Definition.—Plagonida with six radial spines, arising in two opposite divergent groups from the two poles of a common central rod.
The genus Plagonium differs from the preceding closely allied Hexaplagia in the remarkable peculiarity, that the six radial spines do not arise from one common central point, but from the two poles of a common horizontal middle rod; three divergent spines on each pole. The skeleton of Plagonium exhibits therefore the same remarkable form which is found in the isolated spicula of numerous Beloidea (e.g., Lampoxanthium punctatum, Sphærozoum punctatum), and bears the same relation to Hexaplagia that Plagonidium does to Tetraplagia.
1. Plagonium sphærozoum, n. sp. (Pl. [91], fig. 6).
Spines straight and stout, three-sided prismatic, about twice as long as the common middle rod, irregularly branched or nearly verticillate, with short thorny branches. Similar to the single spicula of some species of Sphærozoum and Lampoxanthium.
Dimensions.—Length of the spines 0.12, of the middle rod 0.06.
Habitat.—Equatorial Atlantic, Station 347, surface.
2. Plagonium lampoxanthium, n. sp.
Spines irregularly curved, slender, cylindrical, six to eight times as long as the common middle rod, in the proximal half smooth, in the distal half covered with short thorns. (Similar to an isolated spiculum of Lampoxanthium punctatum or of Sphærozoum variabile, Pl. [4], fig. 5.)
Dimensions.—Length of the spines 0.18, of the middle rod 0.03.
Habitat.—North Pacific, Station 240, surface.