Page:Scientific results HMS Challenger vol 18 part 1.djvu/745

Rh 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.

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.

Dimensions.—Radius of each arm 0.18, basal breadth 0.04, breadth of the branches 0.02.

Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.

Definition.—One odd arm different in size or form from the two other arms, which are paired; distance between them different; fundamental form of the shell therefore a bilateral, isosceles triangle.

3. Chitonastrum jugatum, n. sp. (Pl. 43, fig. 14).

Dictyastrum jugatum, Haeckel, 1881, Prodromus et Atlas (pl. xliii. fig. 14).

Arms very different; odd arm club-shaped, twice as long as broad, at the blunt distal end twice as broad as at the base; its axis is perpendicular to the common axis of both paired arms, which are only two-thirds as long, not so broad, and in the distal half divided into two branches; the anterior branch is straight, nearly horizontal, the posterior shorter and curved backwards.