Habitat.—Indian Ocean, Ceylon, Belligemma, Haeckel, surface.
11. Rhopalastrum arcticum, n. sp. (Pl. [43], fig. 6).
Distance between the paired arms half as large as their distance from the odd arm, which is a little larger. All three arms of the same form, lanceolate, twice to three times as long as broad, twice as broad in the middle as at either end. Each arm with twelve to fourteen transverse septa, at the distal end with a bunch of conical spines, and one single, very large, pyramidal, terminal spine.
Dimensions.—Radius of each arm (without spine) 0.17, greatest breadth of it 0.05 to 0.06.
Habitat.—Arctic Ocean, lat. 83° 19' N., North Polar expedition of the "Alert."
Genus 229. Hymeniastrum,[[268]] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.
Definition.—Porodiscida with three simple, undivided, chambered arms, connected by a patagium; triangular shell regular, with three equal arms and three equal angles.
The genus Hymeniastrum was founded by Ehrenberg (1847) with a very incomplete diagnosis, and hitherto known only by one single species, figured by him as Hymeniastrum pythagoræ (Mikrogeol., 1854, Taf. xxxvi. fig. 31). This form occurs in two different states, externally quite identical; in one state the central disk (as figured, loc. cit.), is a simple lens or hollow disk, containing a medullary shell or "central chamber"; in the other state the central disk is composed of two concentric rings surrounding the "central chamber." We retain here the name Hymeniastrum for this latter state, expressed in the diagnosis given above, and call the former state (the Coccodiscid) Hymenactura (compare above, p. 473). One practical advantage, obtained in this way, is that all genera of Discoidea ending with "-astrum" belong to the Porodiscida. Hymeniastrum differs from Dictyastrum by the possession of a patagium, and from Euchitonia by the equal size of the angles and the arms.
Subgenus 1. Hymenastrella, Haeckel.
Definition.—Arms with blunt ends, without terminal spines.