Cosmioneis Guide
Credit: Rex Lowe
  1. Striae uniseriate, radial
  2. Valve face flat, with broad valve mantle
  3. Central area elliptic
  4. Internal proximal raphe ends anchor-shaped
  5. External proximal raphe ends expanded

Valves of species in the genus Cosmioneis are lanceolate to elliptic in outline, with capitate to rostrate ends. The valve face is flat, and the valve mantles are relatively broad. Striae are uniseriate and radial, and are composed of round to oval areolae. Striae become irregular near the central area. The axial area is narrow and linear, except at the central area, where the central area is expanded and ellipitic. Externally, the proximal raphe ends are expanded. Internally, the proximal raphe ends are anchor-shaped. Internally, the areolae are covered by hymenes. Living cells possess two H-shaped plastids.

Cosmioneis is a small genus found in alkaline, aerophilic habitats of North America. Some species are common in coastal waters. The genus includes the species C. pusilla (W. Smith) D.G.Mann and Stickle and its allies.