The principal shell variation in the snail Cochlicella acuta is described in terms of banding, extra-band opacity and ground colour. A nomenclature like that used for Cepaea is adopted for the banding and a simple descriptive one, using the terms continuously opaque and discontinuously opaque, for extra-band opacity. Ground colour is either lacking, a condition which produces shells of white appearance, or present which makes them amber coloured. The results of breeding experiments are reported, which show that much of the shell polymorphism is controlled by genes located at closely linked loci. The shell polymorphism in Cochlicella acuta is compared with that in Cepaea. Possible factors determining supergene frequencies in Cochlicella acuta are discussed. Visually selective predation by rooks, Corvus frugilegus is suggested as the influence responsible for the evolution of morphs that are cryptic in certain environments. Direct evidence of such evolutionary change is reported from sub-fossil material collected from successive strata below the present surface of fixed dunes at a site in Cornwall.