The diatoms presented in this chapter belong to the class Bacillariophyceae and in one of three orders, Bacillariales, Rhopalodiales, and Surirellales. All of the species may be motile and possess relatively complex and advanced raphe systems that occupy specialized siliceous structures within the cell wall. In both the Bacillariales and Surirellales the raphe occupies a keel that is elevated from the cell surface. The presence of a "keeled" raphe in these diatom genera effectively elevated this locomotory structure from the valve surface allowing more intimate contact between the raphe and substrata that may have fine-scale irregularities, such as unconsolidated fine sediments like silt (Round, 1981). Thus, these diatoms are often found in the epipelon. The keeled raphe enables these organisms to move efficiently through epipelic habitats, and species of this group often reach their maximum abundance in the epipelon (see Chapter 2 for details on habitats). Epipelic habitats are most abundant in slow-moving streams or in lentic environments such as lakes, ponds, and wetlands (Burkholder, 1996; Goldsborough and Robinson, 1996). Epipelic communities, containing populations of diatoms from these two families, can also be found in quiet pools of swift streams where slower current velocities allow fine sediments to fall from suspension.