Definition.—Phacodiscida without radial spines on the margin of the disk.
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.
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. 33, 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. 33, 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. 33, 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.
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. 35, 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.
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.
Definition.—Surface of the disk smooth, without radial ribs or spines.
1. Phacodiscus rotula, n. sp. (Pl. 35, 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. 35, 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.
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. 35, 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.
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. 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.
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. 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.—Phacodiscida 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).
Definition.—Phacodiscida with simple medullary shell and with two radial spines on the margin of the disk, opposite in the equatorial axis.
The genus Sethostylus opens the series of the Heliosestrida or of those Phacodiscida in which a constant number of radial spines (two to eight) is more or less regularly disposed on the margin of the disk. All these marginal spines lie in the equatorial plane of the lens, and have in the same species a rather constant number and similar size, a certain form and disposition. Sometimes they incline more or less to irregular variations. In Sethostylus there are only two spines, opposite in the equatorial diameter of the lens; it corresponds to Stylocyclia among the Coccodiscida, and to Xiphodictya among the Porodiscida.
Definition.—Margin of the disk simple, smooth, without equatorial girdle and without a corona of marginal spines.
1. Sethostylus distyliscus, n. sp. (Pl. 31, fig. 9).
Disk with smooth surface, three times as broad as the medullary shell. Pores regular, circular; nine to ten on the radius of the disk, in the outer rows cylindrical. Margin of the disk simple, smooth, thin, without peculiar equatorial girdle, radially striped by the prominent beams of the peripheral series of pores. Both marginal spines of equal size, pyramidal, sulcated, about as long as the radius of the disk and as broad at the base as the medullary shell.
Dimensions.—Diameter of the disk 0.13 to 0.16, of the medullary shell 0.04 to 0.05; length of both opposite spines 0.06 to 0.08, basal breadth 0.03 to 0.04.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
2. Sethostylus dicylindrus, n. sp. (Pl. 31, fig. 10).
Disk with smooth surface, four times as broad as the medullary shell. Pores regular, circular; sixteen to eighteen on the radius of the disk. Margin of the disk simple, smooth, thick, without peculiar equatorial girdle. Both marginal spines cylindrical, about as long as the diameter of the disk, about twice as broad as a single pore. (Walls of the disk in the central part twice to three times as thick as in the peripheral part.)
Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.05; length of both spines 0.2 to 0.3, breadth 0.008.
Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.
Definition.—Margin of the disk with a solid equatorial girdle or a corona of radial spines.
3. Sethostylus dentatus, n. sp. (Pl. 34, fig. 1).
Heliostylus dentatus, Haeckel, 1881, Prodromus et Atlas (pl. xxxiv. fig. 1).
Disk with smooth surface, six times as broad as the medullary shell. Pores regular, circular; eighteen to twenty on the radius of the disk. Margin of the disk with a broad solid equatorial girdle, which bears fifty to sixty strong conical teeth on the periphery; teeth about as long as the diameter of the medullary shell. Both opposite marginal spines cylindro-conical, about as long as the diameter of the disk, and as broad as the girdle.
Dimensions.—Diameter of the disk 0.3, of the medullary shell 0.05; length of both main spines 0.2 to 0.3, breadth 0.02.
Habitat.—Pacific, central area, Station 272, depth 2600 fathoms.
4. Sethostylus serratus, n. sp. (Pl. 34, fig. 2).
Heliostylus serratus, Haeckel, 1881, Prodromus et Atlas (pl. xxxiv. fig. 2).
Disk with smooth surface, four times as broad as the medullary shell. Pores regular, circular; fourteen to sixteen on the radius of the disk. Margin with a broad solid equatorial girdle, which bears forty to fifty strong conical teeth on the periphery; teeth about as long as the radius of the medullary shell. Both opposite marginal spines spindle-shaped, about as long as the diameter of the disk, and as broad as the girdle.
Dimensions.—Diameter of the disk 0.3, of the medullary shell 0.075; length of both main spines 0.2 to 0.3, breadth 0.03.
Habitat.—South Atlantic, Station 332, depth 2200 fathoms.
5. Sethostylus hastatus, n. sp.
Disk with smooth surface, two and a half times as broad as the medullary shell. Pores irregular, roundish; ten to twelve on the radius of the disk. Margin with a broad solid equatorial girdle, which bears on the periphery six conical teeth, three on each half between the two main spines, which are cylindrical, longer than the diameter of the disk, and half as broad as the girdle. (This species can be derived from Heliosestrum, two opposite spines of the eight marginal spines being much stronger developed than the other six.)
Dimensions.—Diameter of the disk 0.15, of the medullary shell 0.06; length of both main spines 0.2, breadth 0.01.
Habitat.—North Atlantic, Station 354, surface.
6. Sethostylus spicatus, n. sp.
Disk with spiny surface, four times as broad as the medullary shell. Pores irregular roundish, with many unequal conical spines between them. Margin with an equatorial girdle, composed of three to four concentric series of conical radial spines, about as long as the radius of the medullary shell. Both main spines very large, cylindrical, longer than the diameter of the disk, and as broad as the radius of the medullary shell.
Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.05; length of the main spines 0.3 or more, breadth 0.025.
Habitat.—Fossil in the rocks of Barbados, Haeckel; living in the depths of the Equatorial Atlantic, Station 348, depth (2450) fathoms.
Definition.—Phacodiscida with double medullary shell and with two radial spines on the margin of the disk, opposite in the equatorial axis.
The genus Phacostylus differs from the foregoing Sethostylus by the duplication of the medullary shell, and bears the same relation to it as Phacodiscus does to Sethodiscus, or as Amphicyclia in the Coccodiscida does to Stylocyclia.
Definition.—Margin of the disk simple, smooth, without equatorial girdle, and without a corona of marginal spines.
1. Phacostylus amphistylus, n. sp. (Pl. 31, fig. 12).
Disk with smooth surface, four times as broad as the outer, and twelve times as broad as the inner medullary shell. Pores regular, circular; twelve to fourteen on the radius of the disk. Margin of the disk simple, smooth, without spines and equatorial girdle. Both polar spines conical, longer than the diameter of the disk, furrowed at the base, and twice as broad as the inner medullary shell.
Dimensions.—Diameter of the disk 0.16 to 0.18, of the outer medullary shell 0.045, of the inner 0.015; length of the polar spines 0.2, basal breadth 0.03.
Habitat.—Pacific, central area, Station 272, depth 2600 fathoms.
2. Phacostylus amphixiphus, n. sp.
Disk with smooth surface, three times as broad as the outer, and eight times as broad as the inner medullary shell. Pores irregular, roundish; ten to eleven on the radius. Margin of the disk simple, smooth, without spines and equatorial girdle. Both polar spines sword-shaped triangular, two-edged, about as long as the radius of the disk, and as broad at the base as the inner medullary shell.
Dimensions.—Diameter of the disk 0.15, of the outer medullary shell 0.05, of the inner 0.02; length of the polar spines 0.08, basal breadth 0.02.
Habitat.—Indian Ocean, Madagascar, Rabbe, surface.
Definition.—Margin of the disk with a solid equatorial girdle or a corona of radial spines.
3. Phacostylus amphipyramis, n. sp.
Disk with spiny surface, four and a half times as broad as the outer, and fourteen times as broad as the inner medullary shell. Pores irregular, roundish; eight to ten on the radius. Margin of the disk with a corona of irregular, radial spines. Both opposite polar spines pyramidal, twice as long as broad, and nearly as long as the radius of the disk.
Dimensions.—Diameter of the disk 0.22, of the outer medullary shell 0.05, of the inner 0.016; length of the polar spines 0.1, basal breadth 0.05.
Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.
4. Phacostylus caudatus, n. sp. (Pl. 32, fig. 6).
Astrosestrum caudatum, Haeckel, 1881, Prodromus et Atlas (pl. xxxii. fig. 6).
Disk with smooth surface, two and a half times as broad as the outer, and six times as broad as the inner medullary shell. Pores regular, circular; six to seven on the radius of the disk. Margin with a solid equatorial girdle, and irregularly bordered with eight to ten conical spines; two opposite of these are much longer than the others. (This species can be derived from Astrosestrum, two opposite marginal spines being much more strongly developed than the six to eight others.)
Dimensions.—Diameter of disk 0.12, of the outer medullary shell 0.05, of the inner 0.02; length of the polar spines 0.1 to 0.25, basal breadth 0.025.
Habitat.—North Pacific, Station 244, depth 2900 fathoms.
5. Phacostylus maximus, n. sp.
Disk with smooth surface, five times as broad as the outer, and ten times as broad as the inner medullary shell. Pores regular, circular; twenty to twenty-two on the radius. Margin with a solid equatorial girdle, bearing on the periphery one hundred to one hundred and twenty plain teeth, and two very large polar spines, which are cylindrical, longer than the diameter of the disk, and as broad at the furrowed base as the radius of the outer medullary shell. (Similar to Sethostylus dentatus, Pl. 34, fig. 1, but much larger, and with a double medullary shell.) 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.—Phacodiscida 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 size and at unequal distances, one odd spine as long as the radius, both paired spines as long as the diameter of the disk; the odd angle between the latter is smaller. Spines pyramidal, with broad edges.
Dimensions.—Diameter of the disk 0.16, of the medullary shell 0.05, pores 0.008.
Habitat.—Fossil in the rocks of Barbados.
Definition.—Phacodiscida with simple medullary shell and with four radial spines on the margin of the disk, crossed in the equatorial plane.
The genus Sethostaurus exhibits four marginal spines, which form commonly a more or less regular cross in the equatorial plane. Sometimes the size and disposition of the four spines become more or less different, and also the angles between them vary; the regular rectangular cross passes over into a bilateral or irregular form. The medullary shell is simple. The same cross-form of the disk is seen in Staurocyclia among the Coccodiscida, and in Staurodictya among the Porodiscida.
Definition.—Margin of the disk without a solid equatorial girdle or a corona of spines.
