64 GENERA OF SHELLS. 



C. acutissima, albilabris, angistoma, labyrinthus, lucerna, 

 inflata, Gualteriana, bicolor, mauritiana, Madagascarensis, 

 marginata, lychnuchus,planata, planaria, hispidula, lapicida, 

 albella, elegans. 



Anastoma. Shell orbicular, spire convex, 

 obtuse ; aperture rounded, dentated on both 

 sides, within ringent, looking towards the 

 spire ; lip reflected. 



The singular position of the aperture, directed up- 

 wards towards the spine of the shell, is the character- 

 istic of this genus, and peculiar to it. 



A. depressa, globulosa.* 



Helicina. Shell subglobose, imperforate; 

 aperture entire, semi-oval ; columella callous, 

 transverse, flattened, margin acute, forming an 

 angle at the inferior base of the hp ; operculum 

 horny. 



The helicinae resemble small neritae, but the latter 

 are sea shells. They are distinguished from the helices 

 by their transverse columella, whicli is callous, de- 

 pressed, and thin at the lower part. They are land 

 shells, and inhabit warm climates. 



H. neritella, striata, fasciata, viridis, — major, orbiculata, 

 occulta. 



* PoLYGYRA . — Say. Shell discoidal, more or less carinated on 

 the upper edge of the whorls ; umbilicated ; aperture longer than 

 broad 5 lips thickened, toothed, or folded and continued, folds 

 concave beneath ; pillar lip raised above the preceding whorl, and 

 concave beneath. 



P. auriculata, avara, septemvolva, plicata. 



