Lindbergia 15: 27, figs. 3D, E, 12, 14. 1989
Plants medium-sized to very large, green, brownish, variegated green and red, entirely red, or brownish to blackish red. Stems pinnate or irregularly branched, ± in one plane; hyalodermis absent, central strand absent; pseudoparaphyllia broad, foliose; rhizoids or rhizoid initials on stem, at or just before leaf insertions; axillary hair distal cells 2–6, hyaline. Stem leaves ovate to broadly ovate, abruptly narrowed to apex, suddenly curved distally, concave or strongly so, usually distinctly plicate; base not or hardly decurrent; margins finely, irregularly denticulate near apex, otherwise entire; apex acute or acuminate; costa single, ending beyond mid leaf; alar cells not differentiated; medial laminal cell walls thin or slightly incrassate, porose or not. Sexual condition dioicous; inner perichaetial leaves plicate; vaginula with paraphyses present. Capsule with annulus separating; exostome external surface reticulate basally, margins slightly dentate distally. Spores 10–25 µm.
North America, West Indies, n South America, Eurasia, possibly Africa.
Species 2 (2 in the flora).
Hamatocaulis is characterized by the lack of a central strand or hyalodermis, thin and relatively poorly developed cortex, undifferentiated alar cells, and plicate leaves from an erect base suddenly curved. The last feature is found also in Scorpidium. Hamatocaulis is one of the genera of Calliergonaceae in which red pigment is frequently present in parts of the plants. When only parts of the leaves are red, the pigment is mainly found in a sub-basal transverse band. Hamatocaulis is found in mineral-rich to intermediately mineral-rich, but usually not calcium-rich habitats.