3. Lithocyclia ocellus, Ehrenberg.
Lithocyclia ocellus, Ehrenberg, 1854, Mikrogeol., Taf. xxxvi. fig. 30; Abhandl. d. k. Akad. d. Wiss. Berlin, 1875, Taf. xxix. fig. 3.
Phacoid shell three times as broad as the medullary shell, surrounded by numerous (seven to eleven) chambered rings, which are divided by piercing radial beams each into sixty to ninety chambers. Margin of the disk smooth. Pores regular, circular; nine on the radius of the phacoid shell, one single pore on each chamber.
Dimensions.—Diameter of the disk (with eleven rings) 0.22, of the phacoid shell 0.1, of the medullary shell 0.035.
Habitat.—Fossil in the rocks of Barbados.
4. Lithocyclia monococcus, n. sp.
Stephanopyxis dubiosa (?), Bury, 1862, Polycystins of Barbados, pl. xiii. figs. 1, 2.
Phacoid shell four times as broad as the medullary shell, surrounded by numerous (five to eight) chambered rings, which are divided by piercing radial beams each into fifty to seventy chambers. Margin of the disk thickened, thorny. Pores regular, circular; seven on the radius of the phacoid shell, one single pore on each chamber.
Dimensions.—Diameter of the disk (with eight rings) 0.18, of the phacoid shell 0.12, of the medullary shell 0.03.
Habitat.—Pacific, central area, Station 267, depth 2700 fathoms; also fossil in the rocks of Barbados.
5. Lithocyclia heteropora, n. sp.
Phacoid shell two and a third times as broad as the medullary shell, surrounded by five to nine chambered rings, which are divided by piercing radial beams each into fifty to seventy chambers. Margin of the disk smooth. Pores very different in the inner and outer part of the surface; in the phacoid shell larger, regular, circular, eight on its radius, in the chambered periphery very small and irregular, somewhat spongy.
Dimensions.—Diameter of the disk (with nine rings) 0.2, of the phacoid shell 0.13, of the medullary shell 0.055.
Habitat.—Pacific, central area, Station 263, depth 2650 fathoms.
Definition.—Coccodiscida with simple circular margin of the disk, without radial appendages. Medullary shell double.
The genus Coccodiscus has quite the same form and structure as the preceding Lithocyclia, and differs from it only in the double medullary shell, composed of two concentric latticed spheres; sometimes the inner medullary shell is spherical, the outer lenticular; the latter is connected with the lenticular phacoid shell (or cortical shell) by radial beams.
1. Coccodiscus lamarckii, n. sp. (Pl. 36, fig. 1).
Phacoid shell (or lenticular porous cortical shell) very thick walled (as thick as one ring), three times as broad as the outer and seven times as broad as the inner medullary shell, surrounded by two to three chambered rings of equal breadth, each of which is divided by piercing radial beams into thirty-six to forty square chambers. Margin of the disk circular, smooth. Pores regular, circular, of equal size; ten on the radius of the phacoid shell, two on the breadth of each chamber.
Dimensions.—Diameter of the disk (with two rings) 0.2, of the phacoid shell 0.14, outer medullary shell 0.05, inner 0.02.
Habitat.—Western Tropical Pacific, Station 220, depth 1100 fathoms.
2. Coccodiscus darwinii, Haeckel.
Coccodiscus darwinii, Haeckel, 1862, Monogr. d. Radiol., p. 486, Taf. xxviii. figs. 11, 12.
Phacoid shell three times as broad as the outer and nine times as broad as the inner medullary shell, surrounded by five to eight chambered rings of equal breadth, each of which is divided by forty piercing radial beams into forty square chambers of equal size. Margin of the disk smooth, circular. Pores irregular, roundish, of unequal size; eleven on the radius of the phacoid shell, one to two on each chamber.
Dimensions.—Diameter of the disk (with eight rings) 0.32, of the phacoid shell 0.11, outer medullary shell 0.036, inner 0.012.
