Definition.—Porodiscida with a simple central chamber, surrounded by a single concentric ring, which is divided by radial beams into two to six or more radial chambers, without radial spines on the margin.
The genus Archidiscus begins the long and polymorphous series of the Cyclodiscaria or of those Discoidea which do not possess the peculiar "phacoid shell" characteristic of the three preceding families, united as "Phacodiscaria." As already mentioned above, both these groups are probably of independent origin, derived from the Sphæroidea in different ways (compare pp. 402, 405, &c.). Among all Cyclodiscaria Archidiscus is the most simple, and probably the common ancestral form, from which the other genera may be derived.
Definition.—Ring with two chambers, separated by two radial beams.
1. Archidiscus dioniscus, n. sp.
Ring circular, connected with the central chamber by two radial beams, opposite in one axis, therefore two equal semicircular ring chambers. (This primitive form has an interesting reference to Saturnalis, Pl. 13, fig. 16, and differs from it only in the lattice-work covering both faces of the lenticular disk, the margin of which forms the ring.)
Dimensions.—Diameter of the ring 0.05, of the central chamber 0.016.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
2. Archidiscus dithalamus, n. sp.
Ring roundish, connected with the central chamber by two radial beams, not opposite in one axis; both semicircular ring chambers more or less unequal, one of them larger than the other, and sometimes much more prominent. (If this prominence increase, we can regard it as the beginning of spiral convolutions, Discospira.)
Dimensions.—Diameter of the ring 0.06, of the central chamber 0.014.
Habitat.—Central Pacific, Station 266, depth 2750 fathoms.
Definition.—Ring with three chambers, separated by three radial beams.
3. Archidiscus trioniscus, n. sp.
Ring triangular, roundish, equilateral, connected with the central chamber by three radial beams at equal distances (120°); therefore three equal ring chambers. (Resembles the central part of the disk of Tripodictya trigonaria, Pl. 42, fig. 8, and may be the ancestral form of it.)
Dimensions.—Diameter of the ring 0.04, of the central chamber 0.014.
Habitat.—Central Pacific, Station 265, depth 2900 fathoms.
4. Archidiscus trithalamus, n. sp.
Ring irregular, roundish, connected with the central chamber by three radial beams at unequal distances; therefore three ring chambers of different size. (If these differences be important, they introduce a spiral convolution in the further development of the Porodiscus arising from it.)
Dimensions.—Diameter of the ring 0.06, of the central chamber 0.016.
Habitat.—South Pacific, Station 288, surface.
Definition.—Ring with four chambers, separated by four radial beams.
5. Archidiscus stauroniscus, n. sp. (Pl. 48, figs. 9, 9a).
Ring regular, square, connected with the central chamber by four radial beams at equal distances, opposite in pairs in two axes perpendicular one to another; therefore four equal ring chambers (or congruent quadrants of the ring). Resembles the central part of the disk of Staurodictya medusa, &c. (Pl. 42, figs. 1-3); also of Staurodictya gracilis, Ehrenberg, 1875 (Abhandl. d. k. Akad. d. Wiss. Berlin, Taf. xxiii. fig. 3).
Dimensions.—Diameter of the ring 0.05, of the central chamber 0.016.
Habitat.—North Pacific, Station 253, depth 3125 fathoms.
6. Archidiscus tetroniscus, n. sp.
Ring elliptical, of rhomboidal fundamental form, connected with the central chamber by four radial beams, halving the sides of the rhombus and opposite in pairs in two axes which are not perpendicular one to another; therefore four ring chambers in pairs different, two opposite equal and larger than the two others.
Dimensions.—Diameter of the ring 0.05 to 0.07, of the central chamber 0.016.
Habitat.—Central Pacific, Station 263, depth 2650 fathoms.
