Viewed from the top, this is all that is visible of a living pen shell, Pinna muricata, in its natural sandy habitat. A narrow 50mm (2 in) slit, flush with the sand and not much more than 10-15mm wide reveals its checkered mantle. The rest of the translucent bivalve’s shell tapers to a fine point some 76mm (3 inches) beneath the sand.
6 November 2015