The two species of Flat Periwinkle have long been confused.

Best separated by examination of the penis of male animals, they also tend to differ in ecology: Littorina obtusata lives higher on the shore, mainly on Ascophyllum nodosum, whereas Littorina fabialis lives lower down, usually on Fucus serratus. Both can occur on Fucus vesiculosus on rhe mid-shore.

An extended version of the paper can be purchased from the Conch. Soc. (The Conchological Society of Great Britain and Ireland) http://www.conchsoc.org/

There is also an online version on Flickr with lots of photographs.

Finally Martin Willing gives an illustrated summary in British Widlife.