Page:Scientific results HMS Challenger vol 18 part 1.djvu/641

Rh size and at unequal distances, one odd spine as long as the radius, both paired spines as long as the diameter of the disk; the odd angle between the latter is smaller. Spines pyramidal, with broad edges.

Dimensions.—Diameter of the disk 0.16, of the medullary shell 0.05, pores 0.008.

Habitat.—Fossil in the rocks of Barbados.

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

The genus Sethostaurus exhibits four marginal spines, which form commonly a more or less regular cross in the equatorial plane. Sometimes the size and disposition of the four spines become more or less different, and also the angles between them vary; the regular rectangular cross passes over into a bilateral or irregular form. The medullary shell is simple. The same cross-form of the disk is seen in Staurocyclia among the Coccodiscida, and in Staurodictya among the Porodiscida.

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

1. Sethostaurus orthostaurus, n. sp. (Pl. 31, figs. 1, 2).

Disk with smooth surface, three times as broad as the medullary shell. Pores regular, circular; thirteen to fourteen on the radius of the disk. Margin simple, smooth, without girdle. Four crossed spines equal, pommel-shaped, angular, contracted at the base, scarcely as long as the diameter of the medullary shell.

Dimensions.—Diameter of the disk 0.16, of the medullary shell 0.05; length of the crossed spines 0.04, breadth 0.02.

Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.

2. Sethostaurus conostaurus, n. sp. (Pl. 34, figs. 7, 8).

Disk with spiny surface, three times as broad as the medullary shell. Pores irregular, roundish; seven to eight on the radius. Margin simple, smooth, without girdle. Four crossed spines subregular, equal, conical, about as long as the diameter and as broad at the base as the radius of the medullary shell. (Fig. 7 exhibits the normal form, fig. 8 an abnormality with five spines.)

Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.06; length of the crossed spines 0.05, basal breadth 0.03.

Habitat.—South Atlantic, Station 333, surface.