10. Without an operculum (a horny or Hmy structure attached to the body 



and used to close the aperture of the shell) 11. 



With an operculum 45. 



11. Shell minute, with two whorls, w^idcr than high; base of columella 



broadly expanded; Alabama 



l\[eoplanorhis tantillus Pilsbry 

 With three or more whorls 12. 



12. Shell left'handed (coiled counter'clockwise) or else disc-shaped, coiled 



almost or entirely in one plane 13. 



Shell normally right-handed (coiled clockwise), globular to high 32. 



13. Shell left-handed or apparently right-handed, usually coiled in one plane, 



but occasionally with only the small upper whorls so coiled — Ramshom 



Snails 14. 



Shell left-handed; spire acute — Tadpole Snails 26. 



14. With small teeth back within the aperture; seldom over %" wide; 



widely distributed 15. 



Aperture not toothed; size various 16. 



15. Lip thin 



Planorhida armigera (Say) Toothed Ramshorn 

 {Segmentina armigera (Say) ) 

 Lip with a thickened crest within 



Planorhida crassilahris (Walker) 

 (Segmentina crassilahris Walker) 



16. Shell left-handed 17. 

 Shell apparently right-handed 19. 



17. With only the small upper whorls coiled in one plane, so that the shell is 



Physa-shaped; Florida 



Helisoma scalare (Jay) 

 Shell disc-shaped; west to the Rockies 18. 



18. Lip slightly flaring; whorls four and one-half or more, and rather loose; 



smaller whorls distinct 



Helisoma campanulutum (Say) Bell-mouth Ramshorn 

 (Planorhis campanulatus Say) 

 Aperture not flared; coils tight and four and one-half or less; smaller 

 whorls blending 



Helisoma trivolvis (Say) Common Ramshorn 

 (Planorhis trivolvis Say) 



19. Aperture flaring; lip thickened; whorls keeled on both sides, with the 



periphery curved or flat; widely distributed 



Helisoma anceps (Menke) Keeled Ramshorn 

 (Planorhis antrosus Conrad) 

 (Planorhis hicarinatus Say) 

 Lip simple 20. 



20. Shell flat below, with the base of the aperture in the same plane as the 



base of the shell; southern states, Florida to Texas 

 Drepanotrema cultratum (Orb) 

 Shell more or less flattened below, with the base of the aperture below 

 the base of the shell 21. 



115 



