or flattened oval, without col- 

 umella, the left margin of the aperture resembling a half partition. 



I. FRESH WATER SHELLS. 



Navicella. Shell elliptical or oblong, convex above, summit straight, 

 depressed to the margin, concave beneath; the left lip flattened, sharp, nar- 

 row, without teeth, transverse. A solid, flat operculum, having an acute and 

 lateral tooth. 



N. elliptica, 



N. lineata, 



N. tessellata, 



N. porcellana. 



Neritina. Shell thin, semiglobular or oval, flattened beneath, not urn- 

 bilicated; aperture semicircular, the left margin flattened, sharp; no teeth or 

 crenulations on the surface of the right margin. Operculum furnished with a 

 lateral tooth. 



N. perversa, 



' pulligera, 



' dubia, 



' zebra, 



' zigzag, 



' gagates, 



' lugubris, 



II. MARINE SHELLS. 



Nerita. Shell solid, semiglobular, flattened beneath, not umbilicated; 

 aperture entire, semicircular, the left margin flat, septiform, acute, often 

 toothed; teeth or crenulations on the inner surface of the right lip. Oper- 

 culum with a projecting tooth. 



Natica. Shell subglobular, umbilicated; aperture entire, semicircular; 

 left lip oblique, not toothed, callous; the callosity modifying the umbilicus, 



