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Animated coloured scanning electron micrograph (SEM) of centric fossil diatom frustule. This fresh water diatom frustule came from deposits found at Kalamath Falls, Oregon. This freshwater centric diatom is found in fresh water streams, ponds and lakes. Diatoms are a type of algae (Chromophyta, Bacillariophyceae). They consist of two valves, made of silica (hydrated silicon dioxide), which fit together like the top and bottom of a box. This SEM shows the surface of one valve and the broad girdle connecting it to the other valve. The exterior of the valves are often highly decorative, with pores, spikes or other types of projections. It is an important alga in the fresh water food chain. .