1: ribs have sloping surface with raised adapical edge.
2: intervening furrows wide and shallow.

Other identification criteria used for specimen M.

Filament of penis long and tapering without mucronate tip on un-anaesthetized live specimen (see M8599 02 Littorina arcana M8599. 13 mm high. N.W. Anglesey, Wales. March 2014. Specimen M. ).

Previous work by Hannaford Ellis on the same shore as this find showed that L. arcana were thin-shelled, heavily ridged and multicoloured with tessellations or bands. This banded specimen fits the shell description. In contrast, L.saxatilis s.s. had thick smooth unicolorous shells. N.B. these shell differences only apply on this shore; neighbouring shores had different shell characteristics that did not enable separation of the spp.

No full account yet of this species.
marinvert.senckenberg.science/image-browse/7215763361…