The family Auriculidæ is represented in Britain only by Carychium minimum; a very small, semi-transparent, white and glossy shell found under mossy stones and other moist places. Common, but sharp eyes are needed to find it.
We now come to the freshwater shells, which we capture best by means of a perforated scoop, whether they are on the waterweeds or hidden in the sand or mud of the bottom.
It may be noted that all freshwater shells are greenish-brown which is an excellent protective colouring as rendering them less visible among water weeds to the fish, which devour them greedily.
The family of Limnæidæ (or lake dwellers) has the sub-families, Planorbis, Physa, Limnæa, and Ancylus. In the Planorbinæ (i.e. flat-coiled) the only representative of the genus Segmentina is Segmentina nitida, a small, quoit-shaped, keeled, semi-transparent, light brown shell, with internal divisions like those of a nautilus which are visible from the outside of the shell. Local. Found in stagnant or sluggish water. The genus Planorbis contains the sub-genera Hippeutis, Gyraulus, Gyrorbis, Coretus, and Bathyomphalus.
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.