Page:Scientific results HMS Challenger vol 18 part 1.djvu/768

560 with numerous short conical spines and one longer terminal spine. (Resembles Stauralastrum rhopalophorum, Pl. 45, fig. 1, but with six rays instead of four.)

Dimensions.—Radius of each arm 0.3, basal breadth 0.03, distal breadth 0.08.

Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.

Definition.—Arms more or less different in size or form; shell bilateral.

3. Hexalastrum orchidaceum, n. sp. (Pl. 44, fig. 5).

Hexactura orchidacea, Haeckel, 1881, Prodromus, p. 459.

Arms different in length, so that two unequal opposite odd arms determine the main axis, and the four other arms lie on both sides of this as two different pairs. The proportion of their relative length is the following:—anterior lateral arms five, anterior odd arm six; posterior lateral arms seven, posterior odd arm eight. Each arm is club-shaped, two to three times as long as broad, and divided into six to eight joints by five to seven transverse septa; its distal end is armed with a terminal spine and twice as broad as its base.

Dimensions.—Radius of the posterior odd arm 0.4, of the anterior odd arm 0.3; of the posterior lateral pair 0.35, of the anterior lateral pair 0.25; basal breadth 0.08, distal breadth 0.16.

Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.

Definition.— with six simple, undivided, chambered arms, connected by a patagium.

The genus Hexinastrum differs from its ancestral form Hexalastrum by the development of a patagium between the arms. The only observed species is regular.

1. Hexinastrum geryonidum, n. sp. (Pl. 44, fig. 4).

Hexalastrum geryonidum, Haeckel, 1879, Atlas (pl. xliv. fig. 4).

Disk quite regular with six radii; all six arms of the same size and form, at their broad, convexly rounded, smooth end five times as broad as at their narrow base, and little longer than broad. Each arm is divided by eight transverse septa into nine simple joints or chambers of the same height; the breadth of the distal chambers increases rapidly. The regular, hexagonal, central disk exhibits four concentric rings around the central chamber. Patagium between the arms incomplete, with concavely fluted edge.

Dimensions.—Radius of each arm 0.15, basal breadth 0.016, distal breadth 0.08; radius of the central disk 0.04.

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