Part 6

Stenogyra (Cochlicopa) lubrica (i.e. slippery).--Very common in moss and under stones or logs. Much like the previous species, but no denticles and fewer whorls, and broader mouth.

Stenogyra (Cæcilioides) acicula.--If this word is supposed to be Latin it would mean either "like to a blind worm" or "like to a lettuce"! Cæcus, however, being Latin for blind, the allusion is no doubt to the fact that this wholly subterranean species is eyeless. The only British representative of a large family of carnivorous molluscs. I have found it on Saxon bones when unearthed, and in crevices of limestone underground, but it is generally found dead amongst the rejectamenta on the banks of rivers. It is a pretty, glossy white shell, 5 millimetres in height by 1 in breadth.

I may notice here two other land shells, although they scientifically are grouped amongst the fluviatile Gasteropoda.

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.

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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.

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.