Habitat.—Mediterranean, Messina, surface.
3. Coccodiscus goethei, n. sp. (Pl. 36, fig. 2).
Phacoid shell two and a half times as broad as the outer and five times as broad as the inner medullary shell, surrounded by three to seven chambered rings of equal breadth, each of which is divided by piercing radial beams into sixty to eighty chambers. Margin of the disk thorny. Pores regular, circular; eight on the radius of the phacoid shell, a single one on the breadth of each chamber.
Dimensions.—Diameter of the disk (with seven rings) 0.25, of the phacoid shell 0.1, of the medullary shell 0.04, inner 0.02.
Habitat.—South Atlantic, Station 332, depth 2200 fathoms.
Definition.—Coccodiscida with solid radial spines on the margin of the circular disk, situated in its equatorial plane (without chambered arms).
Definition.—Coccodiscida with two opposite solid radial spines on the margin of the circular disk. Medullary shell simple.
The genus Stylocyclia opens the series of the Stylocyclida or of those Coccodiscida in which the margin of the chambered disk is armed with solid radial spines, situated in its equatorial plane, but without chambered arms. Stylocyclia is the most simple form of this subfamily, and bears only two marginal spines, opposite in one equatorial axis of the disk. The medullary shell is simple. This genus corresponds to Xiphodictya in the family Porodiscida. The genus was previously known only by one single species described by Ehrenberg.
1. Stylocyclia dimidiata, Ehrenberg.
Stylocyclia dimidiata, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 84, Taf. xxix. fig. 4.
Phacoid shell three times as broad as the medullary shell, surrounded by five to eight chambered rings of equal breadth, divided by eighty to ninety piercing radial beams into square chambers. Pores subregular, circular; six to seven on the radius of the phacoid shell, a single one on each chamber. Both opposite marginal spines strong, club-shaped, their thickness decreasing from the margin towards the centre of the disk. (The figure of Ehrenberg is very incomplete.)
Dimensions.—Diameter of the disk (with eight rings) 0.25, of the phacoid shell 0.11, of the medullary shell 0.035.
Habitat.—Fossil in the Tertiary rocks of Barbados; living in the depths of the Equatorial Atlantic, Station 348, depth (2450) fathoms.
2. Stylocyclia prionacantha, n. sp. (Pl. 37, fig. 6).
Phacoid shell two and a half times as broad as the medullary shell, surrounded by five to six chambered rings, which are divided by fifty to sixty piercing radial beams into square chambers. Pores in the thick-walled phacoid shell regularly circular, increasing in size from the centre; eight to nine on its radius. Pores on the surface of the chambered girdle smaller, very irregular, two to three on each ring. Both marginal spines longer than the diameter of the disk, with broad serrated edges.
Dimensions.—Diameter of the disk (with five rings) 0.23, of the phacoid shell 0.12, of the medullary 0.05.
Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.
3. Stylocyclia excavata, n. sp. (Pl. 37, fig. 8).
Phacoid shell four times as broad as the medullary shell, surrounded by four chambered rings, which are divided by twenty to thirty radial beams into broad chambers. The height of the rings increases strongly from the centre, so that the fourth ring is two and a half times as high as the first. Pores irregular, roundish; five to six on the radius of the phacoid shell, one to two on the breadth of each ring. Margin of the disk smooth. Both marginal spines thin and long, cylindrical, arising from the medullary shell, longer than the diameter of the disk.
Dimensions.—Diameter of the disk (with four rings) 0.25, of the phacoid shell 0.1, of the medullary shell 0.025.
Habitat.—Fossil in the rocks of Barbados. Indian Ocean, Cocos Islands, Rabbe.
4. Stylocyclia amphacantha, n. sp.
Phacoid shell twice as broad as the medullary shell, surrounded by five chambered rings, divided by piercing beams each into forty to fifty chambers. Pores regular, circular; eight to nine on the radius of the phacoid shell, one on each chamber. Margin of the disk thorny. Both marginal spines strong, conical, as long as its radius.
