Planorbis is a genus of freshwater air-breathing snails, aquatic pulmonate gastropod mollusks in the family Planorbidae, the ram's horn snails, or planorbids. All planorbids have sinistral or left-coiling shells.
To complicate matters further however:
Once you understand that a planorbid shell is sinistral, you can hold the shell with the aperture on the left and facing you, then the sunken spire side of the shell will be uppermost.
To repeat: the side of the shell which is in fact the spire (a sunken spire) faces down in the living animal, contrary to what is the case in almost all other shelled gastropods.
Its shell coils in a flat plane.
Peter Fowler. Farming in the First Millennium AD: British agriculture between Julius Caesar and William the Conqueror.(Book Review)
Mar 01, 2004; PETER FOWLER. Farming in the First Millennium AD: British agriculture between Julius Caesar and William the Conqueror. xviii+393...