Definition.—Porodiscida with three forked, chambered arms, without a patagium. (Arms and angles between them either equal or unequal.)
The genus Chitonastrum differs from its ancestral form, Dictyastrum, by the bifurcation of the distal ends of the arms. The few species of this genus are partly regular (like Dictyastrum), partly bilateral (like Rhopalastrum). If the number of species increases much, these two subgenera may be separated into two genera: Chitonastrella corresponding to the former, Chitonastromma to the latter.
Subgenus 1. Chitonastrella, Haeckel.
Definition.—All three arms of the same size and form, equidistant; fundamental form of the shell therefore an equilateral triangle.
1. Chitonastrum triglochin, n. sp.
All three arms equal and equidistant. Each arm has the form of an isosceles triangle, twice as high as broad; the truncated apex of the triangle is inserted into the large 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, simple in its basal half, double in its distal half. Axes of the six branches straight. (Resembles Trigonastrum regulare, Pl. [43], fig. 16, but differs in the absence of a patagium.)
Dimensions.—Radius of each arm 0.24, greatest breadth 0.11, basal breadth 0.03.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
2. Chitonastrum bathybium, n. sp.
All three arms equal and equidistant, in the basal two-thirds simple, rectilinear, three times as long as broad, in the distal third forked, both branches equal, straight, blunt, half as broad as the basal part.