Dimensions.—Diameter of the disk (with five rings) 0.24, of the phacoid shell 0.1, of the medullary shell 0.05.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
Definition.—Coccodiscida with two opposite solid radial spines on the margin of the circular disk. Medullary shell double.
The genus Amphicyclia has the same form and structure as Stylocyclia, and differs from it only in the double concentric medullary shell. It bears therefore to the latter the same relation that Coccodiscus does to Lithocyclia.
1. Amphicyclia chronometra, n. sp. (Pl. 38, fig. 1).
Phacoid shell three times as broad as the outer and nine times as broad as the inner medullary shell, surrounded by two broad chambered rings, which are divided into irregular chambers by fifty to sixty radial beams of different distance. Margin of the disk thorny, lacerated. Pores irregular, roundish; twelve to sixteen on the radius of the phacoid shell, two to three on the breadth of each ring. The two opposite marginal spines strong, prismatic, with prominent edges, about as long as the diameter of the disk.
Dimensions.—Diameter of the disk (with two rings) 0.24, of the phacoid shell 0.15, outer medullary shell 0.05, inner 0.017.
Habitat.—Pacific, central area, Stations 265 to 268, depths 2700 to 2900 fathoms.
2. Amphicyclia amphistyla, n. sp. (Pl. 37, fig. 7).
Stylocyclia amphistyla, Haeckel, 1879, MS. et Atlas (pl. xxxvii. fig. 7).
Phacoid shell thin walled, two and a half times as broad as the outer and seven times as broad as the inner medullary shell, divided by eighty to ninety radial beams into irregular chambers, which are stratified in four to five floors. Pores regular, circular; eight to nine on the radius of the phacoid shell, two on the breadth of each chamber. Both marginal spines cylindrical.
Dimensions.—Diameter of the disk (with seven rings) 0.27, of the phacoid shell 0.1, outer medullary shell 0.04, inner 0.014.
Habitat.—North Atlantic, Canary Islands, Station 354, surface.
3. Amphicyclia pachydiscus, n. sp. (Pl. 38, fig. 2).
Phacoid shell very thick, spongy, twice as broad as the outer and five times as broad as the inner medullary shell, divided by fifty to seventy radial beams into subregular chambers, which are stratified in five to six floors. Pores irregular, roundish; ten to twelve on the radius of the phacoid shell, two to three on the breadth of each chamber. The two opposite marginal spines quadrangular prismatic, very long, as broad as the radius of the inner medullary shell. Fig. 2 exhibits a vertical section (slide) through the centre.
Dimensions.—Diameter of the disk (with five rings) 0.22, of the phacoid shell 0.1, outer medullary shell 0.05, inner 0.02.
Habitat.—South Atlantic, Station 332, depth 2200 fathoms; fossil in Barbados.
Definition.—Coccodiscida with three solid radial spines on the margin of the circular disk. Medullary shell simple.
The genus Trigonocyclia (only known by one single species, once observed) bears on the margin of the chambered disk three solid radial spines, at equal distances one from another. It corresponds to the genus Tripodictya amongst the Porodiscida.
1. Trigonocyclia triangularis, n. sp. (Pl. 37, fig. 5).
Phacoid shell three times as broad as the medullary shell, connected with it by six equidistant radial beams, three of which are prolonged into marginal spines. In the equatorial plane only one single chambered ring, divided by forty-four radial beams into narrow chambers. Pores large, irregular, roundish; five on the radius of the phacoid shell, two on each chamber. Surface and margin of the disk spiny. Three strong conical marginal spines, divergent at equal angles, as long as the radius of the disk.
Dimensions.—Diameter of the disk 0.16, of the phacoid shell 0.12, of the medullary shell 0.04.
Habitat.—Indian Ocean, Zanzibar, depth 2200 fathoms, Pullen.
