A Roman snail with a scalariform shell - an innocuous defor-
mity. Picture: Claudia Horstmann.
The Roman Snail's Shell.
Normal and scalariform shell in comparison: Cepaea nemoralis.
Picture: Gudrun Ruf.
In general the ratio between width and height of a snail's shell is largely specific for a certain snail species. So a Roman snail's (Helix pomatia) shell is about as wide, as it is high, in a dimension of about 30 to 50 mm. (See also "Identification by Shell Characters").
Among many snail species there may, though, be a shell deformation, in which the shell is atypically much higher than normal.
The shell whorls are not tightly coiled, like in normal type snails, but stretched along the shell's longitudinal axis and set apart from each other like staircase steps, without a suture between the whorls.
After the Latin "scala" for a staircase this deformation is called a scalarid or scalariform shell. Apart from Helix snails, scalarid shells can occur among other snail species, even among certain species of sea snails. For the snail, its deformed shell does not impose a life threat, the snail is as viable as any normal type snail.
So far it remains unknown, how the scalarid deformation of a snail's shell develops. While Geyer wrote in 1927: "by an externally induced disturbance of the suture", much points towards a genetic aberration, as a consequence of which mantle and visceral sac do not grow tightly coiled, but stretched, and so does the atypically elongate shell formed like a staircase.