Page:Scientific results HMS Challenger vol 18 part 2.djvu/36

912 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.

Definition.— 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. Perhaps we find here one of the oldest and simplest types of their "triradial or cortinar structure" (compare above, p. 902).

1. Plagoniscus tripodiscus, n. sp. (Pl. 91, fig. 4).

Spines three-sided prismatic, thorny. Apical spine nearly straight, verticillate, with four to five verticils of three thorny branches, tapering towards the apex. Three basal spines somewhat shorter, curved, with three thorny edges.

Dimensions.—Length of the apical spine 0.2, of the basal spines 0.15.

Habitat.—Central Pacific, Station 263, surface.

2. Plagoniscus euscenium, n. sp.

Spines three-sided prismatic, thorny, with dentate edges, and three to six verticils of three short branches. Apical spine straight, with six verticils, nearly twice as long as the three curved basal spines, each of which bears three verticils; the basal verticils larger and ramified. Resembles somewhat Euscenium eucolpium, Pl. 53, fig. 12, but has no latticed shell.

Dimensions.—Length of the apical spine 0.3, of the basal spines 0.16.

Habitat.—North Pacific, Station 247, surface,