Definition.—Coccodiscida with four solid radial spines on the margin of the disk, crossed in two equatorial diameters perpendicular one to another. Medullary shell simple.
The genus Staurocyclia is characterised by four radial spines on the margin of the disk, which are opposite in pairs and situated in two equatorial diameters, commonly perpendicular one to another. They form therefore a rectangular cross (rarely more or less irregular). The genus corresponds exactly to Staurodictya in the family Porodiscida. The medullary shell is simple.
1. Staurocyclia cruciata, n. sp. (Pl. 37, fig. 1).
Phacoid shell two and a half times as broad as the medullary shell, surrounded by six to eight regular rings, which are divided by fifty to seventy piercing beams into square chambers. Pores regular, circular; six on the radius of the phacoid shell, one single pore on each chamber. Four crossed radial spines, arising from the medullary shell by thin bases, prolonged over the dentated margin as four strong quadrangular swords, as long as the radius of the disk.
Dimensions.—Diameter of the disk (with eight rings) 0.23, of the phacoid shell 0.09, of the medullary shell 0.035.
Habitat—Pacific, central area, Station 267, depth 2700 fathoms.
2. Staurocyclia serrata, n. sp.
? Haliomma sp., Bury, 1862, Polycystins of Barbados, pl. xxii. fig. 2.
Phacoid shell twice as broad as the medullary shell, surrounded by three to four spongy rings, which are divided by thirty to forty radial beams into irregular chambers. Pores in the phacoid shell regular, circular, five on its radius; in the chambered spongy girdle much smaller and irregular. Margin dentated, with four crossed, very large, quadrangular spines, as long as the radius of the disk, with wing-shaped, serrated edges.
Dimensions.—Diameter of the disk (with four rings) 0.2, of the phacoid shell 0.1, of the medullary shell 0.05.
Habitat.—Fossil in the rocks of Barbados.
3. Staurocyclia phacostaurus, n. sp. (Pl. 37, figs. 2, 3).
Phacostaurus pyramidalis, Haeckel, 1879, MS. (as a separate genus).
Phacoid shell two and a quarter times as broad as the medullary shell, connected with it by twelve radial beams (four equatorial and eight divergent) and surrounded by one single ring, which is divided by forty radial beams into regular chambers. Pores regular, honeycomb-like in the phacoid shell, seven on its radius; smaller on the equatorial ring, three on its breadth. Four marginal spines pyramidal four-sided, as long and as broad at the base as the ring.
Dimensions.—Diameter of the disk (with one ring) 0.14, of the phacoid shell 0.11, of the medullary shell 0.048.
Habitat.—South Atlantic, Station 335, depth 1425 fathoms.
4. Staurocyclia magniducis, n. sp. (Pl. 37, fig. 4).
Coccostaurus magniducis, Haeckel, 1881, MS. et Atlas (pl. xxxvii. fig. 4).
Phacoid shell twice as broad as the medullary shell, connected with it by numerous radial beams and surrounded by eight chambered rings, which are divided by one hundred to one hundred and twenty piercing radial beams into small chambers. Pores subregular, circular; ten on the radius of the phacoid shell, two on the breadth of each chamber. Margin of the disk armed with numerous bristle-shaped radial spines, as long as the breadth of the chambered girdle. Four very large crossed spines, nearly as long as the diameter of the disk, quadrangular, with four dentated edges; at the club-shaped distal end thorny, as broad as the medullary shell and three times as broad as at the narrow base. I name this splendid species in the honour of H.R.H. the Grand Duke of Saxe-Weimar, Carl Alexander, the magnanimous protector of arts and sciences, the rector magnificentissimus of the University of Jena.
Dimensions.—Diameter of the disk (with eight rings) 0.27, of the phacoid shell 0.11, of the medullary shell 0.05; length of the four crossed club-spines 0.2, basal breadth 0.02, distal breadth 0.06.