7. Archidiscus tetrathalamus, n. sp.
Ring irregular, quadrangular, connected with the central chamber by four radial beams of increasing unequal length; therefore all four ring chambers of different size, gradually increasing in the following quadrants of the ring. Important as an ancestral type of such spiral and semi-spiral forms as Staurodictya cruciata (Pl. 42, figs. 4, 12, &c.) and Stylodictya clavata, Stylodictya stellata, &c., of Ehrenberg, 1875 (Abhandl. d. k. Akad. d. Wiss. Berlin, Taf. xxiii. figs. 2, 7, 8, 9).
Dimensions.—Diameter of the ring 0.05 to 0.08, of the central chamber 0.06.
Habitat.—South Pacific, Station 295, depth 1500 fathoms.
Definition.—Ring with five chambers, separated by five radial beams.
8. Archidiscus pentoniscus, n. sp.
Ring pentagonal or nearly circular, regular, connected with the central chamber by five radial beams of equal length and at equal distances (72°); therefore all five chambers of the ring of equal size and similar form. (Resembles the central part of the disk of Pentinastrum asteriscus, Pl. 44, fig. 2.)
Dimensions.—Diameter of the ring 0.05, of the central chamber 0.015.
Habitat.—North Pacific, Station 244, surface.
Definition.—Ring with six chambers, separated by six radial beams.
9. Archidiscus hexoniscus, n. sp. (Pl. 48, figs. 10, 10a).
Ring regular, hexagonal, or nearly circular, connected with the central chamber by six radial beams of equal length and at equal distances (60°); therefore all six chambers of the same size and form. (Resembles the central part of the disk of Hexinastrum geryonidum, Pl. 44, fig. 4.)
Dimensions.—Diameter of the ring 0.06, of the central chamber 0.018.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
10. Archidiscus pyloniscus, n. sp. (Pl. 48, figs. 11, 11a).
Ring triangular, connected with the central chamber by six radial beams at alternating distances; therefore three larger chambers (of looser network) alternate with three smaller chambers (of denser network); pores of the former twice to three times as large as those of the latter. This species is of peculiar importance, as an immediate transitional form to the Pylodiscida. If these three larger ring chambers lose their few lattice-beams and so became open gates, we get Triolene or Triopyle, the original forms of the Pylodiscida.
Dimensions.—Diameter of the ring 0.05, of the central chamber 0.015.
Habitat.—Central Pacific, Station 266, depth 2750 fathoms.
11. Archidiscus hexathalamus, n. sp.
Ring irregular, roundish, or hexagonal, connected with the central chamber by six radial beams of unequal increasing length; therefore all six ring chambers of gradually increasing size (beginning a spiral convolution, original form of some Discospira).
Dimensions.—Diameter of the ring 0.05 to 0.07, of the central chamber 0.014.
Habitat.—South Pacific, Station 300, depth 1375 fathoms.
Definition.—Ring with seven or more chambers, separated by seven or more radial beams.
12. Archidiscus octoniscus, n. sp.
Ring circular or regular octagonal, connected with the central chamber by eight equidistant radial beams; therefore eight ring chambers of equal size. (Compare the central part of the disk of Porodiscus quadrigatus, Pl. 41, fig. 3.)
Dimensions.—Diameter of the ring 0.04, of the central chamber 0.014.
Habitat.—Central Pacific, Station 268, depth 2900 fathoms.
13. Archidiscus polythalamus, n. sp.
Ring circular, connected with the central chamber by nine to ten or more radial beams at nearly equal distances. Ring chambers nine to ten, more or less equal, sometimes also eleven to twelve, more different. (This species is very variable, and may perhaps be divided into a number of different "transformistic species.")
Dimensions.—Diameter of the ring 0.04 to 0.06, of the central chamber 0.015.
Habitat.—Central Pacific, Stations 265 to 274, depths 2350 to 2925 fathoms.
Definition.—Porodiscida with a simple central chamber, surrounded by one single concentric ring, which is divided by radial beams into two to six or more radial chambers; margin of the disk armed with radial spines.
