Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.05, of the pores 0.005.
Habitat.—South Pacific, Station 285, depth 2375 fathoms.
Genus 182. Phacodiscus,[[221]] Haeckel, 1881, Prodromus, p. 457.
Definition.—Phacodiscida with double medullary shell and simple margin of the circular disk, without surrounding equatorial girdle and without radial spines.
The genus Phacodiscus differs from the ancestral genus Sethodiscus only in the duplication of the medullary shell, and has therefore the same relation to it that Thecosphæra in the Sphæroidea exhibits to Carposphæra. Both former discoidal genera differ from the two latter spheroidal by the lenticular flattening of the cortical shell.
Subgenus 1. Phacodiscinus, Haeckel.
Definition.—Surface of the disk smooth, without radial ribs or spines.
1. Phacodiscus rotula, n. sp. (Pl. [35], fig. 7).
Disk with smooth surface, four and a half times as broad as the outer and fourteen times as broad as the inner medullary shell. Pores regularly circular; sixteen to eighteen on the radius of the disk. Margin of the lens very thick, truncated, nearly as broad as the outer medullary shell.
Dimensions.—Diameter of the disk 0.21, of the outer medullary shell 0.045, of the inner 0.015; pores 0.008.