Page:Scientific results HMS Challenger vol 18 part 1.djvu/713

Rh Dimensions.—Diameter of the disk (with six rings) 0.14; breadth of each ring 0.01 to 0.012; pores 0.002 to 0.006.

Habitat.—Pacific, central area, Station 272, depth 2600 fathoms.

Definition.— with three solid, equidistant, radial spines on the margin of the circular or triangular disk.

The genus Tripodictya exhibits three radial spines on the margin of the disk, divergent at equal angles; rarely in some specimens the angles differ more or less. Perhaps this genus bears a near relation to the Dictyastrida (or to the Euchitonida with three chambered arms—Dictyastrum, Euchitonia, &c.).

1. Tripodictya trigonaria, n. sp. (Pl. 42, fig. 8).

All rings of the disk concentric, triangular, with three equal convex sides, all of the same breadth; first and second rings with three simple chambers, third and fourth rings with six chambers, fifth ring with twelve chambers. Central chamber also equilateral triangular, from its three corners arise three piercing perradial beams; from the second ring arise three interradial beams, alternate with the latter; from the fourth ring between these and the former arise six adradial beams. Pores subregular, two on the breadth of each ring. Three marginal spines pyramidal, as long as the radius of the disk and as broad as one ring.

Dimensions.—Diameter of the disk (with five rings) 0.11; breadth of each ring 0.011; pores 0.004.

Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.

2. Tripodictya triacantha, n. sp. (Pl. 42, fig. 7).

All rings of the disk concentric, circular, of the some breadth, connected by very numerous irregular radial beams, the number of which increases towards the periphery. Pores irregular, roundish, two on the breadth of each ring. Three marginal spines spindle-shaped, as long as the radius of the disk, and in the middle part as broad as one ring.

Dimensions.—Diameter of the disk (with eight rings) 0.2; breadth of each ring 0.012; pores 0.004.

Habitat.—North Pacific, Station 253, depth 3125 fathoms.

3. Tripodictya tribelonia, n. sp. (Pl. 42, fig. 9).

All rings of the disk, or a part of them, not concentric, spirally convoluted, of equal breadth, connected by numerous irregular, interrupted radial beams, the number of which increases from the centre. (In one marginal view of the disk, Pl. 42, fig. 9, the disk seemed to be composed of seven