Mereschkowsky 1903      Category: Symmetrical biraphid
TYPE SPECIES: Placoneis gastrum (Ehrenberg) Mereschkowsky




Image Credit: Ditmar Metzeltin

CLASS: Bacillariophyceae
  ORDER: Cymbellales
    FAMILY: Cymbellaceae

  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).

Cite This Page:
Spaulding, S., Edlund, M., and Metzeltin, D. (2008). Placoneis. In Diatoms of the United States. Retrieved April 26, 2018, from

Contributor: Sarah Spaulding | Mark Edlund | Ditmar Metzeltin - December 2008
Reviewer: Sam Rushforth - June 2010


May 1st, 2011 - correction of type species

The generitype had been posted as Placoneis exigua (Gregory) Mereschkowsky on this website. It was corrected to Placoneis gastrum (Ehrenberg) Mereschkovsky based on Cox (2003) and Jahn (2004) as of May 1, 2011. - S. Spaulding