The genus "Didymosphenia" M. Schmidt.

For the first time specimens of the type material of Echinella geminata Lyngbye 1819 are documented with light- and scanning-electron-microscopical photographs. This is the reference point for various other presented morphodemes of Didymosphenia geminata originating from five continents and subcontinents or floristic regions respectively. 22 species can be differentiated morphologically among which 11 plus one subspecies are proposed and described here as new to science. Formerly established taxa, for a part in a lower taxonomical rank than recently, are: Didymosphenia clavaherculis (Ehrenberg) Metzeltin & Lange-Bertalot 1995, D. curvata (Skvortzow & Meyer) Metzeltin & Lange-Bertalot 1995, D. curvirostrum (Tempère & Brun) M. Schmidt 1899, D. dentata (Dorogostaisky) Skvortzow & Meyer 1937, D. fossilis Horikawa & Okuno 1944, D. geminata (Lyngbye) M. Schmidt 1899 ssp. geminata, D. lineata Skabichevsky 1983, D. pumila Metzeltin & Lange-Bertalot 1995, D. siberica M. Schmidt 1899, D. subcapitata (Skvortzow & Meyer) stat. nov., D. tatrensis Mroziska, Czerwik-Marcinkowska & Gradziski 2006. Proposed as taxa new to science are: Didymosphenia clavamagna, D. coronata, D. crassiporata, D. geminata ssp. crassa, D. grunowii, D. laticeps, D. laticollis, D. mongolica, D. nipponica, D. skvortzowii, D. strelnikovae.