Narella Gray, 1870: 49.— Cairns & Bayer, 2009: 43 (synonymy and discussion).— Cairns, 2012: 14.— Taylor & Rogers, 2017: 4.—Cairns, 2018: 20–21.
Stachyodes Wright & Studer in Studer, 1887: 49.— Versluys, 1906: 86 –88.
Calypternius Wright & Studer in Studer, 1887: 49–50.
Type Species. Primnoa regularis Duchassaing & Michelotti, 1860, by monotypy.
Diagnosis. Colonies branched dichotomously (laterally or equal), pinnately, in a lyrate fashion, or unbranched. Polyps arranged in whorls, all polyps facing downward in contracted condition. Each polyp covered with three (rarely four) pairs of abaxial body wall scales (one pair of basals, one or two pairs of medials, and one pair of buccals) and a variable number of pairs of smaller adaxial scales, nonetheless leaving the adaxial face largely naked. Articular ridge not present on basal scales. Paired infrabasal scales usually present. Opercular scales keeled on inner surface. Coenenchymal scales thin and slightly imbricate or thick and mosaic in placement, and sometimes prominently ridged.
Discussion. Although keys exist to the Narella species from various geographic regions (see Cairns 2018 for a summary), a comprehensive key, preferably a tabular key, is sorely needed for the 50 species in the world, and is currently being constructed by Cairns and Taylor (in prep.). The last comprehensive key to the species was that of K̹kenthal (1919, 1924), which included only 18 species. Most of the currently recognized species are listed by Taylor & Rogers (2017).
The order in which species are treated below are grouped by presence or absence of a ridge on the dorsolateral basal edge, followed by the colony form, simplest to most complex.
Distribution. All ocean basins except for Arctic, 128–4594 m.