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DescriptionColony pinnate with the main stem straight and thick and the side branches alternate. The side branches increase in length towards the colonies base, giving a triangular outline to the apex of the colony. The hydrothecae are arranged in opposite pairs and are attached to the side branches for about 2/3 of their length. The outer rim of the hydrotheca is smooth and flared and there is a single, rounded opercular flap. Both the male and female gonothecae are small, quadrangular in shape and taper towards the base. Each 'corner' of the capsule forms a projection which curves inwards towards the centre of the theca. The colour of the colony in life is a distinctive chocolate-brown. Colony typically 100-130mm in height by 60mm in width. On Rathlin Island this species occurs in the company of several other rather rare hydroids, namely Diphasia nigra, Polyplumaria flabellata, Diphasia fallax.