Page:Scientific results HMS Challenger vol 18 part 1.djvu/752

544 principal arm with twelve joints, one and a half times as long as the anterior (with nine joints) and twice as long as the two lateral arms (each with six joints). The form and structure of the arms in this species are nearly the same as in Tessarastrum straussi (Pl. 45, fig. 8); but the arms are broader in the middle, and are not connected by a patagium.

Dimensions.—Radius of the principal posterior arm 0.2, of the anterior 0.15, of each lateral arm 0.1; greatest breadth (in the width) 0.01, basal breadth 0.03.

Habitat.—North Atlantic, Færöe Channel, Gulf Stream, surface, John Murray.

Definition.— with four simple, undivided, chambered arms, connected by a patagium; square shell a regular cross, with four equal arms and four right angles between them.

The genus Histiastrum, quite insufficiently characterised by Ehrenberg (1847), was afterwards (1875) illustrated by the figures of two different fossil species. One of these, Histiastrum ternarium, with three arms, belongs to Hymeniastrum; the other, Histiastrum quaternarium, is here retained as the true, typical representative species of the genus. It differs from its ancestral form Stauralastrum, by the possession of a patagium, from Tessarastrum by the regular square form of the shell.

Definition.—Distal ends of the arms blunt, without terminal spines.

1. Histiastrum quadrigatum, n. sp. (Pl. 46, fig. 3).

Arms at their distal end nearly as broad as long, and four times as broad as at their narrow base; their lateral edges concave, their terminal edge convex, without spines. Each arm is divided by seven to eight convex transverse septa into eight to nine simple, broad chambers. Central disk with three to four rings, about as broad as the fifth chamber. Patagium complete, connecting all the lateral edges of the arms.

Dimensions.—Radius of each arm 0.15, basal breadth 0.03, terminal breadth 0.12.

Habitat.—Equatorial Atlantic, Station 347, surface.

2. Histiastrum excisum, n. sp.

Arms four times as long as broad at their base, and twice as broad at their rounded blunt distal end as at their base; their lateral edges rectilinear, divergent. Central disk with three to four rings,