Diagnosis:
Bornet et Flahault (1888): Frondes irregulariter et vage divisae; fila saepius plura in vagina communi inclusa. Heterocystae intercalares.

Bornet et Flahault (1888) sub Hormothamnion: Thallus e filis longitudinaliter concretis formatus, nunc in stratum expansus, nunc filiformis erectus, laciniato-ramosus, membrana communi nulla circumdatus. Vaginae membranacea tenues, saepe muco involutae et confluentes, hyalinae. Trichomata moniliformia. Heterocystae intercalares. Sporae nondum visae.

Descriptions:
Komárek (1992): Filamentous - thallous; filaments isopolar, contain one or more trichomes placed together and more or less parallely within one sheath, joined and coiled into the spread, macroscopic, gelatinous up to leathery, irregularly clathrate or compact, dirty green mat. Trichomes uniserial not branched (but after disintegration they remain parallely situated in the common sheath = similarity with false branching of Blennothrix or Coleodesmium), isopolar, of the same width along the whole trichome, without narrowed and elongated terminal cells, clearly constricted at cross walls. Sheaths firm, more or less thin, structureless, colourless, limited,up to 30 μm wide; sheaths with more trichomes sometimes divaricate, ramify and later split from one another, but remain in one thallus, more or less parallely and later irregularly arranged in one gelatinous mat. Cells barrel-shaped, longer or shorter than wide, pale blue-green. Heterocytes intercalar, barrel-shaped up to cylindrical. Akinetes arise solitary, always
paraheterocytic, elongated, ellipsoidal.

Komárek (1992) sub Hormothamnion: Filamentous - thallous; filaments composed from one to more parallely arranged trichomes enveloped by limited, thin or thick, firm or gelatinous sheaths, single or many together, joined more or less in a mucilaginous mat or forming fasciculate or clustering colonies. Trichomes simple, not branched, not attenuated to the ends, of the same width along the whole length, uniserial, moniliform, clearly constricted at cross walls, probably isopolar and metameric. Sheaths colourless, structureless. Cells subspherical or barrel-shaped, not elongated to the ends. Heterocytes interca1ar, probably only single, several in a trichome. Akinetes not known.

Reproduction strategies, life cycles, cell division:
Komárek (1992): Cells divide cross-wise by binary fission; without meristematic zones, all cells capable to divide. Reproduction probably by hormogonia.

Komárek (1992) sub Hormothamnion: Reproduction was not described. Cells divide evidently cross-wise along the whole trichome, reproduction probably by hormogonia.

Ecology, ecophysiology, ecological significance:
Komárek (1992): The type species known from stagnant freshwater swamp,pools and ponds with vascular submerse plants (forming mats on water plants, rarely secondary floating). Recorded from several very distant areas (Europe, India, Japan, S. Argentine), but never well documented; the identity of all these records should be controlled.

Komárek (1992) sub Hormothamnion: Both species are marine, growing on the muddy bottom of sea beaches or among marine algae. One species was described from Pacific islands, the second,one (type species) was recorded from Caribbean district, Canary Islands and from Mauritius.

List of species:
Hydrocoryne neocaledonica Couté, Tell et Thérézien 1999. Cryptogamie, Algol. 20(4): 334
Hydrocoryne spongiosa Schwabe ex Bornet et Flahault 1888. Ann. Sci. Nat. Bot., ser. 7, 5: 128

Unclear taxa:
Anabaena vaginicola Fritsch et Rich 1929
Hormothamnion convolutum Collins et Harvey 1917
Hormothamnion enteromorphoides Grunow ex Bornet et Flahault 1888
Hormothamnion solutum Bornet et Grunow ex Bornet et Flahault 1888