The genus Axodiscus differs from the preceding Archidiscus only in the shape of the margin of the small lenticular disk, which is armed with a variable number of radial spines, indicating certain axes or radii of the shell. If these marginal spines at certain equal distances from the margin branch and their distal ends become united by these branches forming a concentric second ring, the genus passes into Porodiscus. The different number and disposition of the marginal spines are probably very important, as determining the later development of two to four or more radii in the different genera of Porodiscida.
1. Axodiscus stylophorus, n. sp.
Ring circular, with two equal semicircular chambers, connected with the central chamber by two opposite radial beams, which are prolonged outside into two strong conical spines.
Dimensions.—Diameter of the ring 0.05, of the central chamber 0.016.
Habitat.—Central Pacific, Station 266, depth 2750 fathoms.
2. Axodiscus triradiatus, n. sp.
Ring regular, triangular, equilateral, connected with the central chamber by three equidistant radial beams, which are prolonged outside into three short conical spines. (Differs from Archidiscus trioniscus in the possession of marginal spines.)
Dimensions.—Diameter of the ring 0.04, of the central chamber 0.013.
Habitat.—Central Pacific, Station 267, depth 2700 fathoms.
3. Axodiscus staurophorus, n. sp.
Ring regular, square, connected with the central chamber by four radial beams, which lie opposite in pairs in two perpendicularly crossed axes, and are prolonged outside into four delicate cylindrical spines.
Dimensions.—Diameter of the ring 0.045, of the central chamber 0.015.
Habitat.—North Pacific, Station 244, depth 2900 fathoms.
4. Axodiscus hexagonus, n. sp.
Ring regular, hexagonal, connected with the central chamber by six radial beams at equal distances, which are prolonged outside into six strong pyramidal spines.
Dimensions.—Diameter of the ring 0.06, of the central chamber 0.018.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
5. Axodiscus trigonus, n. sp.
Ring regular, triangular, equilateral, connected with the central chamber by six radial beams of alternating length and distance; therefore three smaller (perradial) chambers (with denser and darker network) alternating with three larger (interradial) chambers (with looser and finer network). On the margin three strong conical radial spines, arising in the radius of the larger chambers. (Differs from Archidiscus pyloniscus, Pl. 48, fig. 11, mainly in the strong angular spines on the three corners of the triangular shell.)
Dimensions.—Diameter of the ring 0.05, of the central chamber 0.014.
Habitat.—South Pacific, Station 296, depth 1825 fathoms.
6. Axodiscus octogonus, n. sp.
Ring circular or nearly octagonal, connected with the central chamber by eight radial beams at nearly equal distances, which are prolonged outside into eight short conical spines. (Differs from Archidiscus octoniscus in the marginal prolongation of the eight beams.)
Dimensions.—Diameter of the ring 0.04, of the central chamber 0.013.
Habitat.—Central Pacific, Station 274, depth 2750 fathoms.
7. Axodiscus spinosus, n. sp.
Ring circular, connected with the central chamber by ten to twelve radial beams at nearly equal distances, which are prolonged outside into short conical spines of variable length. (May be regarded as an aculeate variety of Archidiscus polythalamus.)
Dimensions.—Diameter of the ring 0.04 to 0.05, of the central chamber 0.014.
Habitat.—Central Pacific, Stations 265 to 274, depths 2350 to 2950 fathoms.
Definition.—Porodiscida without radial appendages of the disk (solid spines or chambered arms on the margin), and without peculiar oscula on the margin of the disk, which is composed of two to four or more concentric rings.
Definition.—Porodiscida with simple circular disk, composed of several rings (without radial appendages or peculiar oscula on the margin of the disk).
