_Cyclostoma (Pomatias) elegans._--Common on calcareous soils, especially chalk. A spiral shell of 4-1/2 whorls, suture very deep. Mouth circular (whence its name) and provided with a thick shelly operculum which closes the orifice when the animal retires by means of an elastic ligament. This and the next species are our only land shells provided with an operculum, and this shows their derivation from the marine Gasteropoda (_e.g._ whelk and winkle). Perhaps all shells were originally marine, but some became first amphibious and then terrestrial. It is quite unlike any other of our land shells.
_Acicula lineata_ is a very small shell, the size of the Pupæ; mainly northern in distribution. Feeds on liverworts and fungi. Very local; 6 or 7 whorls. Mouth pear-shaped, with a horny operculum.
The Family Succinea really ranks with the land shells, as belonging to the sub-order Pulmonata or lung-breathing molluscs. It is, however, amphibious, and hibernates in the mud at the bottom of a ditch.
_Succinea putris_ (it is the mud, not the animal, which is putrid!) is called the Amber Snail from the colour of its shell, which is unlike any other. Common on flags, etc., at the edges of ditches and ponds.
_Succinea elegans._--Difficult to distinguish from the former, but the animal is darker and the shell more slender, with a deeper suture and a narrower mouth.
_Succinea oblonga_ is local and rare. Generally found near the sea. Much smaller than the other Succineas, and easily mistaken for the young of other species. Colour dull greenish.
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.