Placoneis Guide
Credit: Ditmar Metzeltin
  1. Valve linear to lanceolate
  2. Apices rostrate to capitate
  3. Striae uniserate
  4. Stigmata present or absent

The valve outline of Placoneis is linear to lanceolate, sometimes with rostrate or capitate ends. The areolae are loculate and internally occluded. Areolae form uniseriate striae. The axial area is narrow. The proximal raphe ends are straight and slightly expanded externally. The plastid is large, distinct, and divided into two X-shaped plates.

Placoneis contains a number of species that grow in epipelic habitats of lakes and streams. It includes the former Placoneis exigua (Gregory) Mereschkowsky and its allies. The taxonomic position of the genus is unclear, as it has been suggested to hold naviculoid as well as cymbelloid and gomphonemoid affinities (Mann and Stickle 1995, Kociolek and Thomas 2010).