Page:Scientific results HMS Challenger vol 18 part 1.djvu/636

428

Definition.— with simple medullary shell and with two radial spines on the margin of the disk, opposite in the equatorial axis.

The genus Sethostylus opens the series of the Heliosestrida or of those Phacodiscida in which a constant number of radial spines (two to eight) is more or less regularly disposed on the margin of the disk. All these marginal spines lie in the equatorial plane of the lens, and have in the same species a rather constant number and similar size, a certain form and disposition. Sometimes they incline more or less to irregular variations. In Sethostylus there are only two spines, opposite in the equatorial diameter of the lens; it corresponds to Stylocyclia among the Coccodiscida, and to Xiphodictya among the Porodiscida.

Definition.—Margin of the disk simple, smooth, without equatorial girdle and without a corona of marginal spines.

1. Sethostylus distyliscus, n. sp. (Pl. 31, fig. 9).

Disk with smooth surface, three times as broad as the medullary shell. Pores regular, circular; nine to ten on the radius of the disk, in the outer rows cylindrical. Margin of the disk simple, smooth, thin, without peculiar equatorial girdle, radially striped by the prominent beams of the peripheral series of pores. Both marginal spines of equal size, pyramidal, sulcated, about as long as the radius of the disk and as broad at the base as the medullary shell.

Dimensions.—Diameter of the disk 0.13 to 0.16, of the medullary shell 0.04 to 0.05; length of both opposite spines 0.06 to 0.08, basal breadth 0.03 to 0.04.

Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.

2. Sethostylus dicylindrus, n. sp. (Pl. 31, fig. 10).

Disk with smooth surface, four times as broad as the medullary shell. Pores regular, circular; sixteen to eighteen on the radius of the disk. Margin of the disk simple, smooth, thick, without peculiar equatorial girdle. Both marginal spines cylindrical, about as long as the diameter of the disk, about twice as broad as a single pore. (Walls of the disk in the central part twice to three times as thick as in the peripheral part.)

Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.05; length of both spines 0.2 to 0.3, breadth 0.008.

Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.