Subfamily 1. Sethodiscida, Haeckel, 1881, Prodromus, p. 457

Definition.—Phacodiscida without radial spines on the margin of the disk.

Genus 181. Sethodiscus, Haeckel, 1881, Prodromus, p. 457.

Definition.—Phacodiscida with simple medullary shell and simple margin of the circular disk, without surrounding equatorial girdle and without radial spines.

The genus Sethodiscus is the most simple and primitive form of all Phacodiscida, and may be regarded as the common ancestral form of this family. The simple spherical medullary shell is connected by a variable number of radial beams with the lenticular or discoidal cortical shell (or "phacoid shell"). The margin of this latter is quite simple, circular, without solid equatorial girdle or radial spines. From the nearly allied genus Carposphæra of the Sphæroidea, its probable ancestral form, Sethodiscus can be derived simply by lenticular compression of the spheroidal cortical shell.

Subgenus 1. Sethodiscinus, Haeckel.

Definition.—Surface of the disk smooth, without radial ribs or spines.

1. Sethodiscus phacoides, n. sp.

Disk with smooth surface, three times as broad as the medullary shell. Pores regular, circular; fourteen to fifteen on the radius of the disk. (Very similar to Periphæna cincta, Pl. , fig. 4, but without the girdle of the margin.)

Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.07, of the pores 0.005.

Habitat.—Pacific, central area, Stations 270 to 274, in various depths.

2. Sethodiscus macroporus, n. sp.

Disk with smooth surface, twice as broad as the medullary shell. Pores regular, circular, very large; five to six on the radius of the disk. (Remarkable for the extraordinary size of the pores, which reaches half the radius of the medullary shell.)

Dimensions.—Diameter of the disk 0.1, of the medullary shell 0.05, of the pores 0.012.

Habitat.—North Atlantic, Gulf Stream, Færöe Channel, John Murray.

3. Sethodiscus microporus, n. sp.

Disk with smooth surface, four times as broad as the medullary shell. Pores regular, circular, very small; twenty-two to twenty-four on the radius of the disk. (The small pores are scarcely half as broad as the thick bars between them.)

Dimensions.—Diameter of the disk 0.25, of the medullary shell 0.06, of the pores 0.002.

Habitat.—South Pacific, Station 288, surface.

4. Sethodiscus lenticula, n. sp. (Pl. , figs. 1, 2).

Disk with smooth surface, four times as broad as the medullary shell. Pores irregular, polygonal; eight to nine on the radius of the disk. (The pores of the medullary shell, fig. 2, are also irregular, polygonal, or roundish.)

Dimensions.—Diameter of the disk 0.17, of the medullary shell 0.04, of the pores 0.01.

Habitat.—Pacific, central area, Station 274, depth 2750 fathoms.

5. Sethodiscus macrococcus, n. sp. (Pl. , fig. 3).

Disk with smooth surface, two and a half times as broad as the medullary shell. Pores irregular, polygonal; eleven to twelve on the radius of the disk. (The pores of the medullary shell, fig. 3, are regular, circular, with elevated hexagonal frames between them. The figured specimen is a young one, both halves of the biconvex disk being not yet united in the equatorial plane.)

Dimensions.—Diameter of the disk 0.16, of the medullary shell 0.065, of the pores 0.01.

Habitat.—North Pacific, Station 244, depth 2900 fathoms.

6. Sethodiscus micrococcus, n. sp.

Disk with smooth surface, five times as broad as the medullary shell. Pores irregular, roundish, very small; twenty-six to twenty-eight on the radius of the disk. (The pores of the small medullary shell are regularly circular, of the same size as those of the disk.)

Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.04, of the pores 0.002.

Habitat.—South Atlantic, Station 335, depth 1425 fathoms.

Subgenus 2. Sethodisculus, Haeckel.

Definition.—Surface of the disk with elevated radial ribs or spines.

7. Sethodiscus radiatus, Haeckel.

Haliomma radians, Ehrenberg, 1854, Mikrogeol., Taf. xix. fig. 50.

Haliomma radiatum, Ehrenberg, 1854, Mikrogeol., Taf. xxi. fig. 54.

Disk with radiated surface, four times as broad as the medullary shell. Pores regularly circular; nine to ten on the radius of the disk, arranged in radial series (about forty), which are separated by smooth crests, not prominent on the smooth margin.

Dimensions.—Diameter of the disk 0.12, of the medullary shell 0.03, of the pores 0.003.

Habitat.—Fossil in Tertiary deposits of the Mediterranean, Greece, Sicily, Oran, &c.

