Ring regular, square, connected with the central chamber by four radial beams at equal distances, opposite in pairs in two axes perpendicular one to another; therefore four equal ring chambers (or congruent quadrants of the ring). Resembles the central part of the disk of Staurodictya medusa, &c. (Pl. [42], figs. 1-3); also of Staurodictya gracilis, Ehrenberg, 1875 (Abhandl. d. k. Akad. d. Wiss. Berlin, Taf. xxiii. fig. 3).
Dimensions.—Diameter of the ring 0.05, of the central chamber 0.016.
Habitat.—North Pacific, Station 253, depth 3125 fathoms.
6. Archidiscus tetroniscus, n. sp.
Ring elliptical, of rhomboidal fundamental form, connected with the central chamber by four radial beams, halving the sides of the rhombus and opposite in pairs in two axes which are not perpendicular one to another; therefore four ring chambers in pairs different, two opposite equal and larger than the two others.
Dimensions.—Diameter of the ring 0.05 to 0.07, of the central chamber 0.016.
Habitat.—Central Pacific, Station 263, depth 2650 fathoms.
7. Archidiscus tetrathalamus, n. sp.
Ring irregular, quadrangular, connected with the central chamber by four radial beams of increasing unequal length; therefore all four ring chambers of different size, gradually increasing in the following quadrants of the ring. Important as an ancestral type of such spiral and semi-spiral forms as Staurodictya cruciata (Pl. [42], figs. 4, 12, &c.) and Stylodictya clavata, Stylodictya stellata, &c., of Ehrenberg, 1875 (Abhandl. d. k. Akad. d. Wiss. Berlin, Taf. xxiii. figs. 2, 7, 8, 9).
Dimensions.—Diameter of the ring 0.05 to 0.08, of the central chamber 0.06.