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

Other identification criteria used for specimen P.

Filament of penis long and tapering without mucronate tip on un-anaesthetized live specimen (see P8560).

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 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 segregation of the spp.

Dissection showed no patch-like ciliated field between anus and head (as often found on L. saxatilis s.s.) but there was a suggestion of a linear pink ciliated field running next to the edge of the pink prostate (as appropriate for L. arcana).

L. compressa occurs on the same shore but can be eliminated from consideration as the shell is over 6mm high and lacks broad, flat-topped or gently curved, strap-like, spiral ribs with narrow, deeply incised grooves.

No full account yet of this species.
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