Page:Scientific results HMS Challenger vol 18 part 2.djvu/33

Rh 1. Triplagia primordialis, n. sp. (Pl. 91, fig. 2).

Spines straight, of equal size and similar form, equidistant, three-sided prismatic, each with two pairs of opposite lateral branches, which are correspondingly parallel to the two other spines; the proximal branches twice as long as the distal branches.

Dimensions.—Length of each spine 0.2, of the basal branches 0.07.

Habitat.—Central Pacific, Station 271, depth 2425 fathoms.

2. Triplagia triradialis, n. sp.

Spines straight, of equal size and similar form, equidistant, three-sided prismatic, regularly pinnate, with six pairs of opposite pinnulæ, tapering gradually towards the distal ends; the basal pinnulæ are again branched, with straight ramules.

Dimensions.—Length of each spine 0.27, of the basal branches 0.08.

Habitat.—North Pacific, Station 256, surface.

3. Triplagia horizontalis, n. sp.

Spines curved, cylindrical, irregularly branched, with three primary and nine to twelve secondary branches, which are also slightly curved. The angles between the bases of the three spines are equal, but the size and form of the branches different.

Dimensions.—Length of the three spines 0.15 to 0.25, of the basal branches 0.05 to 0.08.

Habitat.—West Tropical Pacific, Station 226, depth 4475 fathoms.

Definition.— with three radial spines, arising from one common central point and corresponding to the edges of a three-sided pyramid.

The genus Plagiacantha agrees with the preceding Triplagia in the simple structure of the triradial skeleton, composed of three diverging radial spines, which are united in a common centre at the oral pole of the central capsule. But whilst the three radial rods of Triplagia lie in one horizontal plane, here they lie in different planes and correspond to the three edges of a flat pyramid. Plagiacantha arachnoides, described in 1856 by Claparède, was the earliest known form of all.