Habitat.—Indian Ocean, Ceylon, Belligemma, surface (Haeckel).
Definition.—Coccodiscida with numerous (five or more, commonly thirty to sixty) solid radial spines on the margin of the circular disk. Medullary shell simple.
The genus Astrocyclia exhibits on the margin of the circular chambered disk a large but variable number of solid radial spines, commonly between thirty and sixty. They are the external prolongations of the inner piercing radial beams, which divide the concentric rings of the disk into chambers. All the spines lie in the equatorial plane of the disk. The genus corresponds to Stylodictya in the family Porodiscida.
1. Astrocyclia solaster, n. sp. (Pl. 36, fig. 7).
Phacoid shell two and a half times as broad as the medullary shell, surrounded by four to six regular rings of equal breadth, which are divided by thirty to forty piercing radial beams into broad chambers. Pores regular, circular; seven on the radius of the phacoid shell, one single pore on each chamber (the size increasing from the centre). Margin with thirty to forty angular, broad, dentate spines, the prolongations of the inner radial beams.
Dimensions.—Diameter of the disk (with six rings) 0.22, of the phacoid shell 0.1, of the medullary shell 0.04.
Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.
2. Astrocyclia stella, Haeckel.
Lithocyclia stella, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 78, Taf. xxix. fig. 2.
Phacoid shell three times as broad as the medullary shell, surrounded by four to eight rings of increasing breadth from the centre, the outer of which are more or less spongy. Each ring is divided by forty to fifty piercing radial beams into chambers of increasing size. Pores regular, circular; a single one on each chamber, ten on the radius of the phacoid shell. On the margin are scattered ten to twenty thick and short spines, four-sided pyramidal, about as long and as broad at the base as the diameter of the medullary shell.
Dimensions.—Diameter of the disk (with eight rings) 0.25, of the phacoid shell 0.09, of the medullary shell 0.03.
Habitat.—Fossil in the rocks of Barbados.
3. Astrocyclia rotula, n. sp.
Phacoid shell four times as broad as the medullary shell, surrounded by one single ring, which is divided by forty radial beams into equal chambers. Pores regular, circular; two on the breadth of the ring, six on the radius of the phacoid shell. Margin with forty short, conical spines, the prolongations of the inner radial beams, twice as long as the breadth of the ring.
Dimensions.—Diameter of the disk 0.13, of the phacoid shell 0.1, of the medullary shell 0.025.
Habitat.—South Atlantic, Station 332, depth 2200 fathoms.
4. Astrocyclia arachnia, n. sp.
Phacoid shell twice as broad as the medullary shell, surrounded by four to six rings of equal breadth, which are divided by twelve to twenty radial beams into broad chambers. Pores regular, circular; two on the breadth of each ring, eight on the radius of the phacoid shell. Margin with twelve to twenty very long and thin, bristle-shaped, radial spines, the prolongations of the inner beams, longer than the diameter of the disk. (Resembles Stylodictya arachnia, but differs in the possession of a phacoid shell.)
Dimensions.—Diameter of the disk (with six rings) 0.22, of the phacoid shell 0.09, of the medullary shell 0.045.
Habitat.—North Atlantic, Canary Islands.
5. Astrocyclia heterocycla, n. sp. (Pl. 36, fig. 8). Phacoid shell two and a half times as broad as the medullary shell, surrounded by three rings of unequal breadth, the first ring as broad as the second and third together. They are divided by fifty to sixty piercing radial beams into chambers, which are square in the second and third rings, and half as large as in the first. The radial beams are alternately thicker and thinner, prolonged into short marginal spines. Pores subregular, circular; eight on the radius of the phacoid shell, two on the breadth of the first ring, a single one on each chamber of the second and third rings.
Dimensions.—Diameter of the disk (with three rings) 0.2, of the phacoid shell 0.12, of the medullary shell 0.05.
Habitat.—Equatorial Atlantic, Station 348, depth (2450) fathoms; also fossil in Barbados.
