72 GENERA OF SHELLS. 



of the outer lip. Operculum with a lateral 

 tooth. 



N. perversa, pulligera, dubia, zebra, zigzag, gagates, lugu- 

 bris, corona, brevispina, crepidularia, auriculata, Domingen- 

 sis, fasciata, lineolata, semi-conica, strigilata, meleagris, 

 virginea, fluviatilis, viridis, Bœtica. 



(2.) Marine shells. 



Nerita. Shell solid, semiglobose, flattened 

 beneath, without umbilicus. Aperture semi- 

 orbicular, entire ; pillar lip flattened, septiform, 

 sharp, often toothed ; teeth or crenulations on 

 the internal face of the outer lip. Operculum 

 with an apophysis. 



The spire is but little elevated above the last whorl. 

 The operculum is crescent shaped, horny or calcareous, 

 and exactly closes the aperture. The nerita differs 

 from neritina by the internal face of the right margin 

 being crenate, and from natica by having- no umbilicus. 



N. exuvia, textilis, undata, peloronta, chlorostoma, atrata, 

 polita, albicilla, chamoeleon, versicolor, Ascensionis, Malac- 

 censis, lineata, scabricosta, plicata, lessellata, signala. 



Natica. Shell subglobose, umbilicate. Aper- 

 ture entire, semi-circular ; pillar lip oblique, 

 not toothed, callous : callosity modifying the 

 umbilicus and sometimes covering it. Outer 

 lip sharp, always smooth within. An opercu- 

 lum. 



Distinct from nerita by the umbilicus, and by the 

 columellar margin not being crenate but smooth and 

 callous, and by the smoothness of the interior of the 

 right lip. 



