Page:Scientific results HMS Challenger vol 18 part 2.djvu/52

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Definition.— with six radial spines, arising in two opposite divergent groups from the two poles of a common central rod.

The genus Plectanium has been derived from Plagonium by concrescence of the meeting branches, and bears therefore to it the same relation that the preceding Hexaplecta exhibits to Hexaplagia. Whilst in these two latter genera the six spines arise from a common central point, they arise here in two divergent groups from the two poles of a horizontal common middle rod, similar to the spicula of many.

1. Plectanium trigeminum, n. sp. (Pl. 91, fig. 11).

Spines straight and stout, six to eight times as long as the common middle rod, three-sided prismatic; in the distal half thickened, with three divergent terminal thorns; in the basal half with three to four verticils of thin lateral branches, which are again ramified, and by their united threads produce an irregular loose framework.

Dimensions.—Length of the spines 0.25, basal breadth 0.005, terminal breadth 0.02; length of the middle rod 0.03.

Habitat.—North Pacific, Station 244, surface.

2. Plectanium ovodimare, n. sp.

Spines straight and stout, three-sided prismatic, about four times as long as the common middle rod, gradually thinned towards the distal end; in the basal half with two to three verticils of forked branches, which are dichotomously ramified, and by their united threads form a loose ovate framework.

Dimensions.—Length of the spines 0.17, of the middle rod 0.04.

Habitat.—North Pacific, Station 236, surface.

3. Plectanium sphærozoum, n. sp.

Spines straight, cylindrical, ten to twelve times as long as the common middle rod, with six to eight verticils of thorny branches, tapering towards the distal end; the branches of the verticils are simple in the distal half, again ramified in the basal half, and here connected together by irregularly branched threads forming a loose framework. (Resembles one spiculum of Sphærozoum verticillatum, Pl. 4, fig. 7, but is more richly branched, with united ramules.)

Dimensions.—Length of the spines 0.2, of the middle rod 0.01.

Habitat.—Central Pacific, Station 266, surface.