Scytosiphon lomentaria (Lyngbye) Link
Description: Plants cylindrical, yellowish-brown or dark brown, to 400 mm long. Unbranched, 3-10 mm wide, narrowing at both ends, attached by a short stalk. Hollow and often with irregular constrictions.
Habitat: most common in upper-shore pools growing on limpets; on rock on wave-exposed shores, where another, more rare entity, Scytosiphon dotyi occurs.
Key characteristics: Hollow and often irregularly constricted; but beware narrower plants with no or very few constrictions (below).