Synopsis of the Genera of the Cenodiscida.

Subfamily 1. Zonodiscida, Haeckel.

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

Genus 175. Cenodiscus,^[214] n. gen.

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

The genus Cenodiscus is the most simple and primitive form of all Discoidea, and represents possibly the common ancestral form of this order. The latticed shell is a simple biconvex lens, and encloses a smaller central capsule of the same form, separated from it by the jelly-veil. Cenodiscus can be derived phylogenetically either from Cenosphæra by lenticular flattening of a simple latticed sphere, or directly from Actissa by formation of a lenticular fenestrated shell around the lentiform central capsule. Possibly also some forms of Cenodiscus can be derived from Sethodiscus by reduction and loss of the medullary shell.

1. Cenodiscus phacoides, n. sp. (Pl. 48, figs. 1, 1a).

Disk with smooth surface, without radial ribs or spines. Margin of the lenticular biconvex disk thin, simple. Pores regular, circular; fifteen to sixteen on the radius of the disk. (Very similar to Sethodiscus phacoides, but without medullary shell.)

Dimensions.—Diameter of the disk 0.2, of the pores 0.005.

Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.

2. Cenodiscus rotula, n. sp.

Disk with smooth surface, without medial ribs or spines. Margin of the disk blunt, very thick, rounded; both faces little convex. Pores regular, circular; thirteen to fourteen on the radius of the disk. (Similar to Phacodiscus rotula, Pl. 35, fig. 7, but without medullary shell.)

Dimensions.—Diameter of the disk 0.16, of the pores 0.006.

Habitat.—North Pacific, Station 241, depth 2300 fathoms.

3. Cenodiscus lenticula, n. sp.

Disk with thorny surface, scattered with small, conical spines. Margin of the lenticular biconvex disk thin. Pores irregular, roundish; ten to eleven on the radius of the disk.

Dimensions.—Diameter of the disk 0.15, of the pores 0.008.

Habitat.—South Pacific, Station 295, depth 1500 fathoms.

Genus 176. Zonodiscus,^[215] n. gen.

Definition.—Cenodiscida with surrounding solid equatorial girdle on the margin of the lenticular disk, without radial spines.

The genus Zonodiscus differs from the preceding Cenodiscus only in the development of a solid siliceous girdle around the keen margin of the lenticular disk. This form can also be derived from Periphæna or Perizona by reduction and loss of the medullary shell. The same girdle formation returns not only in both these Phacodiscida (Pl. 32, fig. 7; Pl. 33, fig. 4), but also in the Porodiscid Perichlamydium.

1. Zonodiscus saturnalis, n. sp.

Disk with smooth surface, without radial spines. Pores regular, circular, fifteen to sixteen on the radius of the disk, in its distal half arranged in fifty to sixty radial series, which are separated by prominent radial crests or ribs. The crests are prolonged into the proximal half of the thin solid equatorial girdle, which is one-third as broad as the radius of the disk.

Dimensions.—Diameter of the disk 0.25, of the pores 0.05.

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

Subfamily 2. Trochodiscida, Haeckel.

Definition.—Cenodiscida with radial spines on the margin of the disk, disposed in the equatorial plane.

Genus 177. Stylodiscus,^[216] n. gen.

Definition.—Cenodiscida with two radial spines on the margin of the disk, opposite in one equatorial axis.

The genus Stylodiscus opens the series of the Trochodiscida or of those Cenodiscida in which the thin margin of the hollow lenticular disk is armed with a number of solid radial spines, situated in its equatorial plane. Stylodiscus is at the same time the most simple form of the Stylodiscida, or of the numerous Discoidea (belonging to different families) in which the disk bears only two spines, opposite in one equatorial axis. Hitherto only two species of this genus have been observed, but they seem to represent two different subgenera.

Subgenus 1. Stylentodiscus, Haeckel.

Definition.—Internal cavity of the shell with centripetal axial rods.

1. Stylodiscus endostylus, n. sp. (Pl. 31, fig. 11).

Disk with smooth surface and dentated margin. Pores irregular, roundish; ten to twelve on the radius of the disk. Marginal teeth conical, short, irregular. Both opposite polar spines cylindrical, longer than the diameter of the disk, and as broad as one larger pore. On the inside of the hollow disk both spines are prolonged into two strong centripetal axial rods, which do not reach the centre. Also a number of smaller centripetal axial rods surrounds the central cavity, so that an original medullary shell (Sethostylus) seems to have been lost (comp. above, p. 410).

