This species forms pale pink (can appear straw-coloured in distal regions) to reddish tufts, 2-12 cm in height, on a wide variety of other algae and hard substrata from the shallow to deep (genetically verified records to 20 m) subtidal (Image A). Thalli are uniaxial ecorticate filaments with blunt to acute tips (Image B), the cells becoming progressively smaller with increasing branch order, but the basal cells of determinate branches are not noticeably smaller than adjacent cells (Images B & C). Determinate branches are irregular in size and branching pattern, positioned in whorls of 1 to 4 on cells of indeterminate filaments (Image C). Gland cells can be present on some individuals, laterally positioned and shorter than their bearing cell (Image D). Tetrasporangia are oval, cruciate (typically decussate) in division, sessile and adaxial on cells of determinate branches (Image E). Cystocarps are spherical and essentially naked (Image F). We have genetically verified collections of this species from Connecticut through to the eastern Canadian Arctic, and BC (Bruce & Saunders in prep). Despite claims of two or three species in the Canadian flora (e.g., Mathieson & Dawes 2017), our genetic survey has uncovered only one highly morphologically variable species. Although there was definitely recent isolation between the Pacific and Atlantic populations, ITS data indicate that these are clearly mixing again in the eastern Canadian Arctic (Bruce & Saunders in prep). Whereas more northerly morphologies of Scagelia pylaisaei could be confused with Pterothamnion plumula (J.Ellis) Nägeli, in New England where these two species overlap, the former is typically densely whorled with 2-4 determinate branches per node while the latter more typically produces opposite determinate branches. Furthermore, gland cells in Scagelia pylaisaei tend to remain lens shaped (Image D) while those in Pterothamnion plumula (J.Ellis) Nägeli can become enlarged and oval to spherical in shape. Careful observation will prevent confusion with species such as Antithamnion cruciatum (C.Agardh) Nägeli, Antithamnion hubbsii E.Y.Dawson and Antithamnionella floccosa (O.F.Müller) Whittick. Image A. Pressed voucher of a specimen collected at Escoumins, QC (GWS006142). Image B. Specimen from New River, NB (EGWS000455) displaying uniaxial construction and blunt and acute tips on neighbouring branches. Image C. Specimen displaying uniaxial construction and 2-3 determinate branches visible per node, differing in size and branching order, arising distally from an axial cell. Basal cells of determinate branches not reduced in size (low intertidal pool on invert, Wallace Cove Lighthouse, NB: EGWS001103). Image D. Close up of a lateral gland cell on a determinate branch cell (EGWS001103). Image E. Decussate division of mature tetrasporangia (EGWS001103). Image F. Naked cystocarp (subtidal (5 m) on shell, Wallace Cove, NB; GWS032383).