Diatoms are unicellular algae in the family Bacillariophceae. These photosynthetic unicellular organisms have a cell wall which consists mainly of silica. Of the 600 known genera, 70% are marine, 17% are freshwater, with a few species occuring in both habitats.
Diatoms are abundant and evolved rapidly, therefore they are good for biostratigraphy. They are sensitive to environmental conditions and are therfore good paleoclimatic and paleoceanographic indicators. Diatoms are very small, typically 20-200um in diameter. They are composed of 2 valves that fit together in an overlapping fashion. The classsification is based on their skeletons, centrales are radial symmetric around structural centers and pennales are bilaterally symmetrical along an axis usually oval, rod like, or triangular in shape. The surface texture is also used to diagnose species and genera amoung the two groups.