Dimensions.—Diameter of the disk 0.25, of the pores 0.005 to 0.02; length of the polar spines 0.3 and more, thickness 0.012.

Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.

Subgenus 2. Stylexodiscus, Haeckel.

Definition.—Internal cavity of the shell without axial rods.

2. Stylodiscus amphistylus, n. sp.

Disk with smooth surface and smooth margin. Pores regular, circular; eight to nine on the radius of the disk. Both opposite polar spines pyramidal, sulcated, about as long as the radius of the disk, and three times as long as broad at the base. Inner cavity of the disk simple, without centripetal rods.

Dimensions.—Diameter of the disk 0.15, of the pores 0.007; length of the polar spines 0.08, basal breadth 0.03.

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

Genus 178. Theodiscus,^[217] n. gen.

Definition.—Cenodiscida with three radial spines on the margin of the disk, placed in its equatorial plane.

The genus Theodiscus is the most simple form of those very numerous Discoidea in which three rays are developed on the margin of the disk; a symbol of the Christian dogma of the Trinity. Commonly the three angles between the three spines are equal, more rarely one angle is larger than the two others which are equal. The shell of some species is nearly spherical (formerly separated by me as Theosphæra), whilst in others it is a flattened biconvex lens.

Subgenus 1. Theodiscoma, Haeckel.

Definition.—Angles between the three radial spines equal (triangle equilateral).

1. Theodiscus divinus, n. sp.

Disk nearly spherical, with smooth surface. Pores regular, circular, hexagonally framed; seven to eight on the radius. Three angles between the spines equal. Spines prismatic, straight, twice as broad as one pore, eight to eleven times as long as the shell diameter.

Dimensions.—Diameter of the disk 0.1, of the pores 0.006; length of the spines 0.8 to 1.1, breadth 0.01.

Habitat.—Central Pacific, Station 265, surface.

2. Theodiscus christianus, n. sp.

Disk nearly spherical, with smooth surface. Pores regular, circular, prolonged into short cylindrical, prominent tubuli, half as high as broad; eight to nine on the radius. Three angles between the spines equal. Spines prismatic, straight, twice as broad as one pore, twelve to fifteen times as long as the shell diameter.

Dimensions.—Diameter of the disk 0.08, of the pores 0.005; length of the spines 1.1 to 1.2, breadth 0.01.

Habitat.—Central Pacific, Station 271, surface.

3. Theodiscus trinitatis, n. sp.

Disk a rather flattened, biconvex lens, about twice as broad as thick. Pores regular, circular, not prolonged into tubuli; five to six on the radius. Three angles between the spines equal. Spines pyramidal, three times as broad as one pore, about twice as long as the shell diameter.

Dimensions.—Diameter of the disk 0.06, of the pores 0.007; length of the spine 0.12, basal breadth 0.02.

Habitat.—Central Pacific, Station 274, depth 2750 fathoms.

Subgenus 2. Theodiscura, Haeckel.

Definition.—Angles between the three radial spines unequal, two paired angles equal, larger or smaller than the odd angle (triangle isosceles).

4. Theodiscus vanitatis, n. sp.

Disk nearly spherical, with smooth surface. Pores irregular, roundish; eight to ten on the radius. Three angles between the spines unequal; one odd angle larger than both others. Spines pyramidal, of unequal size; one odd spine larger, both others smaller than the shell diameter.

Dimensions.—Diameter of the disk 0.12, pores 0.004 to 0.006; length of the odd spine 0.2, of the paired spines 0.1.

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

5. Theodiscus nirvana, n. sp.

Disk a flat biconvex lens, about twice as broad as thick. Pores regular, circular; ten to twelve on the radius. Three angles between the spines unequal; one odd angle smaller than both others. Spines prismatic, very long and thin, twice as broad as one pore, ten to twelve times as long as the shell diameter.

Dimensions.—Diameter of the disk 0.1, pores 0.005; length of the spines 1 to 1.2 or more, breadth 0.01.

Habitat.—Indian Ocean, Ceylon, surface, Haeckel.

Genus 179. Crucidiscus,^[218] n. gen.

Definition.—Cenodiscida with four radial spines on the margin of the disk, crossed in the equatorial plane.

The genus Crucidiscus is the most simple form of the Staurodiscida, or of the numerous Discoidea (belonging to different families) in which the margin of the disk bears four radial spines, lying in the equatorial plane, and crossed at right angles. Whilst commonly the internal shell-cavity of Crucidiscus is quite simple, in one case it bears four centripetal axial rods, as inner prolongations of the outer radial cross-spines, perhaps indications of a lost medullary shell (comp. p. 410).

