Definition.—Pylodiscida with Triopyle-shaped medullary shell and Pylodiscus-shaped cortical shell, which is surrounded by a marginal chambered equatorial girdle.
Genus 251. Discozonium,[[290]] n. gen.
Definition.—Pylodiscida with Triopyle-shaped medullary shell and Pylodiscus-shaped cortical shell, which is surrounded by an equatorial chambered girdle. No peculiar osculum on the margin of the disk.
The genus Discozonium and the following Discopyle make up together the small group of the Discopylida, or those Pylodiscida in which a discoidal shell like Pylodiscus is surrounded by a marginal equatorial girdle; this girdle is divided into twelve to twenty-four or more chambers by radial beams, which are the external prolongations of the radial marginal spines of Pylodiscus. The latter genus bears therefore to Discozonium the same relation as Sethodiscus in the other Discoidea does to Lithocyclia, or Phacodiscus to Coccodiscus.
1. Discozonium cyclonium, n. sp.
Disk circular, lenticular, with smooth margin, three times as broad as the triangular, Triopyle-shaped medullary shell. Three gates of the cortical shell kidney-shaped, twice as broad as the three arms. Chambered equatorial girdle with twenty to twenty-four subregular chambers (similar to Discopyle osculata, Pl. [48], fig. 19, but without any marginal osculum).
Dimensions.—Diameter of the disk 0.2, of the cortical shell 0.14, of the medullary shell 0.07.
Habitat.—Central Pacific, Station 265, depth 2900 fathoms.
2. Discozonium trigonium, n. sp.
Disk triangular, with thorny margin, four times as broad as the triangular, Triopyle-shaped medullary shell. Three gates of the cortical shell egg-shaped, scarcely as broad as the three arms. Chambered equatorial girdle with twenty-four to thirty irregular chambers.