The genus Porodiscus is, next to its ancestral form, Archidiscus, the simplest and most primitive form of the Porodiscida, from which all other genera of this family can be derived. The disk is quite simple, without any marginal appendages, composed of a variable number of rings, commonly of circular form, sometimes more or less polygonal, elliptical, or irregular. In my Monograph (1862, pp. 491, 513) I had separated the species, here united in Porodiscus, into two different genera: Trematodiscus with concentric rings, and Discospira with spiral rings. But the extended study of these very common forms in a great number of specimens in the Challenger collection has convinced me that the separation of those two genera cannot be maintained. In one and the same locality, where one single characteristic disk-form is very common, we find intermingled quite regular disks with only concentric, circular rings (Trematodiscus), and other disks with one single perfect spiral ring (Discospira); and between these a smaller number of specimens, in which the rings of the disk are partly concentric, partly spiral; either the rings of the central part of the disk are concentric, the outer spiral (Perispira), or the proportion is inverse (Centrospira); and sometimes the whole disposition of the concentric and spiral rings is irregular, and the rings often interrupted (Atactodiscus). Therefore it appears more natural to give to all these different forms only the value of subgenera of Porodiscus, as I have already proposed in my Prodromus (1881, p. 459). Even the numerous species of Porodiscus (mainly characterised by the equal or different breadth of the rings, and by the number, form, and size of the connecting radial beams and of the superficial pores) are for the most part very variable and hard to distinguish, as all those characters are not constant. Porodiscus is a quite "transformistic genus."
Definition.—All rings of the disk concentric (commonly circular, rarely a little elliptical or polygonal).
1. Porodiscus orbiculatus, Haeckel.
Trematodiscus orbiculatus, Haeckel, 1862, Monogr. d. Radiol., p. 492, Taf. xxix. fig. 1.
Trematodiscus orbiculatus, Stöhr, 1880, Palæontogr., vol. xxvi. p. 108.
All rings of the disk circular, concentric, of equal breadth, connected by numerous alternating radial beams. Chambers differing little in size, about as large as the central chamber. Pores regular, circular, two to two and a half on the breadth of each ring.
Dimensions.—Diameter of the disk (with nine rings) 0.18; breadth of each ring 0.01; pores 0.003.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, surface; also fossil in Tertiary rocks of Barbados, Sicily, and Nicobar.
2. Porodiscus concentricus, Haeckel.
Trematodiscus concentricus, Haeckel, 1862, Monogr. d. Radiol., p. 493.
Trematodiscus concentricus, Stöhr, 1880, Palæontogr., vol. xxvi. p. 108.
Flustrella concentrica, Ehrenberg, 1838, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 132; Ibid., 1875, p. 72, Taf. xxii. fig. 13.
? Flustrella concentrica, Ehrenberg, 1854, Mikrogeol., Taf. xix. fig. 61, Taf. xx. fig. 42, Taf. xxi. fig. 51, Taf. xxxvi. fig. 29.
All rings of the disk circular, concentric, of equal breadth, connected by numerous piercing radial beams. Chambers different in size, increasing from the centre towards the periphery. Pores regular, circular, one and half to two on the breadth of each ring.
Dimensions.—Diameter of the disk (with eight rings) 0.16; breadth of each ring 0.01; pores 0.003.
Habitat.—Fossil in many Tertiary rocks—Barbados, Sicily, Greece, &c.
Porodiscus haeckelii = Trematodiscus haeckelii, Bütschli, 1882, L. N. 41, Taf. xxiv. figs. 5a, 5b.
3. Porodiscus flustrella, n. sp. (Pl. 41, fig. 1).
Trematodiscus flustrella, Haeckel, 1866, MS. Canar. Ins.
All rings of the disk circular, concentric, of equal breadth, connected by numerous piercing radial beams. Chambers different in size, increasing from the centre towards the periphery. Pores very irregular, polygonal, or roundish, one to three on the breadth of each ring.
Dimensions.—Diameter of the disk (with six rings) 0.12; breadth of each ring 0.01; pores 0.002 to 0.006.
Habitat.—Cosmopolitan; Atlantic, Pacific, &c., many Stations, surface.
4. Porodiscus sorites, Haeckel.
Trematodiscus sorites, Haeckel, 1862, Monogr. d. Radiol., p. 492, Taf. xxix. fig. 2.
