Page:Scientific results HMS Challenger vol 18 part 2.djvu/48

924 All pinnulæ connected by a few slender bridges, thus producing a delicate network with irregular rhomboidal meshes.

Dimensions.—Length of the spines 0.15, of the basal branches 0.05.

Habitat.—Central Pacific, Station 272, depth 2600 fathoms.

2. Tetraplecta quadricornis, n. sp.

Spines in the basal half straight, in the distal half slightly curved and irregularly branched, with two to three pairs of unequal alternate lateral branches, the distal ends of which are connected by a few slender bows, marking the six edges of an irregular tetrahedron.

Dimensions.—Length of the spines 0.22, of the basal branches 0.14.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

3. Tetraplecta pinigera, n. sp. (Pl. 91, fig. 8).

Spines three-sided prismatic, straight, or in the basal half slightly curved, verticillate; each spine with six to eight three-branched regular verticils, tapering gradually towards the distal end. All branches of each spine lie parallel in three equidistant meridian planes, and are connected by delicate parallel threads, perpendicular to the branches. Therefore the skeleton consists of four pine-shaped trees and twelve delicate triangular wings with rectangular meshes.

Dimensions.—Length of the spines 0.25 to 0.3, of the basal branches 0.1 to 0.15.

Habitat.—Central Pacific, Station 271, 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 Plectaniscus has probably been derived from Plagoniscus by reticular union of the neighbouring branches of the four radial spines, and exhibits therefore to it the same relation that Tetraplecta bears to Tetraplagia. Whilst in these two latter genera the four spines are equal, in the two former genera there is an important difference between a vertical spine (or apical horn) and three divergent (commonly larger) spines, corresponding to the three basal feet of the triradiate. Perhaps, therefore, Plectaniscus is one of the remotest ancestors of the latter; but differs in the absence of a complete lattice-shell.