Page:Scientific results HMS Challenger vol 18 part 1.djvu/643

Rh prismatic with prominent edges, about as long as the diameter of the disk, and as broad as the girdle.

Dimensions.—Diameter of the disk 0.16, of the medullary shell 0.08, length of the crossed spines 0.18, breadth 0.02.

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

7. Sethostaurus gigas, n. sp.

Disk with smooth surface, four times as broad as the medullary shell. Pores regular, circular; twenty to twenty-two on the radius. Margin of the disk with a narrow solid girdle, bearing on the periphery one hundred to one hundred and twenty slender flat pointed teeth. Four crossed spines equal, conical, about as long as the radius of the disk, as broad at their furrowed base as the radius of the medullary shell.

Dimensions.—Diameter of the disk 0.4, of the medullary shell 0.1; length of the crossed spines 0.5, basal breadth 0.05.

Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.

Definition.— with double medullary shell and with four radial spines on the margin of the disk, crossed in the equatorial plane.

The genus Phacostaurus differs from the foregoing Sethostaurus by the duplication of the medullary shell, and bears to it the same relation as Phacodiscus does to Sethodiscus. Also in this genus the cross of the shell is commonly regular, rectangular, sometimes more or less irregular.

Definition.—Margin of the disk without a solid equatorial girdle or a corona of spines.

1. Phacostaurus oceanidum, n. sp. (Pl. 31, fig. 6).

Disk with smooth surface, three times as broad as the outer and eight times as broad as the inner medullary shell. Pores regular, circular; eleven to twelve on the diameter of the disk. Margin smooth. Four crossed spines pyramidal, deeply sulcate, about as long as the diameter of the outer medullary shell, and two-thirds as broad at the base.

Dimensions.—Diameter of the disk 0.15, of the outer medullary shell 0.05, of the inner 0.02; length of the spines 0.05, basal breadth 0.03.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.