Definition.—Plagonida with four radial spines.
Genus 386. Tetraplagia,[[5]] Haeckel, 1881, Prodromus, p. 424.
Definition.—Plagonida with four equal radial spines, arising from one common central point, and corresponding to the four axes of a tetrahedron.
The genus Tetraplagia is one of the most important Plectoidea, and possibly the common ancestral form of many Nassellaria (compare above, p. [901]). The skeleton is composed of four radial rods, diverging from one common centre in different directions, and corresponding to the four axes, which extend from the central point of a tetrahedron to the central points of its four faces. The whole skeleton of Tetraplagia exhibits therefore the same form, which is observed in the single quadriradiate spicula of some Beloidea (in some species of Lampoxanthium, Sphærozoum, &c.).
1. Tetraplagia geometrica, n. sp.
Spines straight, cylindrical, perfectly equal, corresponding in regular disposition exactly to the four geometrical axes of a regular tetrahedron; in the middle arise from each spine two opposite equal lateral branches of half the length of the spine.
Dimensions.—Length of the spines 0.12, of the branches 0.06.
Habitat.—Central Pacific, Station 266, surface.
2. Tetraplagia phænaxonia, n. sp. (Pl. [91], fig. 3).
Spines straight or slightly curved, three-sided prismatic, with irregular short branches arising from the three edges; the branches are thorny, tapering towards the apex.