Diagnostic characters
Shell almost indistinguishable from that of L. saxatilis rudis, but spire relatively smaller, eversion of aperture at base of columella more marked, and shell breadth (in apical view) parallel to apertural edge greater than that at right angles to it. Penis with tapering tip, its glands variable in number and arrangement but usually reaching its curved base. Red colour absent from right part of mantle cavity wall. Prostate does not extend as far as the anus. Oviparous, terminal part of female duct glandular.
Other characters
L. arcana often lives alongside saxatilis rudis but commonly extends a little lower on the beach and prefers more exposed shores. At breeding periods, however, it migrates downshore to a level where its egg masses are not subject to serious desiccation and the possibility of cross-breeding is eliminated (Hannaford Ellis, 1985). Its distribution is still incompletely known but it has apparently the same range in the British Isles as L. s. rudis.