68 TABLE OF GENERA. 



*** Shell smooth, half-ovate ; mouth half circular ; inner lip 



transverse, acute. 

 1. Neritina. 



c. Shell moderately thick ; edge of lips more or less thickened 

 and reflexed. 



* Mouth transverse. 



10. Helix. Shell suborbicular or conical ; mouth lunate, or ovate 

 or circular. 



* * Mouth oblong, longitudinal. 



13. Bulimus. Shell oblong, striated ; mouth toothless. 



14. Zua. Shell oblong, polished ; mouth margined, tooth 



less. 



15. Azeca. Shell oblong, polished ; mouth margined, toothed. 

 21. Carychium. Shell oblong, smooth, white ; mantle oblong, 



margined, sinuous. 

 23. Conovulus. Shell oblong, smooth; mantle narrow; pil- 

 lar obliquely plaited. 



17. Pupa. Shell subcylindrical, striated, blunt ; mouth margined, 



mostly toothed. 



18. Vertigo. Shell subcylindrical, striated, blunt ; mouth 



dextral or sinistral, margined, mostly toothed. 



19. Balea. Shell turreted, striated; apex acute; mouth sinistral, 



not plaited. 



20. Clausilia. Shell fusiform, striated; apex acute; mouth 



sinistral, plaited. 



E. Shell discoidal ; ivhorles revolving nearly on the same line. 

 * Mouth lunate, sinistral, edge not continued. 



30. Planorbis. Cavity simple. 



31. Segmentina. Cavity crossed with transverse ridges. 



* * Mouth circular, dextral, edge continued. 

 5. Valvata. 