Definition.—Coccodiscida with numerous (five or more) solid radial spines on the margin of the circular disk. Medullary shell double.
The genus Coccocyclia has the same form and structure as the foregoing Astrocyclia, and differs from it only in the double concentric medullary shell. It bears therefore to the latter the same relation as Coccodiscus to Lithocyclia.
1. Coccocyclia liriantha, n. sp.
Phacoid shell three times as broad as the outer and ten times as broad as the inner medullary shell, connected with both by six equidistant piercing radial beams. The margin of the phacoid shell is surrounded by one or two equatorial rings, each as broad as the outer medullary shell, and divided by thirty to forty radial beams into narrow chambers. Pores subregular, circular, about nine to ten on the radius of the phacoid shell, three to four on the breadth of each ring. Margin of the disk ciliated, with six regularly disposed radial spines as prolongations of the inner six piercing beams. Each spine cylindrical, delicately crenulated, about as broad as the inner medullary shell, and once to twice as long as the diameter of the whole disk, elegantly undulated, with a conical terminal point.
Dimensions.—Diameter of the disk (with two rings) 0.32, of the phacoid shell 0.2, of the outer medullary shell 0.06, of the inner 0.02.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
2. Coccocyclia heliantha, n. sp. (Pl. 36, figs. 5, 6).
Phacoid shell two and a half times as broad as the outer and six times as broad as the inner medullary shell, surrounded by six to eight rings of nearly equal breadth, the outer somewhat smaller. They are divided by fifty to sixty piercing radial beams into irregular chambers. Pores irregular, roundish; ten on the radius of the phacoid shell, a single one on each chamber of the equatorial girdle. Margin of the disk with numerous strong conical radial spines of different breadths, arranged in several circles, the strongest in the equatorial plane, about as broad as one ring and twice as long. Surface thorny.
Dimensions.—Diameter of the disk (with seven rings) 0.3, of the phacoid shell 0.12, of the outer medullary shell 0.05, of the inner 0.02.
Habitat.—Pacific, central area, Station 268, depth 2900 fathoms; also fossil in Barbados.
Definition.—Coccodiscida with two or more (commonly three or four) chambered radial arms on the margin of the disk, situated in its equatorial plane (with or without a connecting patagium between the arms).
Definition.—Coccodiscida with two opposite chambered arms on the margin of the circular disk, without a connecting patagium.
The genus Diplactura is the simplest form of the Astracturida, or of those Coccodiscida in which the margin of the disk bears chambered arms, situated in its equatorial plane. This subfamily corresponds to the Euchitonida among the Porodiscida, to the Spongobrachida among the Spongodiscida. In Diplactura there are only two free arms, opposite in one equatorial diameter (corresponding to Amphibrachium among the former, to Spongolena among the latter).
Definition.—Arms blunt at the distal end, without radial spines.
1. Diplactura diplobrachia, n. sp.
Phacoid shell twice as broad as the medullary shell, with six pores on its radius, without a perfect chambered ring around it. Arms club-shaped, one and a half times as long as the diameter of the phacoid shell, at the distal end rounded, blunt, as broad as the latter, at the base only half as broad.
Dimensions.—Diameter of the phacoid shell 0.08, of the medullary shell 0.04; length of the arms 0.12, basal breadth 0.04, distal breadth 0.08.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
Definition.—Arms on the distal end armed with a radial terminal spine.
2. Diplactura diploconus, n. sp. (Pl. 38, fig. 5).
Amphiactura diploconus, Haeckel, 1877, MS. et Atlas (pl. xxxviii. fig. 5).
Phacoid shell three times as broad as the medullary shell, with eight pores on its radius, surrounded by a single chambered ring. Arms club-shaped, twice as long as the diameter of the phacoid shell, at the distal end as broad as the latter, at the base only half as broad. Both poles of the common axis of the arms are armed with a strong spindle-shaped terminal spine.
