Bryol. Univ. 2: 355, plate 10, fig. 7. 1827.
Plants small. Stems irregularly branched. Stem and branch leaves similar, triangular or ovate; margins weakly toothed to entire; ecostate or costa single; medial laminal cells more than 5:1. Seta red-brown, 0.8–2 cm. Capsule suberect, inclined, or rarely erect, symmetric; operculum conic; peristome perfect.
North America, Europe, Asia, Atlantic Islands, Pacific Islands.
Species 11 (5 in the flora).
Isothecium occurs mainly in temperate to subtropical regions and grows in dense, pendent wefts with creeping stems. The primary stem leaves are squarrose and distant, and erect-spreading, strongly imbricate, or occasionally wide spreading. The setae are smooth; the peristome is yellow.
|1||Secondary stems irregularly branched, strongly julaceous when dry; stem leaf alar regions extensive, to 1/3 leaf length.||Isothecium cristatum|
|1||Secondary stems irregularly or regularly branched, not julaceous, or if julaceous regularly branched, sometimes dendroid; stem leaf alar regions small, to 1/10 leaf length||> 2|
|2||Secondary stems 3-15 cm, often dendroid, stipe present, flagelliform tertiary branches rare.||Isothecium cardotii|
|2||Secondary stems 1-9 cm, sometimes closely pinnate, stipe short or absent, flagelliform tertiary branches often present||> 3|
|3||Secondary stems pinnate; plants glossy.||Isothecium stoloniferum|
|3||Secondary stems pinnate or irregularly branched; plants not strongly glossy||> 4|
|4||Secondary stems often pinnate, flagelliform branches sometimes present.||Isothecium stoloniferum|
|4||Secondary stems irregularly branched, flagelliform branches absent (present in I. myosuroides)||> 5|
|5||Branchlet leaf apices blunt; margins slightly toothed in apex; stems julaceous.||Isothecium alopecuroides|
|5||Branchlet leaf apices attenuate; margins coarsely toothed; stems julaceous or not||> 6|
|6||Branches of secondary system sharp-tipped; stems not julaceous.||Isothecium myosuroides|
|6||Branches of secondary system blunt; stems julaceous.||Isothecium stoloniferum|