Definition.—Pylonida with simple, spherical or subspherical, central chamber, surrounded by two crossed latticed girdles, one smaller (primary) transverse and one larger (secondary) lateral girdle.
The genus Dizonium differs from the preceding Monozonium in the possession of two crossed elliptical lattice-girdles. The smaller girdle, immediately surrounding the central chamber, is the transverse girdle (the single girdle of Monozonium). On the poles of its major axis (the transverse axis) it is connected with the larger girdle, the elliptical perimeter of which circumscribes the lateral plane. The minor axis of this latter is the major axis of the former. Between the two crossed girdles remain four larger openings or gates, quite as in Tetrapyle. But whilst in Tetrapyle the enclosed medullary shell is a trizonal Larnacilla-shell, here in Dizonium it is a simple spherical or lentelliptical chamber.
Subgenus 1. Dizonaris, Haeckel.
Definition.—Shell smooth or rough, without radial spines or thorns.
1. Dizonium circulare, n. sp.
Central chamber of the shell spherical, smooth, with four to five pores on the half equator. Lateral girdle circular, three times as broad as the former. Four gates semilunar, twice as broad as high. No radial spines.
Dimensions.—Diameter of the central chamber 0.02, of the lateral girdle 0.06; height of the gates 0.02, breadth 0.04.
Habitat.—Pacific, central area, Station 274, surface.
2. Dizonium ellipticum, n. sp.
Central chamber of the shell elliptical, smooth, with three to four pores on the half equator. Lateral girdle elliptical, three times as long and broad as the central chamber. Four gates kidney-shaped, twice as broad as high. No radial spines.