Description: Thallus is an unbranched, narrow, solid cylinder of dark red cells, tapering to a base just one cell wide, with cells forming rhizoidal extensions that help attach the filament to the substrate. Filaments are 10 cm (4 in) long and 0.15 mm (0.006 in) diameter. This group is represented by numerous species in at least two distinct genera that cannot be distinguished morphologically. Habitat: Different species are found on high intertidal rock in spring; mid intertidal rock in summer (different species). Filaments dry as dark skeins flat against the rock. Similar taxa: Eutrichosiphonia paniculata is polysiphonous and usually occurs in the low intertidal.