Brevisira Iconic
  1. Valve face with central areolate area surrounded by radiating striae
  2. Rimoportulae or fultoportulae absent
  3. Valve margins with small, tapered spines

Brevisira is a centric diatom, with central areolae surrounded by radiating striae on the valve face. In contrast to many centric genera, Brevisira lacks both rimoportulae and fultoportulae. The frustules of Brevisira are small, with small tapered spines on the valve margins. Within a single frustule, one valve face may be concave, while the other is convex. The valve mantle is also ornamented with areolae. Living cells form loose colonies, linked by the small, marginal spines.

As compared to the genus Orthoseira, Brevisira lacks unique processes, rimoportulae and fultoportulae. Currently, the genus is monotypic containing only B. arentii. This species is reported to be widespread in dystrophic waters in the eastern US and in Scandinavia, Eastern Europe, New Zealand and Chile.