9. Zygacantha dicopa, Haeckel.
Astrolithium dicopum, Haeckel, 1862, Monogr. d. Radiol., p. 400, Taf. xx. figs. 3, 4.
Spines compressed, two-edged, linear, of nearly equal breadth throughout their whole length, with a prominent middle rib, which in the distal third is cleft into two divergent teeth, ending in the two corners of the broad, obliquely truncated apex. All twenty spines with their central bases grown together and forming one single piece of acanthin—a star with twenty rays.
Dimensions.—Length of the spines 0.12 to 0.2, breadth 0.01 to 0.02.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, many Stations, surface.
Genus 325. Acanthonia,[[365]] Haeckel, 1881, Prodromus, p. 465.
Definition.—Astrolonchida with simple, four-edged, prismatic or pyramidal radial spines, without apophyses; their transverse section is square.
The genus Acanthonia comprises all those Astrolonchida (formerly united with Acanthometron) in which the simple spines either in their whole length or in the greatest part of it are four-edged, with square transverse section. They are sometimes more prismatic (with equal breadth), at other times more pyramidal (with decreasing breadth towards the distal apex). If Acanthometron be the common simple ancestral form of the Acanthonida, then the two-edged Zygacantha, and the four-edged Acanthonia may be regarded as two divergent main lines arising from it; the former leading to the Phractacanthida and Diporaspida, the latter leading to the Stauracanthida and Tessaraspida.
Subgenus 1. Acanthonarium, Haeckel.
Definition.—Spines at the central base, without leaf-cross and without hollow pyramidal compartments, united by the triangular faces of their pyramidal bases, resting one upon another.