Family Description:
Most species are flattened and one to six cells thick. Many have veins or a midrib. Tetrasporangia are tetrahedrally divided and usually occur in sori. Cystocarps are embedded in the plant. Carpospores escape through an ostiole.

Species description:
Common Sea Oak grows to a height of about 10 cm (4 in), but in the Aleutian Islands, Alaska, can grow considerably larger. The delicate blades have three orders of branching, throughout which the midrib is conspicuous. Paired lateral veins become rather indistinct near blade margins. Narrow, sharp teeth up to 2 or 3 mm in length decorate the blade margins; these can expand to form lateral blades. At the base, the main blade narrows to a stipe, which attaches to the holdfast. In addition to the main blade, the holdfast can also develop elongated processes that do not expand into blades themselves.

Tetrasporangia form in elongated patches beside the midrib and veins and also near the blade margins, but not on the marginal teeth (this is an important character to use in distinguishing this species from others in this genus). The cystocarps, which are up to 1 mm in diameter, are scattered over the surfaces of the blades.

This species was named for George B. Rigg, who formerly worked at the University of Washington. He led the "Kelp Expedition" to Alaska in 1913, during which some of the type specimens of this species were collected at Elrington Island in Prince William Sound.

Source:  North Pacific Seaweeds