Explanation of shell characters as means of identification.
Cheese snail - Helicodonta obvoluta (O. F. Müller 1774)
Cheese snail (Helicodonta obvoluta) with a shell full of hairs.
Description: In the Vienna woods, the cheese snail (Helicodonta obvoluta) is a snail often encountered. Its shell, characteristically formed, the spire not ascending, but depressed, gave this snail its name, as it reminds the onlooker of a round loaf of cheese. With its size a little bit smaller than a banded snail, it is also easily recognized in nature with the bare eye.
Apart from the form of a cheese snail's shell as such, the form of the shell's aperture or shell mouth is striking: It is triangular, and because of the bulging lip appears to be even more peculiar. The shell spire's whorls are coiled tightly, like a belt, which gave the snail its name in some other languages, like in German: Riemenschnecke, the belt snail.
Especially among juvenile animals, the shell with its irregularly striped surface is covered with hairs a millimetre long. Among older specimens usually only the scars remain in the shell surface. Apart from their main role of improving control over humidity and adhesion to the wet leaves of the snails' food plants, one task of those hairs seems to be camouflage, as they keep dirt to the shell and so disguise the shell from its environment.
Cheese snail (Helicodonta obvoluta) with a shell full of hairs
not exclusively used as a means for camouflage. [RN]
Pfenninger, M. et al.: "Why do snails have hairs? A Bayesian inference of character evolution".
Cheese snail (on the right) and a juvenile keel back slug (Li-
max cinereoniger). [RN]
During phases of dormancy and during winter, the cheese snail closes its shell aperture with a chalky white lid called an epiphragm (also see: The Roman snail in hibernation).
Dimensions: B: 11 - 15 mm; H: 5 - 7 mm; N: 5 - 6. (Abbreviations).
Habitat and Distribution: Helicodonta obvoluta lives in forests under dead leaves, on fallen logs and between stones. While it is usually found on limestone ground, it is not necessarily restricted to limestone.
The cheese snail occurs in vast parts of Europe: From the Pyrenees, its distribution areas stretches through south and middle France as far as Belgium and the Limburg province in the southern Netherlands. Through the German low mountain ranges, Helicodonta obvoluta occurs as far east as the inner western Carpathians; as already mentioned, in the Vienna woods (the low mountain ranges west and south-west of Vienna), it is among the more common species.
In the south, the cheese snail's distribution area stretches over the Alps though Tuscany and the north-eastern Balkan Peninsula. As a relic of a postglacial warm period, it also lives isolated in south-eastern England (in the South Downs) and Schleswig-Holstein in North Germany.
Sickle Mouth Snail - Drepanostoma nautiliforme Porro 1836
Sickle mouth snail (Drepanostoma nautiliforme) from Gordevio
(Ticino, Switzerland). The view shows the shell tip (apex).
Picture: © Stefan Haller (schneckenfoto.ch).
Description: Concerning the form of its shell, Drepanostoma nautiliforme is one of the most interesting land snail species: From outside, the snail resembles more a nautilus, than a snail. The shell spire on both sides is strongly depressed and the outer whorls overgrow the inner ones, so it looks like the snail was bilaterally symmetric, like the nautilus, a cephalopod, and had two navels (umbilici). The animal itself, however, (and by closer observation also its shell) are asymmetric: The shell in thruth is coiled to the right (dextral).
A juvenile sickle mouth snail.
Picture: © Stefan Haller (Source).
The sickle mouth snail's shell is of a light horn brown colour and its surface is covered by fine hair. The rim of the sickle-shaped aperture (hence the name) is protruding and strongly folded back. Juveniles under 3 mm of size are white.
Dimensions: H: 2.5 - 3 mm; W: 5 - 6 mm; N: 5. (Abbreviations).
Habitat and Distribution: The sickle mouth snail inhabits shady and humid places in mountainous forests. Often it can be found under the leaf litter, under old wood and between humid rock rubble. The sickle mouth snail is more frequent on calcareous soil than on silicate stone.
Drepanostoma nautiliforme is distributed in the south-western Alps, it is found north-west of Torino as far as the Como lake and the Monte Rosa mountains at the western Swiss-Italian border. There it climbs to altitudes of up to 1400 m MSL. Fossils of this species have also been found north of the western Alps, in sediments from the postglacial warm period, in Württemberg and in Bavaria (Franconian mountains).
Threat Situation: As the sickle mouth snail is threatened by forest fires, it has been classified as vulnerable in Switzerland (see also: IUCN Threat Categories).
Drepanostoma nautiliforme species homepage.
Forum Natura Mediterraneo: Drepanostoma nautiliforme.
There is still argument about into which order to put the cheese snails, as a family (Helicodontidae) or a subfamily (Helicodontinae), which would then belong to the leaf snails (Hygromiidae, Helicoidea superfamily). While CLECOM states the Helicodontidae as a family, Mollbase favours the Helicodontinae as a subfamily.
With pictures by Stefan Haller: