Page:Scientific results HMS Challenger vol 18 part 1.djvu/758

550 thickened, club-shaped, with a pyramidal terminal spine. The ends of the arms are connected by a riband-shaped, straight, spongy patagium of the same breadth as the arms. Between the rhomboidal patagium and the arms remain four large rectangular triangles as interbrachial openings.

Dimensions.—Radius of the longer arms 0.2, of the shorter 0.15; basal breadth 0.02; length of the sides of the rhombic patagium 0.25.

Habitat.—Fossil in the rocks of Barbados.

Definition.— with four forked, spongy, or chambered arms, without a patagium; shell regular (not bilateral), with four equal arms crossed at right angles.

The genus Dicranastrum comprises a number of very remarkable, hitherto unknown, Euchitonida, which are rather common in the Pacific (mainly on the surface), and characterised by the bifurcation of the cross-arms of the regular square shell. It bears therefore to its probable ancestral form, Stauralastrum, the same relation that in the triradiate Euchitonida Chitonastrum does to Dictyastrum. The arms are commonly of very delicate structure, more or less spongy.

Definition.—Each cross-arm with two simple branches.

1. Dicranastrum furcatum, n. sp. (Pl. 47, fig. 2).

Arms simply forked, three times as long as broad at their base, with eight blunt ends of the fork-branches. The simple proximal half of each arm about the same size as each branch of the dichotomous distal part, twice as long as broad. Edges of the arms ragged.

Dimensions.—Radius of each arm 0.38, basal breadth 0.12; breadth of the forked part 0.3.

Habitat.—Pacific, central area, Station 271, surface.

2. Dicranastrum dichotomum, n. sp.

Arms simply forked, four times as long as broad at their base; each arm with two blunt branches. The simple proximal part of each arm is three times as long and twice as broad as each branch of the dichotomous distal part. Ends of the arms blunt, truncated.

Dimensions.—Radius of each arm 0.35, basal breadth 0.08; breadth of the forked part 0.2.

Habitat.—South Pacific, Station 281, surface.