Page:Scientific results HMS Challenger vol 18 part 1.djvu/797

Rh and forming a larger lanceolate disk of looser framework. (Similar in form to Amphymenium pupula, Pl. 44, fig. 8, but with an irregular spongy framework and pointed ends.)

Dimensions.—Radius of the arms 0.16, breadth 0.05; major axis of the lanceolate patagium 0.32, minor axis 0.12.

Habitat.—Pacific, central area, Station 270, surface.

Definition.— with three spongy arms on the margin of the circular or triangular disk, without a connecting patagium.

The genus Rhopalodictyum comprises those very common forms of Spongodiscida in which the margin of the disk is provided with three simple, free, spongy arms. It corresponds to Dictyastrum and Rhopalastrum among the Porodiscida, to Trigonactura among the Coccodiscida. The typical species of this genus is Rhopalodictyum abyssorum of Ehrenberg, the only species figured by him. His diagnosis of the genus was very insufficient, and agreed with that of his Dictyastrum (compare above the improved diagnosis of this genus, p. 526, and my Monograph, p. 466).

Definition.—Triangular shell regular, with three arms of equal size and equidistant.

1. Rhopalodictyum abyssorum, Ehrenberg.

Rhopalodictyum abyssorum, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 299, Taf. viii. fig. 17.

Arms of equal size and equidistant, club-shaped, about as long as the diameter of the triangular central disk, and a little longer than the breadth of the pear-shaped rounded distal end. Surface rough.

Dimensions.—Radius of the arms 0.11, basal breadth 0.03, distal breadth 0.06.

Habitat.—Cosmopolitan; Atlantic, Indian, Pacific; tropical zone, surface.

2. Rhopalodictyum truncatum, Ehrenberg.

Rhopalodictyum truncatum, Ehrenberg, 1861, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 301.

? Dictyastrum angulatum, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 289, Taf. viii. fig. 18.

Arms of equal size and equidistant, nearly square, with straight edges, towards the truncated end a little broader, about of the same diameter as the central triangular disk. This species is indicated