Coloured scanning electron micrograph (SEM) of centric fossil diatom frustule (Actinoptychus heliopelta). Known as the Iron cross, this marine centric diatom came from deposits found at Dunkirk, Virginia. Diatoms have delicately sculpted transparent silica shells (frustules) that overlap like the halves of a pillbox, allowing light to penetrate the cell for photosynthesis. Magnification: x140 when shortest axis printed at 25 millimetres.
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