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Definition.—Margin of the disk with a solid equatorial girdle, or a corona of spines.

2. Phacostaurus quadrigatus, n. sp.

Disk with smooth surface, four times as broad as the outer and ten times as broad as the inner medullary shell. Pores irregular, roundish; fifteen to sixteen on the diameter of the disk. Margin with a broad solid equatorial girdle, from which arise four crossed spines, conical, as long as the radius of the disk.

Dimensions.—Diameter of the disk 0.2, of the outer medullary shell 0.05, of the inner 0.02; length of the spines 0.1, basal breadth 0.03.

Habitat.—South Pacific, Station 300, depth 1375 fathoms.

3. Phacostaurus magnificus, n. sp. (Pl. 31, figs. 7, 8).

Astrostaurus magnificus, Haeckel, 1881, Prodromus et Atlas (pl. xxxi. figs.7, 8).

Disk with smooth surface, three times as broad as the outer and nine times as broad as the inner medullary shell. Pores regular, circular; thirteen to fourteen on the diameter of the disk. Margin with a corona of fifty to sixty conical spines of unequal length. Four spines of the cross very large pyramidal, deeply sulcated, longer than the radius of the disk, and as broad at the base as the radius of the medullary shell.

Dimensions.—Diameter of the disk 0.22, of the outer medullary shell 0.07, of the inner 0.025; length of the four spines 0.16, basal breadth 0.04.

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

Definition.— with simple medullary shell and with six radial spines on the margin of the disk placed in the equatorial plane.

The genus Distriactis exhibits on the margin of the lenticular phacoid shell six radial solid spines. Their distance is commonly equal, rarely unequal. Either all six spines are of equal size and similar form, or three larger (perradial) alternating with three smaller (interradial), so that Distriactis appears to be derived from Triactiscus by development of three secondary between three primary spines.

1. Distriactis liriantha, n. sp.

Disk smooth, four times as broad as the medullary shell, with regular, circular pores (about ten on the radius). Six marginal spines of equal size and at equal distances, triangular, sulcate, half as long as the radius. (Similar to Heliosestrum medusinum, Pl. 34, fig. 6.)