The genus Trigonastrum differs from the preceding Chitonastrum, its ancestral form, in the development of a patagium between the arms. It bears therefore to the latter the same relation that Euchitonia does to Rhopalastrum.
Subgenus 1. Trigonastrella, Haeckel.
Definition.—All three arms of the same size and form, equidistant; fundamental form of the shell therefore a regular, equilateral triangle.
1. Trigonastrum regulare, n. sp. (Pl. [43], fig. 16).
Chitonastrum regulare, Haeckel, 1881, Prodromus et Atlas (pl. xliii. fig. 16).
All three arms equal and equidistant. Each arm has the form of an isosceles triangle, twice as high as broad, the truncated apex of which is inserted into the large circular central disk, whilst its distal base (four times as broad) is divided by a deep incision (half as long as the arm). Each arm with ten to twelve joints. Patagium between the arms nearly complete, spongy. (Differs from Chitonastrum triglochin mainly in the possession of a patagium.)
Dimensions.—Radius of each arm 0.24, greatest breadth 0.11, basal breadth 0.03; length of the sides of the regular triangle 0.45.
Habitat.—Pacific, central area, Station 274, surface.
Subgenus 2. Trigonastromma, Haeckel.
Definition.—One odd arm different in form or size from the other two arms, which are paired; distance between them different; fundamental form of the shell therefore an isosceles triangle.