witTi a revision of thè Limnmdoe of Authors. 
351 
Sub-genus Taphius. H. & A. Ad. 1855. 
Shell excavated beneath. Whorls rounded, columella reeti- 
linear. 
Type Taphius andecola. H. & A. Adams. 
Sub-genus Hélisoma. Swains. 1840. 
Shell ventricose, whorls angulated. Spire sunk below thè 
body whorl. 
Type Hélisoma hicarinata — Planorbis bicarinatus. Sby. 
Sub-genus Planorbis. (Guett.) 1756. 
Central tooth bilobed. 
Spire depressed, whorls rounded, last whorl ventricose, aper- 
ture crescentic. 
Type Planorbis corneus. Linn. 
Sub-genus Planorbella. Hald. 1844. 
Shell few whorled, aperture campanulate. 
Type PloMorbella campanulata — Planorbis campanula- 
tus. Say. 
Sub-genus Adula. H. Ad. 1861. 
Whorls rounded and numerous. Deeply umbilicate on 
thè upper and convex on thè under side, aperture campanu- 
late. 
Type Adula multivolvis = Planorbis multivolris. Case. 
Sub-genus Menetus. H. & A. Ad. 1855. 
Shell depressed, whorls rapidly increasing. Periphery angu- 
lated. 
Type Menetus opercularis = Planorbis operctdaris. Gld. 
Sub-genus Gyraulus. Agassiz. 1837. 
Shell rounded above, fiat beneath, whorls few, rapidly in- 
creasing. 
Type Gyraulus albus — Planorbis albus. Miill. 
