Habit and structure. Thallus medium to dark brown, 10–30 cm high, with corticated, more-or-less terete axes bearing alternately pinnate to bipinnate, complanately branched, basally constricted laterals 4–8 cm long, branches (4–)6–8(–10) mm broad, margins smooth to irregular, minutely crenulate. Holdfast unknown; probably epilithic. Structure. Apices circinnate, with the apical and subapical cells dividing rapidly to form a broad apex; axial cells cutting off 5 pericentral cells, 2 lateral on each side and probably one ventral, with 2 pseudopericentral cells cut off from the dorsal lateral pericentral cells and 1–3 cut off on the ventral side. Wings 2 cells thick, the layers overlapping at their ends, cells 35–60 µm in diameter and L/D 2–3, with the outermost cells producing multicellular spines 130–180 µm long. Midrib slight in upper branches, becoming corticated and 2–3 mm thick below. Trichoblasts scattered on surface of blades, 300–500 µm long. Cells multinucleate; rhodoplasts discoid, chained and reticulate in larger cells.
Reproduction. Procarps unknown. Carposporophytes with a basal fusion cell and short, branched, gonimoblast with elongate-clavate carposporangia 45–70 µm in diameter. Cystocarps scattered on branch surface, stalked, ovoid, 0.7–1 mm in diameter; pericarp ostiolate, corticated, 4–5 cells thick. Spermatangia unknown. Tetrasporangial stichidia scattered on the branch surface between midrib and centre of wings, in groups of 1–5, ovate to slightly elongate, compressed, 300–450 µm broad, lightly corticated, tetrasporangia paired, 70–170 µm in diameter.
Distribution. Port Denison to Eyre, W. Aust.
Habitat. A. mamillaris is a deep-water species.
[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 395 (2003)]
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