All rings of the disk circular, concentric, of equal breadth, connected by numerous alternating radial beams. Chambers different in size, increasing from the centre. Pores regular, circular, one to one and a half on the breadth of each ring.
Dimensions.—Diameter of the disk (with six rings) 0.12; breadth of each ring 0.01; pores 0.006.
Habitat.—Mediterranean (Messina), surface.
5. Porodiscus macroporus, Haeckel.
Trematodiscus macroporus, Haeckel, 1879, MS.
Flustrella macropora, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 160.
All rings of the disk circular, concentric, of equal breadth, connected by numerous piercing radial beams. Chambers of different breadth, increasing from the centre. Pores regular, very large, nearly square, one single pore on the breadth of each ring.
Dimensions.—Diameter of the disk (with five rings) 0.11; breadth of each ring 0.01; pores 0.005.
Habitat.—Fossil in the Tertiary rocks of Barbados.
6. Porodiscus microporus, Haeckel.
Trematodiscus microporus, Stöhr, 1880, Palæontogr., vol xxvi. p. 108, Taf. iv. fig. 17.
All rings of the disk concentric, either circular or a little elliptical; the innermost rings of the same breadth as the central chamber, the third ring much broader. Radial beams between them alternating; chambers of different size. Pores very small, subregular, everywhere of the same size, four to five pores on the breadth of each ring.
Dimensions.—Diameter of the disk (with three rings) 0.15; breadth of the inner rings 0.02, of the outer 0.03; pores 0.0016.
Habitat.—Fossil in Tertiary rocks of Sicily, Grotte, Stöhr.
7. Porodiscus ellipticus, Haeckel.
Trematodiscus ellipticus, Stöhr, 1880, Palæontogr., vol. xxvi. p. 108, Taf. iv. fig. 16.
All rings of the disk concentric, either circular or a little elliptical, connected by eight piercing radial beams; central chamber elliptical, of the same breadth as the first ring, broader than the following rings, the breadth of which decreases towards the periphery. Pores irregular, in the inner rings twice as broad as in the outer (third) ring, four to five on the breadth of each ring.
Dimensions.—Diameter of the disk (with three rings) 0.18; breadth of the inner rings 0.03, of the outer 0.02; pores 0.003 to 0.006.
Habitat.—Fossil in Tertiary rocks of Sicily, Grotte, Stöhr.
8. Porodiscus heterocyclus, Haeckel.
Trematodiscus heterocyclus, Haeckel, 1862, Monogr. d. Radiol., p. 493, Taf. xxix. fig. 3.
? Flustrella cyclia, Harting, 1863, Fauna Banda-Zee, p. 11, pl. i. fig. 19.
Trematodiscus heterocyclus, Stöhr, 1880, Palæontogr., vol. xxvi. p. 108.
All rings of the disk concentric, circular, connected by numerous radial beams, which are partly piercing, partly alternating. The breadth of the rings increases gradually from the centre towards the periphery, corresponding also to the size of the pores; two to three circular pores on the breadth of each ring.
Dimensions.—Diameter of the disk (with six rings) 0.17; breadth of the second ring 0.007, of the sixth 0.02; pores 0.002 to 0.006.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface; also fossil in Tertiary rocks of Barbados and the Mediterranean.
9. Porodiscus quadrigatus, n. sp. (Pl. 41, fig. 3).
All rings of the disk concentric, of nearly equal breadth, connected by four interradial beams, perpendicular one to another. The first ring (surrounding the central chamber) with eight chambers, the second only with four (alternating with the four radial beams). Size of the four chambers of each ring increases much towards the periphery. Pores regular, circular, about two on the breadth of each ring.
Dimensions.—Diameter of the disk (with six rings) 0.18; breadth of each ring 0.05; pores 0.004.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
Definition.—The inner rings of the disk concentric (commonly circular); the outer rings spirally convoluted.
10. Porodiscus perispira, n. sp. (Pl. 41, fig. 2).
Perispira perforata, Haeckel, 1881, Prodromus.