Dimensions.—Diameter of the phacoid shell 0.09, of the medullary shell 0.03; length of the arms (without terminal spines) 0.17, basal breadth 0.05, distal breadth 0.09.
Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.
Definition.—Coccodiscida with two opposite chambered arms on the margin of the circular disk, connected by a spongy patagium.
The genus Amphiactura differs from the foregoing Diplactura in the development of a patagium between the arms, and therefore bears the same relation to it as Amphymenium in the Porodiscida does to Amphibrachium, or Spongobrachium in the Spongodiscida does to Spongolene. In this and in the following Discoidea provided with a patagium, this connecting web constantly exhibits a different texture of its framework, which is sometimes more regularly chambered, at other times more irregularly spongy.
1. Amphiactura amphibrachia, n. sp. (Pl. 38, figs. 3, 4).
Phacoid shell three times as broad as the medullary shell, with eight pores on its radius. Arms nearly equilateral triangular, twice as long as the diameter of the phacoid shell, at the truncated distal end as broad as the latter, at the base only one-third as broad. Patagium a circular lenticular disk, enveloping only the basal third of the arms, with three to four concentric circular rings, divided into chambers by about forty radial beams, which are prolonged beyond the margin of the patagium into radial spines. The vertical section (fig. 4) shows that the chambers of each arm (eleven to twelve transverse rows in the radius) are disposed in two layers.
Dimensions.—Diameter of the phacoid shell 0.09, of the medullary shell 0.03; length of the arms 0.18, basal breadth 0.03, distal breadth 0.09.
Habitat.—Pacific, central area, Station 263, depth 2650 fathoms.
Definition.—Coccodiscida with three chambered arms on the margin of the circular or triangular disk, without a connecting patagium.
The genus Trigonactura exhibits three radial arms, which in all known species are separated by three equal angles. The terminal points of the arm-axes are the corners of an equilateral triangle. It corresponds therefore to Dictyastrum among the Porodiscida.
Definition.—Distal end of the arms blunt or truncated, without a terminal spine.
1. Trigonactura pythagoræ, Haeckel.
Astromma pythagoræ, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 66, Taf. xxx. fig. 2.
Phacoid shell circular, twice as broad as the medullary shell, with five pores on its radius, without a completely surrounding chambered girdle. Arms nearly square, at the truncated distal end as broad as long, and scarcely broader than at the base, two-thirds as long as the diameter of the central disk.
Dimensions.—Diameter of the phacoid shell 0.1, of the medullary shell 0.05; length of the arms 0.07, distal breadth 0.07.
Habitat.—Cosmopolitan; Atlantic, Indian, Pacific, in various depths; also fossil in Tertiary rocks of Barbados and Nicobar.
2. Trigonactura rhopalastrella, n. sp.
Stephanastrum sp., Bury, 1862, Polycystins of Barbados, pl. xv. fig. 7.
Phacoid shell twice as broad as the medullary shell, with five pores on its radius, without a perfect chambered ring around it. Arms club-shaped, at the blunt distal end rounded, as long as the diameter of the phacoid shell; their basal breadth is one-fourth, their distal breadth one-half of its length.
Dimensions.—Diameter of the phacoid shell 0.1, of the medullary shell 0.05; length of the arms 0.1, basal breadth 0.025, distal breadth 0.05.
Habitat.—Pacific, central area, Stations 270 to 274, depths 2400 to 2800 fathoms; also fossil in Tertiary rocks of Barbados.
3. Trigonactura trigonobrachia, n. sp.
Phacoid shell two and a half times as broad as the medullary shell, with six pores on its radius, without a perfect chambered girdle. Arms nearly equilateral triangular, twice as long as the diameter of the phacoid shell, at the truncated distal end as broad as its diameter and three times as broad as at the narrow base. (Resembles Hymenactura hexagona, but without a patagium.)
Dimensions.—Diameter of the phacoid shell 0.11, of the medullary shell 0.045; length of the arms 0.2, basal breadth 0.035, distal breadth 0.1.
