62 GENERA OF SHELLS. 



ViTRiNA. Shell small, very thin, depressed, 

 terminated superiorly by a short spire ; last 

 whorl very large ; aperture large, rounded- 

 oval ; left margin arquated, slightly inflexed. 



V. pellucida. 



Order III. — TRACHELIPODA. 



Shells spirivalve, unsheathing. 



Section I. — TRACH. PHYTIPHAGA. 



Aperture of shell entire ; base without any ascending notch 

 or canal. 



COLIMACEA. 



Shell spirivalve, icith no other projecting parts on the exte- 

 rior than the striœ of groivth ; right margin of the aper- 

 ture often recurved or reflexed outicards. 



Helix. Shell orbicular, convex or conoid, 

 sometimes globular, spire not much elevated ; 

 aperture entire, broader than long, very oblique, 

 contiguous to the axis ;. margins disjoined by 

 the prominence of the penultimate whorl. 



The helix is distinguished from the pupa, by the 

 general form of the shell, which is never cylindrical, 

 and by the borders of the aperture being disunited ; 

 from the bulimus, by the aperture being rather trans- 

 verse than longitudinal, and its plane very oblique and 

 almost perpendicular to the axis of the spine ; and 

 from planorbis by the left margin of the aperture 

 being contiguous to the axis of the shell. The margin 

 of the adult helix is always reflected outwards, which 

 it never is in aquatic shells. The helix is readily 



