Genus 176. Zonodiscus,[[215]] 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. [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.
Subfamily 2. Trochodiscida, Haeckel.
Definition.—Cenodiscida with radial spines on the margin of the disk, disposed in the equatorial plane.