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. Twenty-two 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, and D. strelnikovae.