Page:Scientific results HMS Challenger vol 18 part 1.djvu/635

Rh

Definition.— with double medullary shell, without radial spines, but with a solid equatorial girdle around the margin of the lenticular disk.

The genus Perizona differs from its ancestral form Phacodiscus in the development of a thin solid siliceous girdle, lying in the equatorial plane around the margin of the disk. The two genera bear the same relation to each other that Periphæna bears to Sethodiscus. But in the two latter genera the spherical medullary shell is simple, in the two former double.

1. Perizona scutella, n. sp. (Pl. 32, fig. 7).

Disk with smooth surface, in the distal third radiated, four times as broad as the outer, and ten times is broad as the inner medullary shell. Pores subregular, circular; thirteen to fourteen on the radius of the disk, in the marginal part separated by eighty to ninety prominent radial crests, which are prolonged into the proximal half of the solid girdle. Breadth of the girdle equal to that of the inner medullary shell.

Dimensions.—Diameter of the disk 0.25, of the outer medullary shell 0.06, of the inner 0.025; pores 0.007.

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

2. Perizona pterygota, n. sp. (Pl. 32, figs. 8, 8a).

Disk with smooth surface, six times as broad as the outer, and fifteen times as broad as the inner medullary shell. Pores regularly circular; eighteen to twenty on the radius of the disk. Margin much thickened and truncated, abruptly separated in the equatorial circumference from the peripheral sharp margin of the solid girdle; breadth of the girdle equal to that of the outer medullary shell.

Dimensions.—Diameter of the disk 0.25, of the outer medullary shell 0.04, of the inner 0.015; pores 0.006.

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

Definition.— with a constant number of radial spines on the margin of the disk (two, three, four, six, or eight), which are commonly regularly disposed (sometimes more or less irregularly).