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

The genus Periphæna, founded by Ehrenberg in 1873 for the fossil Periphæna decora of Barbados, differs from its ancestral form Sethodiscus in the development of a very thin siliceous solid girdle around the margin of the lenticular disk; this girdle lies in the equatorial plane of the shell, and reappears in similar form in Perichlamydium among the Porodiscida, in Spongophacus among the Spongodiscida, and in Zonodiscus among the Cenodiscida.

1. Periphæna cincta, n. sp. (Pl. 33, fig. 4).

Disk with smooth surface, four times as broad as the medullary shell. Pores regularly circular; fourteen to sixteen on the radius of the disk. Girdle of the margin about half as broad as the radius of the medullary shell, in the distal half structureless, in the proximal half with seventy to eighty short radial ribs.

Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.05, of the pores 0.005.

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

2. Periphæna decora, Ehrenberg.

Periphæna decora, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 80, Taf. xxviii. fig. 6.

Disk with smooth surface, three times as broad as the medullary shell. Pores regular, circular; twenty to twenty-two on the radius of the disk, disposed in radial series. Girdle of the margin nearly as broad as the radius of the medullary shell, in the distal half structureless, in the proximal half with eighty to ninety short radial ribs.

Dimensions.—Diameter of the disk 0.25, of the medullary shell 0.08, of the pores 0.06.

Habitat.—Fossil in Barbados.

3. Periphæna statoblastus, n. sp.

Disk with smooth surface, five times as broad as the medullary shell. Pores regularly circular; twenty-four to twenty-six on the radius of the disk, disposed in radial series, those in the distal half of the disk being separated by piercing radial beams. Girdle of the margin twice as broad as the radius of the medullary shell, in the whole breadth with one hundred and twenty to one hundred and thirty piercing radial ribs, the prolongations of the beams.

Dimensions.—Diameter of the disk 0.3, of the medullary shell 0.06, of the pores 0.04.

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