Page:Scientific results HMS Challenger vol 18 part 2.djvu/47

Rh 3. Plectophora novena, n. sp.

Spines slightly curved, three-sided prismatic, thorny, with two pairs of lateral branches. The six smaller distal branches end freely, whilst the six larger basal branches are connected by slender convex bows. There are therefore nine wide meshes, as in the preceding closely allied species.

Dimensions.—Length of the spines 0.18, of the basal branches 0.08.

Habitat.—North Atlantic, Færöe Channel (Gulf Stream) (John Murray), surface.

4. Plectophora pyramidalis, n. sp.

Spines straight, three-sided prismatic, with three to four verticils of short lateral branches. The branches of the basal verticils are again ramified, and form by connecting bows a delicate loose framework, covering the three sides of a flat pyramid, the three edges of which are the three radial spines.

Dimensions.—Length of the spines 0.2, base of the pyramid 0.16.

Habitat.—Central Pacific, Station 267, surface.

Definition.— with four radial spines.

Definition.— with four equal radial spines, arising from one common central point and corresponding to the four axes of a tetrahedron.

The genus Tetraplecta has been derived from Tetraplagia by union of the neighbouring branches of the four radial spines, diverging from a common point in different directions. In some forms of this genus the four rods seem to correspond exactly to the four axes, which are directed from the centre of a tetrahedron towards its four corners; whilst in other forms the four rods and the angles between them are perhaps not perfectly equal.

1. Tetraplecta tetrahedra, n. sp.

Spines straight, equal, three-sided prismatic, pinnate, each with three or four pairs of opposite straight slender pinnulæ or lateral branches; the pinnulæ of each side are correspondingly parallel.