Thalassiosira Iconic
  1. Valve face flat or tangentially undulate
  2. Fultoportulae one to several near the center valve
  3. Valve margin with spines or occluded processes
  4. Mantle narrow

The valve face of most freshwater species of Thalassiosira is flat, although some are tangentially undulate. The mantle is usually relatively narrow. Striae are radial and not arranged in fascicles. Marginal fultoportulae are positioned near marginal spines, or occluded processes. One to several fultoportulae are also located near the center of the valve face, often in a circular array.

Species of Thalassiosira grow primarily in marine waters. The fultoportulae, or strutted processes, secrete ß-chitin, which is considered to offer resistance to sinking and keep cells buoyant. Some species within the genus are found in estuaries, high conductance waters and rivers, polluted ponds, and other aquatic systems that have been impacted by human activities.