The marine algae of British Columbia, northern Washington, and southeast Alaska: division Rhodophyta (red algae), class Rhodophyceae, order Gigartinales, families Caulacanthaceae and Plocamiaceae

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This is a floristic treatment of the gigartinalean families Caulacanthaceae and Plocamiaceae that occur in southeast Alaska, British Columbia, and northern Washington. Caulacanthaceae is represented by one genus, Caulacanthus Kützing, and one species, the type, C. ustulatus (Mertens ex Turner) Kützing, whereas Plocamiaceae is represented by two genera, Plocamiocolax Setchell, including the type species, P. pulvinata Setchell, and Plocamium Lamouroux, including three species, P. cartilagineum (L.) Dixon, P. oregonum Doty, and P. violaceum Farlow. For each species, we provide a description of its habit and life history, vegetative and reproductive morphology, habitat and seasonality, and list its global and local distributions and the representative specimens examined. Keys are provided to genera and species where appropriate. The habit and salient vegetative and reproductive features that can be used to distinguish the taxon in the local flora are illustrated. Emended diagnoses are provided for the family Caulacanthaceae, for the genus Plocamiocolax, and for the species Plocamiocolax pulvinata and Plocamium oregonum. Reasons are given for not recognizing the northeast Pacific specimens of Plocamium cartilagineum as a distinct subspecies. Plocamium tenue Kylin is considered a synonym of P. violaceum. A neotype is designated for Plocamiocolax pulvinata. Lectotypes are designated for P. pacificum Kylin and for P. tenue Kylin.