1. Sethostaurus orthostaurus, n. sp. (Pl. 31, figs. 1, 2).
Disk with smooth surface, three times as broad as the medullary shell. Pores regular, circular; thirteen to fourteen on the radius of the disk. Margin simple, smooth, without girdle. Four crossed spines equal, pommel-shaped, angular, contracted at the base, scarcely as long as the diameter of the medullary shell.
Dimensions.—Diameter of the disk 0.16, of the medullary shell 0.05; length of the crossed spines 0.04, breadth 0.02.
Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.
2. Sethostaurus conostaurus, n. sp. (Pl. 34, figs. 7, 8).
Disk with spiny surface, three times as broad as the medullary shell. Pores irregular, roundish; seven to eight on the radius. Margin simple, smooth, without girdle. Four crossed spines subregular, equal, conical, about as long as the diameter and as broad at the base as the radius of the medullary shell. (Fig. 7 exhibits the normal form, fig. 8 an abnormality with five spines.)
Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.06; length of the crossed spines 0.05, basal breadth 0.03.
Habitat.—South Atlantic, Station 333, surface.
3. Sethostaurus rhombostaurus, n. sp. (Pl. 31, fig. 4).
Disk with smooth surface, two and a half times as broad as the medullary shell. Pores regular, circular; ten to eleven on the radius. Margin simple, smooth, without girdle. Two opposite spines longer than the diameter of the disk, four times as long as the two others, which only equal its radius. Spines sword-shaped, angular.
Dimensions.—Diameter of the disk 0.14, of the medullary shell 0.06; length of the major spines 0.2, of the minor 0.05, breadth 0.025.
Habitat.—North Atlantic, Station 353, depth 2965 fathoms.
4. Sethostaurus recurvatus, n. sp. (Pl. 31, fig. 3).
Disk with smooth surface, three times as broad as the medullary shell. Pores irregular, roundish; eight to nine on the radius. Margin simple, smooth, without girdle. Four spines cylindrical, irregularly curved, of different sizes; one single very large, much longer than the three others; two opposite lateral spines recurved, hook-shaped.
Dimensions.—Diameter of the disk 0.15, of the medullary shell 0.05; length of the major spine 0.32, of the opposite spine 0.16, of both lateral spines 0.08.
Habitat.—South Pacific, Station 295, depth 1500 fathoms.
Definition.—Margin of the disk with a solid equatorial girdle or a corona of spines.
5. Sethostaurus cruciatus, n. sp. (Pl. 31, fig. 5).
Heliostaurus cruciatus, Haeckel, 1881, Prodromus et Atlas (pl. xxxi. fig. 5).
Disk with smooth surface, three times as broad as the medullary shell. Pores regular, circular; eleven to twelve on the radius. Margin with a solid, radially striped girdle, which bears on the periphery forty to fifty triangular pointed teeth of unequal length. Four crossed spines of equal size and similar form, pyramidal, sulcated, about as long as the radius of the disk, as broad at the base as the girdle.
Dimensions.—Diameter of the disk 0.2, of the medullary shell 0.06; length of the crossed spines 0.1, basal breadth 0.02.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
6. Sethostaurus coronatus, n. sp.
Disk with spiny surface, twice as broad as the medullary shell. Pores irregular, roundish; nine to ten on the radius. Margin of the disk with a solid broad girdle, bearing on the periphery a corona of thirty to forty flat tongue-shaped teeth of different length. Four crossed spines prismatic with prominent edges, about as long as the diameter of the disk, and as broad as the girdle.
Dimensions.—Diameter of the disk 0.16, of the medullary shell 0.08, length of the crossed spines 0.18, breadth 0.02.
Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.
7. Sethostaurus gigas, n. sp.
Disk with smooth surface, four times as broad as the medullary shell. Pores regular, circular; twenty to twenty-two on the radius. Margin of the disk with a narrow solid girdle, bearing on the periphery one hundred to one hundred and twenty slender flat pointed teeth. Four crossed spines equal, conical, about as long as the radius of the disk, as broad at their furrowed base as the radius of the medullary shell.
Dimensions.—Diameter of the disk 0.4, of the medullary shell 0.1; length of the crossed spines 0.5, basal breadth 0.05.
Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.
Definition.—Phacodiscida with double medullary shell and with four radial spines on the margin of the disk, crossed in the equatorial plane.
The genus Phacostaurus differs from the foregoing Sethostaurus by the duplication of the medullary shell, and bears to it the same relation as Phacodiscus does to Sethodiscus. Also in this genus the cross of the shell is commonly regular, rectangular, sometimes more or less irregular.
Definition.—Margin of the disk without a solid equatorial girdle or a corona of spines.
1. Phacostaurus oceanidum, n. sp. (Pl. 31, fig. 6).
Disk with smooth surface, three times as broad as the outer and eight times as broad as the inner medullary shell. Pores regular, circular; eleven to twelve on the diameter of the disk. Margin smooth. Four crossed spines pyramidal, deeply sulcate, about as long as the diameter of the outer medullary shell, and two-thirds as broad at the base.
Dimensions.—Diameter of the disk 0.15, of the outer medullary shell 0.05, of the inner 0.02; length of the spines 0.05, basal breadth 0.03.
Habitat.—Central Pacific, Station 266, depth 2750 fathoms.