Albrecht等(2007)[6]分析過本科部分物種，based on sequences of mitochondrial 18S ribosomal DNA and cytochrome-c oxidase I (COI) genes, and on the basis of the results, they rearranged the taxonomy like this:
The genus Camptoceratops Wenz, 1923 is no longer considered to be a planorbid. It was recognised by Curry (1965, p. 360) as a euthecosomatous pteropod (Heterobranchia) (note by Arie W. Janssen, 092507).
Apical, apertural and umbilical view of the shell of Biomphalaria tenagophila. Scale bar is 3 mm.
The shells of most species in this family are disk-like or button-like, being coiled in one plane, although several groups have shells that are more higher-spired, and some are limpet-like.
All coiled shell Planorbidae are sinistral in their shell coiling, as is proved by their internal anatomy (the respiratory and the genital orifice are situated on the left side), however the animals carry their shells with what would normally be the ventral (i.e. umbilical) surface uppermost, and because of this, the shells appear to be dextral.
Indeed, formerly planorbids were thought to have dextral shells, and so species of this family were figured as if they had dextral shells. Although it is now understood that these species are sinistral in shell coiling, disk-like Planorbid shells are often still shown in illustrations oriented as if they were dextral.
Most species of coiled planorbids have a rather thin and moderately smooth shell, although more distinct sculpture such as a keel occurs in, and is diagnostic of, certain species. In the flat, keeled species, the whorls tend to overlap.
The aperture has a sharp outer lip. A peristome can be present, but often the lip is not thickened nor reflected. Those planorbid species which have a high-spired shell may have a narrow umbilicus, but frequently this is covered by callus.
In height most species vary between 6 mm and 6 cm, however, disk-like shells are usually less than about 2 cm in maximum dimension.
Like all pulmonate aquatic snails, ramshorn shells do not have an operculum to close the shell aperture.
Sinistral shells
Planorbarius corneus. View of the sunken spire (held facing downwards in life)Planorbarius corneus. View of the umbilicus (held uppermost in life)白化了的扁卷螺科物種，令帶有血紅素的身體組織在身體內顯得紅紅啡啡的。
Flat-coiled planorbid gastropod shells are hard to understand in terms of their coiling and orientation. Many of the shells of species in this family are almost planispiral in coiling such that one side of the shell often looks rather like the other side, but it is important to bear in mind that nonetheless there is an umbilical side and a spire side of the shell. In addition these are in fact sinistral shells, despite the fact that the snail carries its shell as if it were a normal dextral shell. To make sense of the shell coiling, the following facts are useful:
In life, these pond snails hold their shells upside down compared to the normal gastropod shell orientation, with the umbilicus facing upwards
The spire of the shell is quite sunken in many species, in addition it is carried facing downwards
The umbilicus of the shell is very wide and shallow
In some species the umbilicus is not as deeply "dished" as the sunken spire is, so superficially it can be hard to tell one from the other
However, once it is understood that the 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. This is a convenience for understanding the shell, but is the opposite of the way the shell is actually carried in life.
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. Because the shell is carried "upside down" like this, the aperture of the shell is angled to face downwards also, so the aperture faces a little towards the spire, not away from it, as is usually the case in other shelled gastropods.
Ancestors of ramshorn snails are known with certainty since the Jurassic period. Modern taxa developed since the Cretaceous.
Species in this family occur worldwide. In Northwest Europe about 20 species are known (including non-indigenous species). In this region, various extinct taxa are known to have occurred, starting in the Jurassic period.
Strong, E. E.; Gargominy, O.; Ponder, W. F.; Bouchet, P. Global Diversity of Gastropods (Gastropoda; Mollusca) in Freshwater. Hydrobiologia. 2008, 595: 149-166.
Glöer, P. Die Süßwassergastropoden Nord- und Mitteleuropas (北歐和中歐的淡水腹足類動物). Die Tierwelt Deutschlands [德國的動物] ?. Hackenheim, Germany: ConchBooks. 2002: 19–20. ISBN 3-925919-60-0（德語）.
Liu L et al. (2010) "The phylogeography of Indoplanorbis exustus (Gastropoda: Planorbidae) in Asia". Parasites & Vectors3: 57. doi:10.1186/1756-3305-3-57.
Mollusc Specialist Group (1996). Culmenella rezvoji. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4. <www.iucnredlist.org>. Downloaded on 08 March 2011.
