1: rectum. 2: reduced prostate (post breeding?). 3: area without ciliated field, usually occupied if L. saxatilis s.s.. 4: ribbed orange ciliated field along edge of prostate. 5: reduced penis (post breeding?). 6: back of head.
Note: ciliated field is in correct position, but larger than expected on L. arcana; may be because prostate, when fully developed, would conceal some of it. Hannaford Ellis proposed the size and position of the ciliated field as a diagnostic feature for segregating L. arcana from L. saxatilis s.s., but other workers have not used it in publications seen by I.F.Smith because they have found it does not apply in all areas, and perhaps because it requires time and skill for the necessary dissection.

Other identification criteria used for specimen Z

Specimen from well defined station where Dytham, Grahame & Mill (1990) found 100% of Rough periwinkles to be Littorina arcana.

Though perhaps not statistically significant, among over 20 000 Rough periwinkles, they found only ten grey ones with a white band which they identified as L. arcana; specimen Z had a partial white band on a grey shell. See L. arcana Z940 07 Littorina arcana Z940. 9.9 mm high. N. Yorkshire, September 2014. Specimen Z. .

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