Definition.—Two opposite arms of the cross larger than the two others.
3. Stephanastrum rhombus, Ehrenberg.
Stephanastrum rhombus, Ehrenberg, 1854, Mikrogeol., Taf. xxxvi. fig. 33; Abhandl. d. k. Akad. d. Wiss. Berlin, 1875, Taf. xxv. fig. 1.
Two arms of the longitudinal axis one and a third times as long as two arms of the transverse axis. All four arms linear, about eight times as long as broad, at their distal end somewhat 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.
Genus 238. Dicranastrum,[[277]] Haeckel, 1881, Prodromus, p. 460.
Definition.—Porodiscida 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.