SHAPE VARIATION IN THE ROUGH PERIWINKLE LITTORINA-SAXATILIS ON THE WEST AND SOUTH COASTS OF BRITAIN

Variation in the shape of the shell in Littorina saxatilis Olivi has b een shown to be due largely to the same variables on both the west and the south coasts of Britain, and it exhibits various dines. Two impor tant aspects are the size of the aperture, which becomes relatively la rger from the Isle of Man southwards to Cornwall and eastwards from De von to the Isle of Wight, and the jugosity of the shell, which increas es with distance from Cornwall both northwards as far as the Isle of M an and eastwards as far as Kent. Superimposed on the dines are domains of shape, notably one in Lewis/Harris, where the shells have a relati vely large aperture, which is long and narrow, coupled with a rather g lobose second whorl. The local and geographical aspects of shell shape variation are discussed.