Page:Scientific results HMS Challenger vol 18 part 2.djvu/377

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Definition.— (vel Dicyrtida multiradiata aperta) with numerous straight or slightly curved radial ribs in the wall of the pyramidal thorax. Network simple. Cephalis commonly without horn.

The genus Sethopyramis and the four following genera form together a peculiar small group (Sethopyramida), characterised by the slender pyramidal shell, in the wall of which numerous (six to nine or more) radial ribs are enclosed. These ribs may be regarded as the terminal feet of a Calpophæna or Halicalyptra, which become connected by transverse bars. Sethopyramis may be derived also directly from Patagospyris by loss of the primary sagittal ring and reduction of the cephalis.

Definition.—Pyramidal shell with six radial main beams (sometimes also five or seven, as individual varieties).

1. Sethopyramis scalaris, Haeckel.

Cornutella scalaris, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 68, Taf. ii. fig. 1.

Shell smooth, slenderly pyramidal, hexagonal. Cephalis large, subglobular, hyaline, without pores (or with very small and scarce pores). Thorax with six straight (or little curved) stout longitudinal ribs or radial beams, which are connected by twenty to thirty complete, hexagonal, horizontal rings. The large meshes so produced are regular, square, their length and breadth equally and gradually increasing towards the mouth.

Dimensions.—Cephalis 0.04 diameter; thorax (with twenty-four transverse rings) 0.4 long, 0.15 broad at the mouth.

Habitat.—Fossil in Barbados.

2. Sethopyramis hexalactis, n. sp.

Shell thorny, of the same form as, and similar structure to, the preceding species, but differing in the spiny surface and the smaller subglobular cephalis, which bears small circular pores and an oblique conical horn twice the length. The whole surface is covered with short conical thorns.

Dimensions.—Cephalis 0.02 diameter; thorax (with twenty transverse rings) 0.25 long, 0.1 broad.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

Definition.—Pyramidal shell with nine radial main beams (sometimes also eight or ten, as individual varieties).