Page:Scientific results HMS Challenger vol 18 part 2.djvu/369

Rh

Definition.—Thorax with six radial ribs (three primary perradial and three secondary interradial beams).

6. Sethophormis hexalactis, n. sp. (Pl. 56, fig. 6).

Cephalis large, hemispherical, with irregular, polygonal pores. Thorax flatly campanulate, with subregular, hexagonal pores, and six stout radial ribs; the lateral connected in pairs. (Fig. 6 exhibits the central part of the shell, seen from below.) In the subcircular collar septum the two anterior (jugular) meshes are much smaller than the two posterior (cardinal) meshes. The odd anterior (sternal) rib is opposite to the posterior (caudal) rib. The two paired ribs of each side (antero-lateral or pectoral and postero-lateral or ventral ribs) diverge from one common point. In the specimen figured the left half of the thorax exhibited the two regular ribs, and the right half three, a posterior accessory branch being developed (transition to Octophormis). This specimen might indeed be called Heptaphormis heptalactis.

Dimensions.—Cephalis 0.02 long, 0.05 broad; thorax 0.08 long, 0.22 broad.

Habitat.—Central Pacific, Station 274, depth 2750 fathoms.

7. Sethophormis hexagonalis, n. sp.

Cephalis small, flat, cap-shaped, with small, regular, hexagonal pores. Thorax flatly conical, nearly discoidal, with larger pores of the same form, and six stout, straight, prominent ribs. These arise at equal distances from the six corners of the hexagonal base of the collar septum. Peristome hexagonal, with a corona of short spines.

Dimensions.—Cephalis 0.015 long, 0.04 broad; thorax 0.06 long, 0.24 broad.

Habitat.—North Atlantic, Station 354, surface.

Definition.—Thorax with eight radial ribs, four secondary interradial beams being interpolated between the four primary perradial beams of Tetraphormis.

8. Sethophormis octalactis, n. sp.

Cephalis large, hemispherical, with irregular, polygonal pores. Thorax flatly campanulate, with regular, hexagonal pores, and eight stout, regularly-disposed, radial ribs reaching the margin at equal distances. Four of these correspond to the four primary ribs of Tetraphormis cruciata, and exhibit at their base a larger pair of basal pores, surrounded by a semicircular ring (Pl. 70, fig. 5); the other four are secondary, interpolated. Peristome circular, simple.

Dimensions.—Cephalis 0.03 long, 0.06 broad; thorax 0.1 long, 0.22 broad.

Habitat.—Indian Ocean, Madagascar (Rabbe), surface.