From the Latin fissio (a split or cleft) and dens (a tooth), referring to the divided peristome teeth of most species.

Plants usually minute (1–3 mm long) to medium-sized, or more than 10 cm (aquatic species). Stems simple or branched, growing from a 2-sided apical cell, except at the earliest stage; rhizoids smooth. Leaves distichous, complanate, equitant, linear to lanceolate, plain to falcate; complex in form, consisting of 2 vaginant laminae clasping the stem and joined only along the costa (vaginant lamina open), or joined above from the costa to the margin (vaginant lamina closed), or with the minor lamina joining by a shorter suture extending part way to the margin (part open); apical lamina above the vaginant lamina small or large; dorsal (abaxial) lamina extending the length of the leaf, reaching the leaf base, or ending above, or rarely decurrent; cells of apical and dorsal lamina thin- to thick-walled, smooth, mammillose, uni- or multipapillose, the surface flat to strongly bulging, sometimes lenticularly (convexly) thickened, usually small, isodiametric, occasionally longer (especially near the leaf base); cells of vaginant lamina often larger, more elongate or broader proximally; marginal cells differentiated or not; costa single, usually well-developed, failing below the apex to short-excurrent, sometimes reduced, absent or nearly so.

Dioicous or monoicous. Perigonia terminal or lateral. Perichaetia terminal, rarely lateral; perichaetial leaves often longer and/or narrower than vegetative leaves, the vaginant lamina open. Setae smooth, infrequently papillose or scabrous, mostly elongate, erect, often geniculate at base. Capsules erect to inclined, cylindrical, symmetrical or asymmetrical. Operculum conical and apiculate to rostrate. Peristome single, endostomate, of 16 teeth, these usually divided to ½–⅔ length (rarely entire to rimose), the arms filamentous. Calyptra cucullate, rarely mitrate.

Fissidens Hedw., Sp. Musc. Frond. 152 (1801). Lectotype: F. bryoides Hedw., fide E.G.Britton, in N.L.Britton, Fl. Bermuda 435 (1916).

Fissidens was divided into several subgenera by Müller (1849) and, subsequently, into sections (Müller, 1901). These were adopted by Brotherus (1924) and emended by Norkett, as described by Gangulee (1971). Bruggeman-Nannenga (1978) made some refinements, and Iwatsuki & Inoue (1984) and Iwatsuki (1985) added two subgenera, including Serridium which included species occurring in Australia. Fissidens subg. Serridium was re­-evaluated by Bruggeman-Nannenga et al. (1994), and sect. Amblyothallia was transferred from subg. Fissidens. Further proposals by Pursell (1988) concerning the circumscription of subg. Fissidens have also been made. The subgeneric divisions are based primarily on gametophyte characters, but peristome attributes correlate reasonably well.

In their most recent reappraisal of the subgenera, Pursell & Bruggeman-Nannenga (2004) include only five subgenera: Section Aloma was elevated to subgenus and contains the greatest number of species in the genus (thanks to the number of multipapillose species); Subgenus Serridium was relegated to subg. Pachyfissidens sect. Pachyfissidens on the basis of costal anatomy and peristome structure; Section Semilimbidium was subsumed in subgen. Aloma; Section Amblyothallia of subgen. Serridium was subsumed in subgen. Pachyfissidens sect. Pachyfissidens; Section Crenularia of subgen. Fissidens was subsumed in subgen. Aloma; and Section Areofissidens was also subsumed in subgen. Aloma. Four subgenera of Fissidens are represented in Australia: Aloma (the largest), Fissidens, Octodiceras and Pachyfissidens.

Subgenus ALOMA Kindb., Eur. N. Amer. Bryin. 2: 165 (1897)

Plants light to sordid green, sometimes with a reddish tinge, particularly the stem and leaf costa, terrestrial, infrequently aquatic, small to medium-sized. Stems monomorphic, unbranched or branched; in section a central strand present or absent. Leaves imbricate or distant, usually pinnately arranged, occasionally palmate; margins ±entire, limbate or occasionally elimbate, limbidium on all laminae or ±confined to vaginant laminae, marginal or intralaminal, occasionally inconspicuous, unistratose or rarely multistratose. Costa variable in length, sometimes absent or nearly so, when present of bryoides-type. Lamina cells distinct or obscure, with or without oil droplets, unistratose to irregularly or regularly bistratose, small to large, firm-walled or large and inflated, thin-walled and collapsing when dry, smooth, mammillose, uni- or multipapillose.

Monoicous; Perigonia variable in position. Perichaetia terminal on main stems and branched, occasionally with axillary naked antheridia and archegonia. Sporophytes 1–6 per perichaetium, yellow, darkening with age; setae mostly elongate; capsules usually exserted, mostly erect and radially symmetrical, stomatose; exothecial cells in c. 32 columns, ±quadrate, the vertical walls thicker than horizontal walls, usually collenchymatous. Peristome of scariosus-type, rarely anomalous. Calyptra cucullate, rarely mitrate, usually smooth, rarely prorate.

Subgenus FISSIDENS Hedw., Sp. Musc. Frond. 152 (1801)

Plants light green to blackish green, terrestrial or aquatic. Stems monomorphic or dimorphic, unbranched or branched, in section a central strand present or absent. Leaves ±crispate when dry, usually imbricate, in few to several pairs, mostly pinnately arranged; margins variable, typically limbate on all laminae or occasionally (e.g. F. taylorii) confined to vaginant laminae, occasionally absent from some or all leaves; limbidium uni- to multistratose. Laminal cells medium-sized, plane or convex, firm-walled, with smooth evenly thickened walls (occasionally lenticularly thickened), smooth, often bulging, distinct to obscure, irregularly quadrate to hexagonal, unistratose to irregularly or regularly bistratose; costa of bryoides-type, with 2 large adaxial and 1 or more large central cells.

