Many short, concentric lines in skirt-shell-layer iridesce blue when light is right. Rise and fall of surface related to ribs has caused light to catch iridescent lines in a way to suggest radiating bands of blue, but appearance changes with angle of light.
Interior has opaque porcellaneous layer that conceals view of colour bands in exterior shell-layer. On some shores in north Yorkshire shells of this size lack opaque porcellaneous layer; perhaps because conditions there, near limit of range in North Sea, are less optimal than shore in Lleyn where this one comes from.

OTHER SPECIES ALBUMS
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