Page:Scientific results HMS Challenger vol 18 part 1.djvu/764

556 13. Myelastrum anomalum, n. sp. (Pl. 47, fig. 9).

All four arms of different size and form; anterior arms broader, posterior longer; one anterior arm trifid, the three other arms bifid; length of the branches unequal; all four angles between the arms unequal. (This anomalous form, seen only once, may be an individual abnormality.)

Dimensions.—Radius of the arms 0.4 to 0.7; constrictions 0.3.

Habitat.—North Pacific, Station 237, surface.

Definition.— with five simple, undivided, chambered arms, without a patagium.

The genus Pentalastrum opens the small series of Euchitonida, in which the shell is not provided with three or four arms, as usual, but with five. All forms of this little group are rare. Some species resemble in their external form and in the articulation of their arms certain forms of Asterida. In Pentalastrum, the most simple genus, the five arms are simple, not forked, and without a patagium. It can be derived from Dictyastrum or Stauralastrum by increase of the number of arms.

Definition.—All five arms equal, with equal angles between them. Shell a regular pentagon.

1. Pentalastrum asteracanthion, n. sp.

All five arms equal, club-shaped, at their thickened obtuse end three times as broad as at their base, twice as long as broad. Angles between the arms equal.

Dimensions.—Radius of each arm 0.2, basal breadth 0.03, distal breadth 0.08.

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

2. Pentalastrum astropecten, n. sp.

All five arms equal, with five to six distinct, simple joints, the basal joint two-thirds as broad as the terminal joint, which bears a strong conical spine. Angles between the arms equal. (Resembles Pentinastrum asteriscus, Pl. 44, fig. 2, but has no patagium.)

Dimensions.—Radius of each arm 0.14, basal breadth 0.024, distal breadth 0.036.

Habitat.—Pacific, central area, Station 263, depth 2650 fathoms.