8. Sethodiscus echinatus, Haeckel.

Haliomma echinatum, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 74, Taf. xxvii. fig. 2.

Disk with radiated thorny surface, three times as broad as the medullary shell. Pores regular, circular; eleven to twelve on the radius of the disk, arranged in radial series (about sixty), which are separated by thorny crests, prominent a little on the dentated margin.

Dimensions.—Diameter of the disk 0.18, of the medullary shell 0.06, of the pores 0.01.

Habitat.—Fossil in the rocks of Barbados.

9. Sethodiscus cristatus, n. sp.

Disk with radiated surface, four times as broad as the medullary shell. Pores regularly circular; sixteen to seventeen on the radius of the disk, arranged in radial series (about eighty), which are separated in the distal half by smooth elevated crests, strongly prominent on the dentated margin. (Very similar to Phacodiscus cristatus, Pl. , fig. 6, but with simple medullary shell.)

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

Habitat.—South Pacific, Station 285, depth 2375 fathoms.

Genus 182. Phacodiscus, Haeckel, 1881, Prodromus, p. 457.

Definition.—Phacodiscida with double medullary shell and simple margin of the circular disk, without surrounding equatorial girdle and without radial spines.

The genus Phacodiscus differs from the ancestral genus Sethodiscus only in the duplication of the medullary shell, and has therefore the same relation to it that Thecosphæra in the Sphæroidea exhibits to Carposphæra. Both former discoidal genera differ from the two latter spheroidal by the lenticular flattening of the cortical shell.

Subgenus 1. Phacodiscinus, Haeckel.

Definition.—Surface of the disk smooth, without radial ribs or spines.

1. Phacodiscus rotula, n. sp. (Pl. , fig. 7).

Disk with smooth surface, four and a half times as broad as the outer and fourteen times as broad as the inner medullary shell. Pores regularly circular; sixteen to eighteen on the radius of the disk. Margin of the lens very thick, truncated, nearly as broad as the outer medullary shell.

Dimensions.—Diameter of the disk 0.21, of the outer medullary shell 0.045, of the inner 0.015; pores 0.008.

Habitat.—North Pacific, Station 224, depth 1850 fathoms.

2. Phacodiscus lentiformis, n. sp. (Pl. , fig. 8).

Disk with smooth surface, three and a half times as broad as the outer, and ten times as broad as the inner medullary shell. Pores regularly circular; twelve to thirteen on the radius of the disk. Margin of the disk sharp, as in a biconvex lens.

Dimensions.—Diameter of the disk 0.25, of the outer medullary shell 0.07, of the inner 0.025; pores 0.01.

Habitat.—Central Pacific, Station 265, depth 2900 fathoms; fossil in Barbados.

3. Phacodiscus grandis, n. sp.

Disk with thorny surface, five times as broad as the outer, and fifteen times as broad as the inner medullary shell. Pores regularly circular; twenty-two to twenty-four on the radius of the disk. Margin of the disk thick, rounded.

Dimensions.—Diameter of the disk 0.45, of the outer medullary shell 0.09, of the inner 0.03; pores 0.01.

Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.

Subgenus 2. Phacodisculus, Haeckel.

Definition.—Surface of the disk with elevated radial ribs or spines.

4. Phacodiscus echiniscus, n. sp.

Disk with spiny surface, four times as broad as the outer, and ten times as broad as the inner medullary shell. Pores irregularly roundish; fourteen to fifteen on the radius of the disk. Margin of the disk sharp, as in a biconvex lens.

Dimensions.—Diameter of the disk 0.3, of the outer medullary shell 0.75, of the inner 0.03; pores 0.007.

Habitat.—South Atlantic, Station 332, depth 2200 fathoms.

5. Phacodiscus clypeus, n. sp. (Pl. , figs. 6, 9).

Disk with radiated surface, four times as broad as the outer, and twelve times as broad as the inner medullary shell. Pores regularly circular; sixteen to eighteen on the radius of the disk; in the distal half disposed in sixty to seventy radial series, which are separated by prominent crests; the sharp margin therefore a little jagged.

Dimensions.—Diameter of the disk 0.2, of the outer medullary shell 0.05, of the inner 0.017; pores 0.006.

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

Genus 183. Periphæna, Ehrenberg, 1873 Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 246.

Definition.—Phacodiscida 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. , 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.

Genus 184. Perizona, Haeckel, 1881, Prodromus, p. 457.

Definition.—Phacodiscida 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. , 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. , 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.