Monoicous. Sporophytes 1–several per perichaetium; setae usually elongate; capsules usually exserted, erect, radially symmetrical to ±inclined to ±arcuate, bilaterally symmetrical, stomatose; exothecial cells ±quadrate to oblong, the vertical walls thicker than horizontal walls, often collenchymatous, in 40 or more columns around mid-capsule diameter. Peristome of bryoides-type, sometimes anomalous. Operculum conical, usually short-rostrate. Calyptra cucullate, infrequently mitrate, usually smooth or occasionally prorate. Spores smooth to finely papillose.

Subgenus OCTODICERAS (Brid.) Broth., Nat. Pflanzenfam. 1(3): 361 (‘1900’) [1901]

Plants yellowish to dark green, large, aquatic, submerged but often periodically emergent, delicate. Stems to 12 cm or more long, usually much-branched, flaccid, lacking a central strand. Leaves numerous, usually distant, long and narrow, lanceolate to linear-lanceolate; apex acute; lamina unistratose, the margins ±entire, elimbate to weakly limbate on lower 1/3 of vaginant laminae; costa ending near the apex or well below, bryoides-type; vaginant laminae acute, ±equal, ending on or near the costa; dorsal lamina reaching to insertion or ending above. Lamina cells unistratose or infrequently and irregularly bistratose, firm-walled, smooth, flat or slightly bulging, quadrate to short-rectangular or hexagonal.

Monoicous. Perigonia and perichaetia gemmiform, axillary or terminal on main stem and elongated axillary branches; sporophytes 1 or more per perichaetium, small, inconspicuous. Setae short. Capsules erect, usually exserted, radially symmetrical. Operculum conical, short-rostrate, occasionally long-rostrate. Peristome reduced bryoides-type, undivided or divided; filaments infrequently absent. Calyptra mitrate or cucullate, smooth.

Subgenus PACHYFISSIDENS (Müll.Hal.) Kindb., Eur. N. Amer. Bryin. 2: 165 (1897)

Plants small to large, erect to decumbent, light to dark green. Stems usually branched, in section a central strand usually present, occasionally absent. Leaves usually imbricate, pinnately arranged, oblong to lanceolate; margins often irregularly, coarsely and distantly serrate distally, elimbate or weakly limbate on proximal parts of vaginant laminae. Costa ending several cells below the apex to short-excurrent, rarely obscured by overlapping chlorophyllous cells, in section of oblongifolius- or taxifolius-type. Lamina cells unistratose, bistratose, rarely multistratose, firm-walled, the walls evenly thickened, mostly somewhat bulging, smooth, plane, infrequently mammillose or multipapillose, irregularly quadrate to irregularly hexagonal.

Monoicous or dioicous. Perigonia mostly gemmiform, basal, axillary, rarely terminal on epiphyllous dwarf males, infrequently terminal on longer stems. Perichaetia terminal on short axillary branches, rarely terminal on main stems. Sporophytes usually 1 per perichaetium, reddish; setae elongate, reddish or yellow, darkening with age; capsules immersed or exserted, erect, radially symmetrical or somewhat inclined, slightly arcuate, bilaterally symmetrical, mostly stomatose; exothecial cells in more than 40 columns around mid-capsule diameter, mostly oblong, the vertical walls thinner than horizontal walls. Operculum conical, long-rostrate. Peristome of taxifolius-, similiretis- or zippelianus-type. Calyptra cucullate, smooth, covering only the rostrum. Spores finely papillose to smooth.

Three sections of Subgenus Pachyfissidens are recognised:

Sect. Pachyfissidens

Sect. Amblyothallia Müll.Hal.

Sect. Crispidium Müll.Hal.

This treatment of the family Fissidentaceae, originally intended for publication in Flora of Australia, is based on a manuscript prepared by Ilma Stone in 1992, and substantially updated in 1998. The authors of that early treatment, Ilma Stone and David Catcheside, are both deceased.

Since the preparation of the original manuscript, the infrageneric classification of Fissidens has altered considerably, along with the recognition of many new taxa and new synonyms, while the monogeneric Nanobryaceae has been subsumed into the Fissidentaceae. Therefore, an up-to-date, illustrated treatment of the most diverse moss genus in the Australian flora was clearly long overdue.

To accommodate recent revisions and records, a new treatment of the Australian Fissidentaceae has been prepared by Rodney D. Seppelt for publication in Australian Mosses Online, drawing heavily on the pioneering work of Ilma Stone. Descriptions of taxa have been expanded, all taxa are or will be fully illustrated, and a new key to species has been constructed. Over many years, Ilma Stone collected hundreds of Fissidens specimens, with particular emphasis on the tropical and subtropical regions of Australia. The subtropics support the richest Fissidens flora, and the majority of Ilma's specimens were of species that have proved to be notoriously variable and difficult to identify with certainty. Many challenges lie ahead, and updates to this treatment can be expected.

R.D.Seppelt & I.G.Stone (2016), Australian Mosses Online 70. Fissidentaceae. Australian Biological Resources Study, Canberra. Version 16 June 2016. http://www.anbg.gov.au/abrs/Mosses_online/70_V2_Fissidentaceae.html