_Planorbis (Hippeutis) fontanus_ is much like Segmentina but has no septa, and is flatter. Common, especially on watercress. Often encrusted with mud.
_Planorbis (Gyraulus) nautileus_ is very small; quoit-shaped, with the upper side flat. Grey and striated. The variety crista has the ridges of the epidermis drawn into points, and is beautiful when seen by a magnifying glass. Common in ponds and ditches.
_Planorbis (Gyraulus) dilatatus_ is a very small shell imported in cotton bales from America, and naturalized in canals in Lancashire. No other of its kind is so small.
_Planorbis (Gyraulus) albus_ is dull white and striated. Flattish above, with spire depressed. Frequently encrusted and black with mud. Common.
_Planorbis (Gyraulus) parvus_ (but not so small as _dilatatus_).--Convex above with a central depression, concave beneath. Suture deep, and umbilicus large. Smooth and glossy. Local.
_Planorbis (Gyrorbis) spirorbis._--Very flat, glossy, brown, whorls 5 to 6. Common in ponds and ditches.
_Planorbis (Gyrorbis) vertex._--Very like the last, but flatter and thinner, and with a prominent keel. More local than _spirorbis_, but sometimes found with it. Whorls 6 to 8.
_Planorbis (Gyrorbis) carinatus._--Larger than _spirorbis_ and _vertex_. Sharply keeled in the centre of the outer margin. Mouth angulated above and below. Local, mainly in the south and east of England.
_Planorbis (Gyrorbis) umbilicatus._--Like the last, but the keel is below and not on the centre. Mouth rhomboidal. More common than _carinatus_.
_Planorbis (Coretus) corneus._--Far the largest species. Dark brown, lighter below. Mouth nearly circular. Spire sunk. In boiling water often exudes a crimson fluid. Common.
_Planorbis (Bathyomphalus) contortus._--Small, 8-whorled, flat above, very convex below. Fairly common in still water. Very compact in appearance.
The sub-family Physa has two genera, Aplecta and Physa.
_Physa (Aplecta) hypnorum_ is a spindle-shaped, very glossy, semi-transparent, dark reddish brown, shell, with 6 to 7 whorls. Not common. Found in still water.
_Physa (Physa) fontinalis._--More common, and found in running as well as in still water. Shorter and more rounded than the last. Shell very thin, greenish horn-colour. Lobes of the mantle expand over the shell. Seen in an aquarium are its perpendicular threads of mucus, up and down which the animals climb.