Generic name: PETALONEMA Berkeley ex Correns, 1889. Flora 72: 321.
Type species: Petalonema alatum Berkeley ex Kirchner 1889
Komárek (1992): Filaments free, usually solitary or in small groups, slightly coiled, creeping on the substrate or a little erected, heteropolar, with morphologically separated base (sometimes with a heterocyte) and free apical end, with false branching (branches lateral, solitary or in pairs, dividing after the disintegration of trichomes at heterocytes). Sheaths firm, very thick, distinctly funnel-like, lamellated, colourless or yellow-brown, limited from outside. Trichomes cylindrical in basal and
middle parts, with elongated, cylindrical or barrel-shaped cells; in terminal parts are cells shorter, barrel-shaped and usually widened; apical cells always rounded. Heterocytes intercalar, solitary, more or less spherical and usually wider than vegetative cells. Akinetes missing.
Genotype differences, molecular
Reproduction strategies, life cycles, cell division:
Komárek (1992): Reproduction by hormogonia, separating from the apical parts of trichomes.
Taxonomic position, higher hierarchy:
Cyanophyceae, Nostocales, Scytonemataceae
Notes to taxonomy, misinterpretations:
Ecology, ecophysiology, ecological significance:
Komárek (1992): Subaerophytic or submersely (freshwater), epilithic, on soil or epiphytic, several species are known from geographically limited areas (tropical species).
Physiology and biochemistry:
Distribution, endemism, problematic citations:
Reference strain:
Infrageneric scheme, species concept:

List of species:

Petalonema alatum
Berkeley ex Kirchner 1898
Petalonema crassum [Nägeli in Kützing 1849] Migula 1907
Petalonema crustaceum Agardh ex Kirchner 1900
Petalonema densum [A. Braun] ex Migula 1907
Petalonema fluminalis Zhao in Zhao et Chen 1991
Petalonema incrustans [Kützing] Komárek 2012. Fottea 12(1):143
Petalonema involvens [A. Braun] Migula 1907
Petalonema pulchrum (Frémy) Geitler 1932
Petalonema velutinum [Rabenhorst] Migula 1907

Petalonema-like and unclear species:
Croatella lithophila Ercegović 1925
Scytonema splendens Fritsch et Rich 1924
Scytonema simmeri Schmidle in Simmer 1900
Scytonema geitleri Bharadwaya 1934
Scytonema junipericola Farlow in Collins et al. 1900
Scytonema torulosum Jao 1939
Tolypothrix arboricola Frémy 1930
Tolypothrix elenkinii Hollerbach 1923
Tolypothrix epilithica (Ercegović) Geitler 1932
Tolypothrix hollerbachii Muzafarov
Tolypothrix sawiczii Kosinskaja 1928
Tolypothrix tibestiensis Behre 1958

List of stains:
Komárek 1992
Application technology:
  2.1 taxonomy: Geitler 1932, Geitler 1942, Jaag 1945, Kondrateva 1968, Komárek 1992
  2.2 cytomorphology:
  2.3 16S rRNA sequencing:
  2.4 biology and life cycles:
  2.5 ecology: