Subfamily 1. Zonodiscida, Haeckel

Definition.—Cenodiscida without radial spines on the margin of the disk.

Genus 175. Cenodiscus, n. gen.

Definition.—Cenodiscida with simple margin of the circular disk, without surrounding equatorial girdle and without radial spines.

The genus Cenodiscus is the most simple and primitive form of all Discoidea, and represents possibly the common ancestral form of this order. The latticed shell is a simple biconvex lens, and encloses a smaller central capsule of the same form, separated from it by the jelly-veil. Cenodiscus can be derived phylogenetically either from Cenosphæra by lenticular flattening of a simple latticed sphere, or directly from Actissa by formation of a lenticular fenestrated shell around the lentiform central capsule. Possibly also some forms of Cenodiscus can be derived from Sethodiscus by reduction and loss of the medullary shell.

1. Cenodiscus phacoides, n. sp. (Pl. , figs. 1, 1a).

Disk with smooth surface, without radial ribs or spines. Margin of the lenticular biconvex disk thin, simple. Pores regular, circular; fifteen to sixteen on the radius of the disk. (Very similar to Sethodiscus phacoides, but without medullary shell.)

Dimensions.—Diameter of the disk 0.2, of the pores 0.005.

Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.

2. Cenodiscus rotula, n. sp.

Disk with smooth surface, without medial ribs or spines. Margin of the disk blunt, very thick, rounded; both faces little convex. Pores regular, circular; thirteen to fourteen on the radius of the disk. (Similar to Phacodiscus rotula, Pl. , fig. 7, but without medullary shell.)

Dimensions.—Diameter of the disk 0.16, of the pores 0.006.

Habitat.—North Pacific, Station 241, depth 2300 fathoms.

3. Cenodiscus lenticula, n. sp.

Disk with thorny surface, scattered with small, conical spines. Margin of the lenticular biconvex disk thin. Pores irregular, roundish; ten to eleven on the radius of the disk.

Dimensions.—Diameter of the disk 0.15, of the pores 0.008.

Habitat.—South Pacific, Station 295, depth 1500 fathoms.

Genus 176. Zonodiscus, n. gen.

Definition.—Cenodiscida with surrounding solid equatorial girdle on the margin of the lenticular disk, without radial spines.

The genus Zonodiscus differs from the preceding Cenodiscus only in the development of a solid siliceous girdle around the keen margin of the lenticular disk. This form can also be derived from Periphæna or Perizona by reduction and loss of the medullary shell. The same girdle formation returns not only in both these Phacodiscida (Pl. , fig. 7; Pl. , fig. 4), but also in the Porodiscid Perichlamydium.

1. Zonodiscus saturnalis, n. sp.

Disk with smooth surface, without radial spines. Pores regular, circular, fifteen to sixteen on the radius of the disk, in its distal half arranged in fifty to sixty radial series, which are separated by prominent radial crests or ribs. The crests are prolonged into the proximal half of the thin solid equatorial girdle, which is one-third as broad as the radius of the disk.

Dimensions.—Diameter of the disk 0.25, of the pores 0.05.

Habitat.—South Atlantic, Station 335, depth 1425 fathoms.