Genus 324. Zygacantha,[[364]] J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 51.
Definition.—Astrolonchida with simple, compressed, and two-edged radial spines, without apophyses; their transverse section is elliptical or rhomboidal.
The genus Zygacantha comprised in the original definition of J. Müller only a single species, Zygacantha furcata, distinguished from the other Acanthometra by forked spines with two long parallel teeth. It seems now advisable to unite in this genus all those Astrolonchida in which the simple spines are two-edged, compressed, or leaf-shaped. The term Zygacantha may be conceived as the general expression of the important fact, that in all Icosacantha the twenty spines are opposite in pairs.
Subgenus 1. Zygacantharium, Haeckel.
Definition.—Spines at the central base without leaf-cross and without hollow pyramidal compartments, united by the opposed triangular faces of their pyramidal bases, resting one upon another.
1. Zygacantha lanceolata, Haeckel.
Acanthometra lanceolata, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 48, Taf. xi. fig. 12.
Acanthometra lanceolata, Haeckel, 1862, Monogr. d. Radiol., p. 380.
Spines lanceolate, from the broader middle part equally thinned towards the two ends. Apex simple. Base pyramidal, without leaf-cross. Each flat lamellar spine exhibits an elevated middle rib (like a lanceolate leaf), and is therefore compressed quadrangular.
Dimensions.—Length of the spines 0.1 to 0.15, greatest breadth (in the width) 0.03 to 0.04.