Planorbis is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of freshwater air-breathing snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s, aquatic

Aquatic animal

An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

 pulmonate gastropod mollusks in the family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Planorbidae

Planorbidae

Planorbidae, common name the ramshorn snails or ram's horn snails, is a family of air-breathing freshwater snails, aquatic pulmonate gastropod mollusks....

, the ram's horn snails, or planorbids. All species in this genus have sinistral or left-coiling shells.

Description

Planorbis shells

Gastropod shell

The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

 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, which means that the shell is coiled more or less flat. When examining the shell or the living animal, it is important to bear in mind the fact that planorbids have sinistral shells.

• In life, these pond snails hold their shells upside down, with the umbilicus facing upward and the spire facing downward (contrary to what is the case in every other kind of shelled gastropod)
• The spire of the shell is quite sunken in many species (rather than being elevated, as is the case in almost all shelled gastropods)
• The umbilicus
  Umbilicus (mollusk)
  The umbilicus of a shell is the axially aligned, hollow cone-shaped space within the whorls of a coiled mollusc shell. The term umbilicus is often used in descriptions of gastropod shells, i.e...
  
   of the shell is extremely wide and shallow
• In many species, the shallow depth of the umbilicus is more or less equivalent to the "dishing" of the sunken spire on the opposite side

Once it is understood that a planorbid shell is sinistral, if the shell is held with the aperture on the left and facing the observer, then the sunken spire side of the shell is uppermost.

To repeat: the side of the shell which is in fact the spire (a sunken spire) faces downwards in the living animal, contrary to what is the case in almost all other shelled gastropods.

Distribution

Worldwide distribution.

The genus is known from the Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 to the recent periods.

Species

Species within the genus Planorbis include:

• Planorbis corinna
  Planorbis corinna
  Planorbis corinna, is a species of minute freshwater air-breathing snail, an aquatic pulmonate gastropod mollusk, or micromollusk in the family Planorbidae, the ram's horn snails, or planorbids...
  
  Gray, 1850
• Planorbis carinatus
  Planorbis carinatus
  Planorbis carinatus is species of freshwater gastropod from family Planorbidae.-Description:Width of shell is 9–15 mm. Its keel is in the centre of the shell.-Distribution:* Not listed in IUCN red list - not evaluated * Czech Republic...
  
  O. F. Müller, 1774
• Planorbis planorbis
  Planorbis planorbis
  Planorbis planorbis is a species of air-breathing freshwater snail, an aquatic gastropod mollusk in the family Planorbidae, the ram's horn snails.-Description:...
  
  (Linnaeus, 1758)
• Planorbis kahuica
  Planorbis kahuica
  Planorbis kahuica, is a species of minute freshwater air-breathing snail, an aquatic pulmonate gastropod mollusk, or micromollusk in the family Planorbidae, the ram's horn snails, or planorbids...
  
  Finlay & Laws, 1931

Species brought into synonymy :

• Planorbis campanulatus Say, 1821 accepted as Planorbella campanulata (Say, 1821)

Further reading

• Powell A. W. B.
  Arthur William Baden Powell
  Dr Arthur William Baden Powell CBE was a New Zealand malacologist, naturalist and palaeontologist, a major influence in the study and classification of New Zealand molluscs through much of the twentieth century. He was known to his friends and family by his third name, "Baden".Powell was born at...
  
   (1979). New Zealand Mollusca, William Collins Publishers Ltd
  HarperCollins
  HarperCollins is a publishing company owned by News Corporation. It is the combination of the publishers William Collins, Sons and Co Ltd, a British company, and Harper & Row, an American company, itself the result of an earlier merger of Harper & Brothers and Row, Peterson & Company. The worldwide...
  
  , Auckland, New Zealand, ISBN 0-00-216906-1