Thallus medium to dark brown, tufted, with numerous axes arising from a clumped base, mostly 0.2–1.5 (–3) cm long, much branched in the lower half with long unbranched filaments surmounting the meristematic regions, usually epiphytic. Filaments 18–30 (–40) µm in diameter below with cells L/B (1.5–) 2–4, of fairly uniform diameter throughout and 20–35 µm in diameter above the upper meristem with cells becoming L/B 3–6 and with few phaeoplasts. Growth from meristems at base of laterals and usually at intervals along main filaments, with occasional other diffuse divisions (especially where sporangia occur). Cells with numerous discoid phaeoplasts, each with a pyrenoid, and aggregated physodes.

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Basionym: Ectocarpus irregularis Kützing

Type locality: Adriatic Sea (Silva, Basson & Moe 1996: 564). Type: On Laurencia obtusa. Notes: According to Womersley (1987), the type specimen may be in L but apparently not located (Clayton 1974: 779).

Taxonomic notes: F. irregularis was referred to Giffordia by Clayton (1974, p. 777), but most plants are typical of Feldmannia in having long unbranched filaments above the upper meristematic zones, and restriction of most sporangia to below these zones; it is, however, more branched in the lower parts than most species of Feldmannia, with distinct meristematic zones at the base of lateral branchlets, and the sporangia are sessile, as in species of Giffordia. Plants which resemble Giffordia have usually lost part of the long unbranched terminal filaments, and seem to be more common in warmer waters; it is, however, a bridging species between Feldmannia and Giffordia.