Multicoloured specimen with spiral lines; might cause misidentification as Littorina compressa if recorder unaware that dark spiral lines are not diagnostic for that species.
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. The shell of this specimen fits the description except for its indistinct ribs. 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 segregation of the spp.

Other identification criteria used for specimen Q.

Female with pallial oviduct; not brood pouch with shelled ova. (distinguishes from L. saxatilis s.s., but L. compressa similar)

No discernible ciliated field.

“Patulous” shell, i.e. no appreciable flare on the base of the aperture (not diagnostic, but patulous shells more frequent on L. arcana than on L. saxatilis s.s.)

No full account yet of this species.
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