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.

Description

Planorbis gastropod shells are hard to make sense of in terms of their coiling and orientation. Most of the shells in this genus are almost planispiral in coiling, and when examining the shell, it is important to bear in mind the fact that all planorbids have sinistral shells.

To complicate matters further however:

• In life, these pond snails hold their shells upside down, with the umbilicus facing upward
• The spire of the shell is quite sunken in many species
• The umbilicus of the shell is very wide, and it in some species the umbilicus only as deeply "dished" as the sunken spire is.

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.

Distribution

World-wide distribution.

The genus is known from the Jurassic to the recent periods.

Species

• Planorbis corinna Gray, 1850
• Planorbis carinatus O. F. Müller, 1774
• Planorbis planorbis (Linnaeus, 1758)
• Planorbis kahuica Finlay & Laws, 1931

See also

• Planorbarius corneus, synonym Planorbis corneus (Linnaeus, 1758). As of 14 July 2007, its taxonomy on ITIS is out of date.

References

• Powell A W B, New Zealand Mollusca, William Collins Publishers Ltd, Auckland, New Zealand 1979 ISBN 0-00-216906-1