Two orders of diatoms are generally recognised, the Pennales and the Centrales. The Pennales or pennate diatoms, have frustules which are elliptical or rectangular in valve or top view, with bilaterally symmetrical sculpture. The latter includes a longitudinal unsilicified groove called a raphe down the middle of each valve, with rows of punctae (pores or perforations) arranged at right angles on either side. The Centrales, or centric diatoms, have frustules which are circular, triangular or quadrate in valve view and rectangular or ovate in girdle or side view. Being mostly planktonic and non-molitile, they lack the raphe. The pillbox-like frustules often have radiating punctae and the valve face can be divided into compartments, alternatively elevated and depressed with punctae of different size and shape.
The earliest recorded diatoms are centric from the Cretaceous strata. Because high proportions of fossil genera and species are still extant, diatoms can be valuable tools in palaeoenviromental studies. Increasing research in deep-sea history has also encouraged their use as biostratigraphic zone fossils. Diatoms constitute the major part of the phytoplankton sea.
The species shown here are from the North Sea and have been photographed as a single frame without stacking with some difficulty to show each best possible taking into account the varying thickness of each. The central centric diatom measures 240μm in diameter.
Microscope - MOTIC B1
Objective - 20X Achromatic