Subgenus 1. Staurentodiscus, Haeckel.

Definition.—Internal cavity of the shell with centripetal axial rods.

1. Crucidiscus endostaurus, n. sp. (Pl. 48, fig. 2).

Disk with smooth surface and smooth simple margin. Pores regular, circular; thirteen to fourteen on the radius of the disk. Four crossed spines conical, strong, longer than the radius of the disk, on the inside prolonged into four thinner centripetal axial rods, which do not reach the centre. In the middle part of the disk also some other short axial rods arise from the inside, not reaching the centre (as in Stylodiscus endostylus, Pl. 31, fig. 11).

Dimensions.—Diameter of the disk 0.16, of the pores 0.004; length of the spines 0.2, breadth 0.014.

Habitat.—North Pacific, Station 253, depth 3125 fathoms.

Subgenus 2. Staurexodiscus, Haeckel.

Definition.—Internal cavity of the shell without centripetal axial rods.

2. Crucidiscus cuspidatus, n. sp.

Disk with smooth surface and simple smooth margin. Pores regular, circular; seven to eight on the radius of the disk. Four crossed spines pyramidal, with prominent edges, somewhat shorter than the radius of the disk, twice as broad at their thick base as one pore. No internal axial rods in the shell cavity.

Dimensions.—Diameter of the disk 0.12, of the pores 0.005; length of the spines 0.05, basal breadth 0.025.

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

3. Crucidiscus echinatus, n. sp.

Disk with thorny surface and thorny margin. Pores regular, circular, six to seven on the radius. Four crossed spines pyramidal (four sided?), about as long as the radius of the disk, as broad at their base as one pore. No internal axial rods. Spines of the surface short, conical.

Dimensions.—Diameter of the disk 0.14, of the pores 0.008; length of the spines 0.08, basal breadth 0.02.

Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.

4. Crucidiscus cruciatus, n. sp.

Disk with smooth surface and broad dentated margin. Pores regular, circular; eleven to twelve on the radius. Four crossed spines pyramidal, sulcated, about as long as the radius of the disk, as broad at their base as the radiated margin, which bears fifty to sixty triangular pointed teeth of irregular size. (Similar to Sethostaurus cruciatus, Pl. 31, fig. 5, but without internal axial rods and medullary shell.)

Dimensions.—Diameter of the disk 0.2, of the pores 0.007; length of the spines 0.1, basal breadth 0.02.

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

Genus 180. Trochodiscus,^[219] n. gen.

Definition.—Cenodiscida with numerous (ten to twenty or more) radial spines on the margin of the disk, situated in its equatorial plane.

The genus Trochodiscus bears on the thin margin of the latticed disk a variable number of radial spines (commonly ten to twenty, sometimes fifty to eighty or more). Their size and disposition are commonly more or less irregular. For the most part all the marginal spines lie in the equatorial plane; but sometimes part of them are crowded into two or four parallel girdles. Several species of this genus are very similar to some species of Heliodiscus, and only differ in the absence of the medullary shell, perhaps in consequence of the phylogenetic loss of it; if this be the case, the former are to be derived from the latter (compare p. 410).

Subgenus 1. Trochodisculus, Haeckel.

Definition.—Bases of the marginal spines free, not connected by a solid equatorial girdle.

1. Trochodiscus cenophacus, n. sp.

Disk with smooth surface. Pores regular, circular; eleven to twelve on the radius. Marginal spines sixteen to twenty, conical, of irregular size and disposition, about three times as long as broad, and as long as the radius of the disk.

Dimensions.—Diameter of the disk 0.15, of the pores 0.006; length of the spines 0.06 to 0.09, basal breadth 0.02 to 0.03.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

2. Trochodiscus medusinus, n. sp.

Disk with smooth surface. Pores regular, circular; nine to ten on the radius. Marginal spines ten to twelve, equilateral triangular, deeply sulcated, about half as long and broad as the radius of the disk. (Very similar to Heliosestrum medusinum, Pl. 34, fig. 6, but without enclosed medullary shell. Compare p. 410.)

Dimensions.—Diameter of the disk 0.2, of the pores 0.01; length of the spines 0.05, basal breadth 0.05.

Habitat.—North Pacific, Station 241, depth 2300 fathoms.

3. Trochodiscus odontotrochus, n. sp.

Disk with smooth surface, in the distal part radially sulcated. Pores irregular, roundish; twelve to fourteen on the radius. Marginal spines very short and numerous (thirty to forty), conical, scarcely one-fourth as long as the radius of the disk, which resembles a wheel with marginal teeth.

