This is a deformed shell of a common Roman snail Helix pomatia. It is called scalarid shell. This deformation happens, when the mantle of the snail is damaged during the embryonic development. Therefore such shells are unobvious and rare. It is teratological specimen in general. “Teratological” means, that it has developed abnormally during its ontogeny – during the development of the individual.
I have never seen scalarid shell of Helix pomatia by naked eye before. I have taken this photo of shell in the Waldstein Riding School, Prague. It is a part of Cabinet of curiosities in the exhibition by František Skála.
Horsák M., Juřičková L. & Picka J. 2013: Měkkýši České a Slovenské republiky. Molluscs of the Czech and Slovak Republics. Kabourek, Zlín, 264 pp. (in Czech and English). page 142.