Genus: PLANORBIS Müller, 1774
Shell relatively large and disk-shaped with a keeled periphery. Two species occur in Britain.
Planorbis planorbis
Linnaeus, 1758
Planorbis planorbis Linnaeus, 1758
Description: Shell rather thick and strong. It is not glossy and it has a paler keel towards the upper part of the periphery, above the mid-line. There are 5 – 6 whorls which increase regularly in size. The greatest breadth of the body whorl constitutes about one-quarter of the total breadth of the shell. The mouth may be slightly expanded and sometimes bears a narrow rib inside.
Size: Height: 2.5 – 4 mm. Breadth: 9 – 18 mm.
Habitat: Ponds, ditches, and in quiet slow-flowing canals and rivers, usually in the shallower parts amongst the water weeds. It is found in most counties of England, and many in Ireland, but is recorded only infrequently from Wales and Scotland.
Planorbis carinatus
Müller, 1774
Planorbis carinatus Müller, 1774
Description: Shell rather thin and glossy, and with a sharp keel at about the mid-line of the periphery, or just above it. The keel is sharper than in P. planorbis, and the greatest breadth of the body whorl is about one-third of the total width of the shell, which is made up of 4½ – 5 whorls. The rate of expansion of the body whorl is a more reliable character for identification than the position of the keel. The mouth aperture is more oval than in P. planorbis, and is more acutely angled at the inner and outer margins.
Size: Height: 1.5 – 3 mm. Breadth: 9 – 15 mm.
Habitat: It is found typically in flowing water throughout most of the British Isles.