Dimensions.—Diameter of the disk 0.18, of the pores 0.004; length of the spines 0.02, basal breadth 0.01.

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

4. Trochodiscus solaris, n. sp.

Disk with smooth surface. Pores subregular, circular; eight to nine on the radius. Marginal spines very numerous (sixty to eighty), in two to four girdles densely crowded together, the largest as long as the diameter of the disk, bent and conical. (Similar to Heliodiscus solaster, Pl. 34, fig. 4, but without medullary shell.)

Dimensions.—Diameter of the disk 0.15, of the pores 0.008; length of the spines 0.05 to 0.15, basal breadth 0.01.

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

5. Trochodiscus echiniscus, n. sp.

Disk with spiny surface. Pores irregular, roundish; ten to eleven on the radius. Marginal spines conical, very numerous (forty to fifty), irregular in size and disposition, the largest half as long as the radius of the disk. (Similar to Heliodiscus echiniscus, Pl. 34, fig. 5, but without medullary shell.)

Dimensions.—Diameter of the disk 0.18, of the pores 0.005 to 0.015; length of the spines 0.02 to 0.04, basal breadth 0.01.

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

Subgenus 2. Pristodiscus, Haeckel.

Definition.—Bases of the marginal spines connected by a solid siliceous equatorial girdle.

6. Trochodiscus stellaris, n. sp. (Pl. 48, fig. 3).

Disk with smooth surface. Pores subregular, circular; seven to eight on the radius. Marginal spines twelve to sixteen, triangular, of subregular size and disposition, about half as long as the radius of the disk, connected at their broad base by a solid equatorial girdle of half the breadth; girdle and spines radially striped.

Dimensions.—Diameter of the disk 0.25, of the pores 0.015; length of the spines 0.04 to 0.06, basal breadth 0.02 to 0.04.

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

7. Trochodiscus cingillum, n. sp.

Disk with smooth surface, in the distal part radially sulcated. Pores regular, circular; twelve to thirteen on the radius. Marginal spines twenty to twenty-four, triangular, of equal size and at regular distances, only one-fourth as long as the radius of the disk, connected at their broad base by a solid equatorial girdle of the double breadth. (Similar to Heliodiscus cingillum, Pl. 33, fig. 7, but without medullary shell.)

Dimensions.—Diameter of the disk 0.22, of the pores 0.004; length of the spines 0.02, basal breadth 0.02

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

Family XIX. Phacodiscida, Haeckel (Pls. 31-35).

Phacodiscida, Haeckel, 1881, Prodromus, p. 456.

Definition.—Discoidea with simple extracapsular phacoid shell (or lenticular latticed cortical shell), connected by radial beams with an intracapsular, simple or double, concentric medullary shell, without chambered equatorial girdles.

The family Phacodiscida comprises a large number of splendid forms (about a hundred species), which agree with the preceding Cenodiscida in the possession of the characteristic extracapsular "phacoid shell," but differ from them in having one or two intracapsular "medullary shells"; these concentric spherical medullary shells are connected with the lenticular cortical shell or phacoid shell by means of radial beams perforating the central capsule. The Phacodiscida bear therefore the same relation to the Cenodiscida that the Disphærida and Trisphærida do to the Monosphærida.

Formerly several species belonging to the family were described by Ehrenberg and Johannes Müller, but not distinguished from the Sphæroidea, genus Haliomma (e.g., Haliomma sol et Haliomma humboldtii of the former, Haliomma amphidiscus of the latter). For these oldest known species I constituted in 1862 my genus Heliodiscus (Monogr. d. Radiol., p. 436). Some other genera were afterwards (1875) figured by Ehrenberg as Periphæna and Chilomma. The rich material of the Challenger revealed this family as very polymorphic and widely distributed, so that in my Prodromus (1881, p. 457) I could enumerate eighteen different genera of Phacodiscida. This number is here reduced to fifteen, uniting several of them into one genus as "subgenera."

