The genus Dictyastrum is the simplest form of the Trigonastrida, or of the Porodiscida, in which the margin of the central disk is furnished with three chambered arms. In Dictyastrum these are quite simple and regular, without a patagium, separated by equal angles, so that the whole shell represents a regular, equilateral triangle, if we connect the distal points of the arms by lines. The genus Dictyastrum, founded by Ehrenberg in 1860, differs from his Rhopalodictyum—after his own diagnosis—only by an insignificant difference in the form of the simple arms, which is scarcely a specific character. I therefore apply this name here in the above amended sense, seeing that the only figured species of Ehrenberg (Dictyastrum angulatum) occurs in two different, but externally very similar forms: one of these is a true Porodiscid (Dictyastrum) with two porous covering-plates and concentric rings; the other is a true Spongodiscid (Rhopalodictyum) with quite spongy, irregular network, and is probably identical with the Rhopalodictyum truncatum of Ehrenberg.
Subgenus 1. Dictyastrella, Haeckel.
Definition.—Arms with blunt ends, without terminal spines.
1. Dictyastrum angulatum, Ehrenberg.
Dictyastrum angulatum, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 289, Taf. viii. fig. 18.
Arms nearly square with straight edges, towards the truncated end a little broader, about the same diameter as the triangular central disk. The figure of Ehrenberg seems to represent a Spongodiscid (Rhopalodictyum angulatum), but in the same locality (Philippine Sea) occurs also a true Dictyastrum of quite the same form, but with three to four concentric rings of the central disk, and with jointed arms.
Dimensions.—Radius of each arm (length from the centre to the distal end) 0.13; breadth of the truncated end 0.1.
Habitat.—Tropical Pacific, Philippine Sea, Station 200, depth 250 fathoms.
2. Dictyastrum bandaicum, Haeckel.
Rhopalastrum bandaicum, Harting, 1863, Mikr. Fauna Banda-Zee, p. 16, Taf. iii. fig. 45.