The majority of snail shells are normally dextral (the coiling of the shell is right-handed). This means that when the shell is held with its aperture facing the observer and its apex pointing up, the aperture will be on the right. Occasionally, one may chance upon a freak individual whose shell is coiled in a direction opposite to that of its conspecifics. The Helix aspersa shell shown here, from the Smithsonian's National Museum of Natural History, is one such example.
Abnormally coiled shells have been of interest to me1. About a year ago, I had a post about a sinistral snail in M. C. Escher's woodcut Plane Filling II. I have also mentioned species with shells that are normally sinistral, including the freshwater snail genus Physa and the land snail Chondrus tournefortianus of Turkey.
In yet another post, I asked the question, are there any normally sinistral slug species? I still don't have a definite answer.
1. Örstan, A. & Welter-Schultes, F. 2002. A dextral specimen of Albinaria cretensis (Pulmonata: Clausiliidae). Triton, No. 5, pp. 25-28. Pdf