GENUS XXIX. HELIX. 129 



out at pleasure into a heap of little bladders, and which assists 



it in swimming. 



Linnaeus describes 59 species, Gmelin has added 168, and 



Dr. Turton 26. Total, 253. 



Lamarck divides this genus into eight, as follow : 



Janthisa. Shell nearly globular, transparent ; the aperture 

 triangular, w^ith an angular hollow or notch on the margin 

 of the right lip (see Helix janthina, pi. 8. /. 107). 



BuLiMUS. Shell oval or oblong, having the last whorl of 

 the spire larger than the former ; the aperture entire, 

 longer than it is broad; the pillar smooth, the base entire 

 and not spread out (see Helix oblonga, pi. 8. /. 1 08). 



LiYMNMA. Shell oblong, rather tapering ; the aperture entire, 

 longer than wide; the lower part of the right lip re- 

 ascending and turned back into the aperture, and form- 

 ing a very oblique fold on the pillar (see Helix stagnalis, 

 pi. 8. /. 109). 



Melania. Shell turrited ; aperture entire, longer than wide, 

 and spreading at the base of the pillar, which is smooth 

 (see Helix Amanda, pi. 8. /. 117). 



Ampvllaria. Shell globular, ventricose, umbilicated at the 

 base ; the aperture entire and longer than w ide, without 

 any thickening on the left lip (see Helix ampiillacea, 



rLs.f. 112). 



Flanorbis. Shell wheel-shaped; spire not projecting, flat- 

 tened or sunk in ; the aperture entire, longer than wide, 



