L. obtusata.
Citrina colour form; exterior yellow, interior white. Usually the commonest colour of L. fabalis (See 26Dof 26 Differentiation L. obtusata/L. fabalis ) on shores well sheltered from waves, but not diagnostic as it also occurs commonly on L. obtusata, such as this, on the same shores.
Menai Strait, North Wales. February 2011.
Full account at: 02 Differentiation L. obtusata/L. fabalis

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