Habitat.—North Pacific, Station 244, depth 2900 fathoms.
4. Trigonactura lanceolata, n. sp.
Phacoid shell three times as broad as the medullary shell, with eight pores on its radius, surrounded by one perfect chambered ring. Arms lanceolate, three times as long as the diameter of the phacoid shell, and four times as long as broad in the middle part, at both ends very narrow, blunt.
Dimensions.—Diameter of the phacoid shell 0.09, of the medullary shell 0.03; length of the arms 0.25, breadth in the midst 0.06.
Habitat.—South Atlantic, Station 332, depth 2200 fathoms.
Definition.—Distal end of the arms armed with a radial spine.
5. Trigonactura triacantha, n. sp. (Pl. 38, figs. 6, 7).
Phacoid shell two and a half times as broad as the medullary shell, with eight pores on its radius, surrounded by one perfect chambered ring. Arms nearly lanceolate, in the middle part one and a half times as broad as at both ends, twice as long as the phacoid shell, at the distal end with a strong pyramidal terminal spine. Through the spongy framework of each arm shine sixteen to eighteen transverse bars (as septa of the joints) and six to eight longitudinal piercing beams.
Dimensions.—Diameter of the phacoid shell 0.09, of the medullary shell 0.038; length of the arms 0.2, greatest breadth 0.08.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
6. Trigonactura trigonodiscus, n. sp.
? Astromma sp., Bury, 1862, Polycystins of Barbados, Taf. xv. fig. 2.
Phacoid shell triangular, twice as broad as the spherical medullary shell, with six to seven pores on its radius, surrounded by one chambered ring. Arms equilateral triangular, as long as the diameter of the central disk and at the base half as broad, slowly decreasing in breadth towards the distal end, which is armed with a strong pyramidal spine.
Dimensions.—Diameter of the phacoid shell 0.1, of the medullary shell 0.05; length of the arms 0.12, basal breadth 0.07.
Habitat.—Pacific, central area, Station 268, depth 2900 fathoms; also fossil in Barbados.
7. Trigonactura trixiphos, n. sp.
Phacoid shell circular, twice as broad as the medullary shell, without a completely surrounding chambered ring, with four pores on its radius. Arms club-shaped, about as long as the diameter of the central disk, at the base half as broad as long, at the rounded distal end broader, and armed with a strong pyramidal terminal spine, nearly as long as the arm itself.
Dimensions.—Diameter of the phacoid shell 0.08, of the medullary shell 0.04; length of the arms (without terminal spine) 0.08, basal breadth 0.04, distal breadth 0.06.
Habitat.—Fossil in the rocks of Barbados.
Definition.—Coccodiscida with three chambered arms on the margin of the circular or triangular disk, connected by a spongy patagium.
The genus Hymenactura differs from the foregoing Trigonactura in the spongy patagium between the arms, and bears therefore to it the same relation that among the Porodiscida Hymeniastrum does to the simpler Dictyastrum. The oldest known species of this genus is Hymenactura pythagoræ, described by Ehrenberg as Hymeniastrum pythagoræ, but differing from this in the structure of the central disk.
Definition.—Distal end of the arms blunt or truncated, without terminal spines.
1. Hymenactura archimedis, n. sp. (Pl. 38, fig. 8).
Phacoid shell three times as broad as the medullary shell, with eight pores on its radius. Arms nearly trapezoidal, somewhat longer than the diameter of the central disk, at the truncated distal end nearly as broad, at the base only half as broad. In each arm eleven to twelve transverse rows of square chambers, each of which exhibits on the surface one large pore. Patagium enveloping the basal half of the arms, with four to five convex rows of chambers, forming together a circular concentric disk.
Dimensions.—Diameter of the phacoid shell 0.11, of the medullary shell 0.035; length of the arms 0.12, basal breadth 0.05, distal breadth 0.1.
Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.