Dimensions.—Length of the spines 0.15, of the basal branches 0.03.
Habitat.—Tropical Atlantic, Station 332, surface.
3. Tetraplagia abietina, Haeckel.
Plagiacantha abietina, var. quadrispina, Richard Hertwig, 1879, Organismus d. Radiol., p. 73.
Spines straight, three-sided prismatic, verticillate, with six to eight verticils of three simple straight branches; the branches of each edge are parallel, tapering towards the distal end. R. Hertwig regards this species only as a four-spined variety of his three-spined Plagiacantha abietina; but a specimen, observed by me in Corfu, exhibited all the characters of Tetraplagia.
Dimensions.—Length of the spines 0.2, of the basal branches 0.07.
Habitat.—Mediterranean (Messina, Corfu), surface.
Genus 387. Plagoniscus,[[6]] n. gen.
Definition.—Plagonida with four unequal radial spines, arising from one common central point; one vertical or apical spine opposed to three divergent or basal spines.
The genus Plagoniscus agrees with the preceding Tetraplagia (its probable ancestral form) in the possession of four radial spines, diverging from one common central point. But whilst in this latter all four spines are equal, corresponding exactly to the four axes of a tetrahedron, here in Plagiocarpa an important difference exists between one vertical or apical spine and three other divergent basal spines; these latter corresponding probably to the three "feet," the former to the single "apical horn" of the majority of Nassellaria. Perhaps we find here one of the oldest and simplest types of their "triradial or cortinar structure" (compare above, p. [902]).