The Medullary Shell of the Phacodiscida, or the intracapsular latticed shell, is either simple and spherical, or double, composed of two concentric spheres, which are united by a variable number of radial beams. We could distinguish therefore as two subfamilies the Carpodiscida (with simple medullary shell) and the Thecodiscida (with double concentric medullary shell); the former corresponding to the Carposphærida (or Dyosphæria), the latter to the Thecosphærida (or Triosphæria). But as this difference seems not to be so important as the different shape of the disk margin, we prefer this latter as a character for the separation of subfamilies. The form of the medullary shells is commonly quite spherical (as also in the above mentioned Sphæroidea); sometimes it is a little compressed in the same direction as the enclosing lenticular phacoid shell. The diameter of the latter is commonly three to four times as large as the diameter of the medullary shell; if this be double (in the Thecodiscida) then the diameter of the outer medullary shell is commonly three to four times as large as that of the inner. The radial beams connecting the two seem to be very variable in number and disposition (compare Pl. 31, fig. 8; Pl. 32, figs. 3, 4a, 7, 8a; Pl. 33, figs. 2, 3; Pl. 35, figs. 4, 8, 9, &c.).

The Radial Beams, which connect the medullary shell with the cortical or "phacoid shell," and which pierce the membrane of the lenticular central capsule, are commonly aggregated into two polar bunches around the shortened axis of the disk (compare Pl. 31, fig. 8; Pl. 32, figs. 3, 8; Pl. 33, figs. 2, 3, &c.). Their number seems to be usually between ten and thirty. More rarely piercing radial beams lie also in the equatorial plane, and then commonly as inner prolongations of the outer marginal spines; so we find two opposite in one axis, in Heliostylus (Pl. 34, figs. 1, 2), or four opposite in pairs in two crossed axes, in Phacostaurus (Pl. 31, figs. 1, 2, 7). Often the thickness of the beams increases from the centre towards the periphery.

The Phacoid Shell, or the lenticular extracapsular cortical shell, exhibits in the Phacodiscida quite the same general character as in the Cenodiscida, above described (p. 410). Commonly the equatorial diameter of the lens is twice to three times as large as the vertical diameter or the shortened "main axis." The convexity of both opposite faces is either quite even to the sharp margin (Pl. 31, fig. 2; Pl. 33, figs. 2, 5), or the central part of the lens is more strongly vaulted, and often the margin is thickened or truncated (Pl. 31, fig. 10; Pl. 35, fig. 7). The surface of the lens is commonly smooth, but sometimes also covered with bristle-shaped radial spines (Pl. 34, figs. 3, 5; Pl. 32, figs. 2, 3); rarely these spines are prolonged and branched (Pl. 35, figs. 3, 5). The pores of the phacoid shell are usually quite regular, circular, and regularly arranged, either in more concentric or in more radial rows; the latter are sometimes separated by radial crests arising towards the margin (Pl. 35, fig. 6). If the wall of the phacoid shell be much thickened, the pores in its central part are shorter and cylindrical, in its marginal part longer and conical (Pl. 31, fig. 7; Pl. 32, fig. 1).

The Margin of the Lens of the Phacodiscida is very polymorphic, and serves mainly for the separation of genera. In the first subfamily, Sethodiscida, it is either quite simple (Pl. 35, figs. 6-8) or surrounded by a thin solid equatorial girdle, the basal part of which is often radially striped (Pl. 32, figs. 7, 8). In the second subfamily, Heliosestrida, we find on the margin a small number of radial spines in the equatorial plane regularly disposed, either two opposite in one axis (Pl. 31, figs. 9-12) or four opposite in pairs in two crossed axes (Pl. 31, figs. 1-8), or eight opposite in pairs in four axes, crossed at angles of 45° (Pl. 34, figs. 3, 6); in the latter case we can sometimes distinguish (as in many Medusæ) four larger perradial spines alternating with four smaller interradial spines. Often in one and the same species occur abnormalities in number and disposition of the radial spines, three or five spines instead of four, or also seven or nine spines instead of eight; often both halves of the disk become asymmetrical. If the number of the marginal spines exceed eight to ten, they commonly become very variable in size and irregular in disposition; these variations characterise the third subfamily, Heliodiscida. Commonly also here all spines lie in the equatorial plane; but sometimes they become crowded in several parallel circles on both sides of the margin (Pl. 32, fig. 1). The form of the marginal spines is commonly conical or flattened triangular, often also pyramidal or deeply furrowed (Pl. 31, figs. 6-9). Very rarely the spines are fenestrated (Pl. 35, fig. 1); only in one genus (Heliodrymus) they are all or partly branched (Pl. 33, fig. 9; Pl. 35, figs. 3, 5).

The peculiar development of the phacoid shell has been already described by J. Müller (compare my Monograph, 1862, pp. 156, 438).

The Central Capsule of the Phacodiscida is everywhere circular, lenticular, envelops the medullary shell, and is enclosed by the phacoid shell, perforated by the radial beams, which connect the latter to the former.

Synopsis of the Genera of the Phacodiscida.