Our British Snails Part 3

In the family Stenogyra we have three genera, Azeca, Cochlicopa, and Caecilioides (with also the imported _Stenogyra Goodallii_, found only in pine-houses).

_Stenogyra (Azeca) tridens_ is a small chrysalis-shaped, solid but semi-transparent sh.e.l.l, horn-coloured, with 3 denticles. Not rare in moist places.

_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 (Caecilioides) acicula._--If this word is supposed to be Latin it would mean either "like to a blind worm" or "like to a lettuce"! _Caecus_, 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 sh.e.l.l, 5 millimetres in height by 1 in breadth.

I may notice here two other land sh.e.l.ls, although they scientifically are grouped amongst the fluviatile Gasteropoda.

_Cyclostoma (Pomatias) elegans._--Common on calcareous soils, especially chalk. A spiral sh.e.l.l of 4-1/2 whorls, suture very deep.

Mouth circular (whence its name) and provided with a thick sh.e.l.ly operculum which closes the orifice when the animal retires by means of an elastic ligament. This and the next species are our only land sh.e.l.ls provided with an operculum, and this shows their derivation from the marine Gasteropoda (_e.g._ whelk and winkle). Perhaps all sh.e.l.ls were originally marine, but some became first amphibious and then terrestrial. It is quite unlike any other of our land sh.e.l.ls.

_Acicula lineata_ is a very small sh.e.l.l, the size of the Pupae; mainly northern in distribution. Feeds on liverworts and fungi. Very local; 6 or 7 whorls. Mouth pear-shaped, with a h.o.r.n.y operculum.

The Family Succinea really ranks with the land sh.e.l.ls, 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 sh.e.l.l, 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 sh.e.l.l 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 Auriculidae is represented in Britain only by _Carychium minimum_; a very small, semi-transparent, white and glossy sh.e.l.l found under mossy stones and other moist places. Common, but sharp eyes are needed to find it.

We now come to the freshwater sh.e.l.ls, 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 sh.e.l.ls 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 Limnaeidae (or lake dwellers) has the sub-families, Planorbis, Physa, Limnaea, and Ancylus. In the Planorbinae (_i.e._ flat-coiled) the only representative of the genus Segmentina is _Segmentina nitida_, a small, quoit-shaped, keeled, semi-transparent, light brown sh.e.l.l, with internal divisions like those of a nautilus which are visible from the outside of the sh.e.l.l. Local. Found in stagnant or sluggish water. The genus Planorbis contains the sub-genera Hippeutis, Gyraulus, Gyrorbis, Coretus, and Bathyomphalus.

_Planorbis (Hippeutis) fonta.n.u.s_ 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 gla.s.s. Common in ponds and ditches.

_Planorbis (Gyraulus) dilatatus_ is a very small sh.e.l.l imported in cotton bales from America, and naturalized in ca.n.a.ls 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, sh.e.l.l, 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. Sh.e.l.l very thin, greenish horn-colour. Lobes of the mantle expand over the sh.e.l.l.

Seen in an aquarium are its perpendicular threads of mucus, up and down which the animals climb.

_Limnaea (Amphipeplea) glutinosa._--Very local. Somewhat like _Ph.

fontinalis_, but larger and more thin. In young specimens the mantle covers the sh.e.l.l, and in adults the animal is not wholly contained in the sh.e.l.l.

_Limnaea (sub-genus Radix) involuta._--Only found in one Irish tarn.

Whorls envelop the spire. Very thin, pale amber.

_Limnaea (Radix) peregra._--The most common and variable of all our freshwater sh.e.l.ls. Spire pointed. Somewhat amphibious. Found practically over the whole of the Eastern Hemisphere.

_Limnaea (Radix) auricularia._--Mouth very large, with outer lip widely reflected. Very common and fine in the Thames. Spire very short, apex sharp.

_Limnaea (sub-genus Limnophysa) stagnalis._--The largest of the genus.

Common, except in Wales. Sh.e.l.l greyish, spire long and tapering to a point; 12210 teeth on its lingual ribbon. See the ill.u.s.tration on p.

57, which also shows above two specimens of _Paludina contecta_, one being covered (as freshwater sh.e.l.ls often are) by a vegetable growth, which obscures the marking.

_Limnaea (Limnophysa) pal.u.s.tris._--Sh.e.l.l tapering, somewhat solid, brown, much smaller than stagnalis. Common in slow or stagnant water.

Some varieties much darker than the type.

_Limnaea (Limnophysa) truncatula._--Like the last in shape, but much smaller, and with a deeper suture. Common, and fond of being out of the water. A parasite of this mollusc causes "fluke" in sheep which have taken it in by drinking or by eating gra.s.s by the side of ponds and ditches.

[Ill.u.s.tration: _Paludina contecta_ (two) and _Limnaea stagnalis_ on water-weeds.]

_Limnaea (Omphiscola) glabra._--Also amphibious. About the same size as _truncatula_. Local. Inner lip rather thick and reflected on the base of the penultimate whorl.

_Limnaea (Ancylus) fluviatilis._--"Freshwater limpet." Sh.e.l.l, rather limpet-like, with a hooked apex (whence its generic name), adheres to stones or piles in running water. Common. I once dredged a large water-beetle with three of these sh.e.l.ls adhering to its wing-cases; thus it would be transported to fresh habitats.

_Limnaea (Acroloxus) lacustris._--Like the former but more local, and preferring sluggish or still waters. Sh.e.l.l more oblong, thinner, and apex twisted to the left instead of to the right as in _fluviatilis_.

The sub-order Pectinibranchiata (comb-like gill) contains the genera Neritina, Paludina, and Valvata, in all of which there are two tentacles with eye at the base, and an operculum to the sh.e.l.l.

_Neritina fluviatilis._--Solid, glossy, chequered brown, white, and purple (but also a lemon-coloured variety). Operculum semi-lunar, orange, with a projection which serves as a lock to keep the operculum in position. Not rare in England; on stones in running water. See ill.u.s.tration below, which also shows above _L. (Ancylus) fluviatilis_.

_Paludina (Vivipara) contecta._--Sh.e.l.l dark green with darker bands.

Conical. Suture very deep. Operculum h.o.r.n.y. Viviparous. Local.