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.
Genus 235. Histiastrum,[[274]] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.
Definition.—Porodiscida 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.
Subgenus 1. Histiastrella, Haeckel.
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.