Page:Scientific results HMS Challenger vol 18 part 1.djvu/620

412 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. 35, 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.

Definition.— 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. 32, fig. 7; Pl. 33, 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.

Definition.— with radial spines on the margin of the disk, disposed in the equatorial plane.

Definition.— with two radial spines on the margin of the disk, opposite in one equatorial axis.