Page:Scientific results HMS Challenger vol 18 part 1.djvu/640

432 Dimensions.—Diameter of the disk 0.4, of the outer medullary shell 0.08, of the inner 0.04; length of the polar spines 0.5, basal breadth 0.04.

Habitat.—Fossil in the rocks of Barbados.

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

The genus Triactiscus exhibits on the margin of the lenticular phacoid shell three radial solid spines, the distance between which is sometimes equal, at other times unequal. Either all the three spines are of equal size and similar form, or one odd arm is smaller than the two paired arms. The triradial form of this genus is repeated in Tripodocyclia among the Coccodiscida, and in Tripodictya among the Porodiscida. The medullary shell of Triactiscus is simple.

1. Triactiscus tripyramis, n. sp. (Pl. 33, fig. 6).

Disk with smooth surface and smooth margin, three times as broad as the medullary shell. Pores regular, circular; twelve to thirteen on the radius of the disk. Three radial spines of nearly equal size and equidistant, or one odd spine a little smaller, and the opposite angle (between the paired spines) also smaller. Spines pyramidal, with broad prominent edges, one to one and a half times as long as the radius of the disk, as broad at the base as the radius of the medullary shell.

Dimensions.—Diameter of the disk 0.15; of the medullary shell 0.05, pores 0.006.

Habitat.—Pacific, central area, Station 272, depth 2600 fathoms.

2. Triactiscus tricuspis, n. sp. (Pl. 33, fig. 5).

Disk with smooth surface and smooth margin, five times as broad as the medullary shell. Pores irregular, roundish; ten to eleven on the radius of the disk. Three radial spines of unequal size and at unequal distances, very short, conical, not longer than the radius of the medullary shell.

Dimensions.—Diameter of the disk 0.15, of the medullary shell 0.03; pores 0.004 to 0.008.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

3. Triactiscus tripodiscus, Haeckel.

Haliomma triactis, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 236, Taf. xxviii. fig, 4.

Disk with thorny surface and spiny margin, three times as broad as the medullary shell. Pores regular, circular; eight to nine on the radius of the disk. Three radial spines of different