Planothidium Guide
Credit: Eduardo Morales, Marina Potapova, Ditmar Metzeltin
  1. Frustules heterovalvar - one valve with a raphe, the other lacking a raphe
  2. Striae multiseriate
  3. Terminal raphe fissures long, curved to secondary side

Frustules of Planothidium are heterovalvar, with slightly concave raphe valve and convex rapheless valve. Valves are elliptic to lanceolate, with rounded, rostrate or capitate apices. The striae are multiseriate. The raphe valve possess radiate striae and a prominent raphe. The terminal raphe fissures are long, curved and deflected unilaterally.

Many Planothidium species, but not all, possess an asymmetric central area on the rapheless valve. Internally, this central area bears a rimmed depression, and in some cases, a cavum is also present. Note that alternate terms for the cavum are "cave" and "hood". The structure of the rimmed depression and cavum were not recognized until the they were examined under the scanning electron microscope. Previously, the structures were termed ‘hufeisenförmige Fleck” in German, or “hoof-mark” or "horseshoe structure" in English.

Cells have an adnate growth habit, in which they are attached by the raphe valve face to the substrata. The genus was split from Achnanthidium by both Round and Bukhtiyarova (1996) and Lange-Bertalot (1997). Details of sexual reproduction in P. lanceolatum are described in Geitler (1977).