Ring circular, connected with the central chamber by two radial beams, opposite in one axis, therefore two equal semicircular ring chambers. (This primitive form has an interesting reference to Saturnalis, Pl. [13], fig. 16, and differs from it only in the lattice-work covering both faces of the lenticular disk, the margin of which forms the ring.)
Dimensions.—Diameter of the ring 0.05, of the central chamber 0.016.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
2. Archidiscus dithalamus, n. sp.
Ring roundish, connected with the central chamber by two radial beams, not opposite in one axis; both semicircular ring chambers more or less unequal, one of them larger than the other, and sometimes much more prominent. (If this prominence increase, we can regard it as the beginning of spiral convolutions, Discospira.)
Dimensions.—Diameter of the ring 0.06, of the central chamber 0.014.
Habitat.—Central Pacific, Station 266, depth 2750 fathoms.
Subgenus 2. Trioniscus, Haeckel.
Definition.—Ring with three chambers, separated by three radial beams.
3. Archidiscus trioniscus, n. sp.