Stereodon richardsonii Mitten, J. Linn. Soc., Bot. 8: 42. 1864 (as richardsoni); Calliergon macounii Karczmarz; C. obtusifolium Karczmarz; C. subgiganteum Kindberg

Plants medium-sized to large, brownish, yellow, or sometimes green. Stems regularly radially pinnate; axillary hair apical cell short- to long-linear. Stem leaves rounded ovate, broadly rounded ovate, or rounded ovate-cordate; costa single, usually branched or distally 2-fid, 53-119 µm wide near base, 1/2-9/10 leaf length; alar region sharply delimited, triangular or usually oblong, from margins 60-80% distance to costa; medial laminal cells 50-133 × (7-)8-13(-14) µm. Sexual condition autoicous. Spores 17-31 µm.

Wet, ± mineral-and nutrient-rich fens, ditches, shores, floating or submerged in lakes; low to high elevations; Greenland; Alta., Man., N.B., Nfld. and Labr., N.W.T., Nunavut, Ont., Que., Sask., Yukon; Alaska, Maine, Mich., Minn., Wyo.; n Eurasia; Pacific Islands (New Zealand).

Calliergon richardsonii is easily separated from other members of the genus by its comparatively short, usually branched or 2-fid stem leaf costa. The species has smaller alar regions than C. cordifolium and C. giganteum, which aids in separating it from occasional specimens of the other two that have some leaves with a slightly shorter costa than usual. Due to somewhat less spreading branch leaves and a less regular branching pattern, the plants do not become spruce treelike as in C. giganteum. Differences between 5. C. megalophyllum and C. richardsonii are provided with the former. Molecular evidence (M. Stech, pers. comm.) suggests that C. richardsonii consists of two entities, the status of which is currently being investigated.