Cornu aspersum, like a majority of terrestrial gastropods, is normally dextral [the shell opening is to the right when held spire upwards]. However, accidents of nature do happen, and a rare reverse coiled specimen is found such as the sinistral 27 mm. specimen below [pictured on the far left] which was discovered in Newark, England in 1978.

Sinistral Cornu aspersum (Müller, 1774) Brown Garden Snail
 
    When an individual is particularly lucky an acquaintance arrives with a live sinistral/dextral pair that were found in his garden such as the two illustrated below which were collected in Arcata, California and subsequently photographed by Dan and Hiromi Yoshimoto of Eureka, California. The specimens measure about 26 mm. Images Copyright 2006, Dan and Hiromi Yoshimoto.

Cornu aspersum (Müller, 1774) Brown Garden Snail

sinistral/dextral pair

Cornu aspersum (Müller, 1774) Brown Garden Snail

sinistral/dextral pair

Cornu aspersum (Müller, 1774) Brown Garden Snail

sinistral specimen
 
References:

Born, I. A., 1780.  Testacea Museiu Caesarei Vindobonensis, quae jussu Mariae Theresiae Augustae. J. P. Kraus, Vienna. ii + 1-442 + xvii [unpaginated indices].

Favanne, J. de and J. G de Favanne, 1780.  La conchyliologie, histoire naturelle des coquillages de mer, d'eau douce, terrestres et fossiles .... par M. Desallier d'Argenville .... troisième édition 3 vols., De Bure, Paris.

Müller, O. F., 1774. Vermium Terrestrium et Fluviatilium seu animalium infusorium, helminthicorum, et testaceorum non marinorum succincta historia. Heinick and Faber, Havnia and Lipsia. xxxvi + 1-214 + x [unpaginated indices].

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