Ring triangular, roundish, equilateral, connected with the central chamber by three radial beams at equal distances (120°); therefore three equal ring chambers. (Resembles the central part of the disk of Tripodictya trigonaria, Pl. [42], fig. 8, and may be the ancestral form of it.)
Dimensions.—Diameter of the ring 0.04, of the central chamber 0.014.
Habitat.—Central Pacific, Station 265, depth 2900 fathoms.
4. Archidiscus trithalamus, n. sp.
Ring irregular, roundish, connected with the central chamber by three radial beams at unequal distances; therefore three ring chambers of different size. (If these differences be important, they introduce a spiral convolution in the further development of the Porodiscus arising from it.)
Dimensions.—Diameter of the ring 0.06, of the central chamber 0.016.
Habitat.—South Pacific, Station 288, surface.
Subgenus 3. Tetroniscus, Haeckel.
Definition.—Ring with four chambers, separated by four radial beams.
5. Archidiscus stauroniscus, n. sp. (Pl. [48], figs. 9, 9a).