Inner rings of the disk (two or three) concentric, outer rings (three or four) spirally convoluted, spiral line simple. All rings nearly of the same breadth, connected by alternating irregular radial beams. Pores irregular, two to three on the breadth of each ring.
Dimensions.—Diameter of the disk (with six rings) 0.17; breadth of each ring 0.016; pores 0.003 to 0.005.
Habitat.—Pacific, central area, Stations 272 to 274, surface.
11. Porodiscus radiatus, n. sp.
Perispira radiata, Haeckel, 1881, Prodromus.
Inner rings of the disk (four or five) concentric, outer rings (three or four) spirally convoluted, spiral line simple. All rings connected by piercing radial beams (eight in the inner half, sixteen in the outer half). Breadth of the rings and of the pores increasing from the centre towards the periphery, three to four pores on the breadth of each ring.
Dimensions.—Diameter of the disk (with eight rings) 0.18; breadth of the second ring 0.006, of the eighth ring 0.02; pores 0.002 to 0.006.
Habitat.—South Atlantic, Station 332, depth 2200 fathoms.
Definition.—The inner rings of the disk spirally convoluted, the outer rings concentric (commonly circular).
12. Porodiscus centrospira, n. sp. (Pl. 41, fig. 6).
Centrospira perispongidium, Haeckel, 1881, Prodromus.
Inner rings of the disk (two or three) spirally convoluted (with simple or double spiral line), outer rings (three or four) concentric, subcircular. All rings nearly of the same breadth, connected by alternating radial beams. Pores subregular, two to three on the breadth of each ring. Lattice-work in the periphery of the disk a little spongy (as in Pl. 41, fig. 11).
Dimensions.—Diameter of the disk (with five rings) 0.15; breadth of each ring 0.015; pores 0.003.
Habitat.—Pacific, central area, Station 267, depth 2700 fathoms.
Definition.—All rings of the disk spirally convoluted, forming parts of a simple or double spiral turning.
13. Porodiscus helicoides, Haeckel.
Discospira helicoides, Haeckel, 1862, Monogr. d. Radiol., p. 514, Taf. xxix. fig. 7.
Trematodiscus helicoides, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 841.
All rings of the disk spirally convoluted around the central chamber; spiral line regular, simple. All rings nearly of the same breadth, connected by numerous alternating radial beams. Chambers little different in size, little longer than broad. Pores of equal size, regular, two on the breadth of each ring.
Dimensions.—Diameter of the disk (with ten rings) 0.2; breadth of each ring 0.01; pores 0.0025.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific; also fossil in the Tertiary rocks of the Mediterranean, Sicily, Oran, &c.
14. Porodiscus spiralis, Haeckel.
Flustrella spiralis, Ehrenberg, 1840, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 210; Mikrogeol., 1854, Taf. xix. fig. 62.
Discospira spiralis, Haeckel, 1862, Monogr. d. Radiol., p. 513.
All rings of the disk spirally convoluted around the central chamber; spiral line regular, simple. All rings nearly of the same breadth, connected by numerous (twelve to sixteen) piercing radial beams. Chambers increasing in size from the centre towards the periphery. Pores irregular, of different size, one and a half to two on the breadth of each ring.
Dimensions.—Diameter of the disk (with eight rings) 0.16; breadth of each ring 0.01; pores 0.004.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific; also fossil in the Tertiary rocks of Barbados, Sicily, Greece, &c.
15. Porodiscus operculina, Haeckel.
Discospira operculina, Haeckel, 1862, Monogr. d. Radiol., p. 514, Taf. xxix. fig. 8.
All rings of the disk spirally convoluted around the central chamber, spiral line regular, simple. Breadth of the rings and length of the chambers increasing in size from the centre towards the periphery. Most part of chambers twice as long as broad. Pores irregular, of very different sizes, two to three on the breadth of each ring.
Dimensions.—Diameter of the disk (with six rings) 0.16; breadth of the rings 0.012 to 0.016; pores 0.002 to 0.006.
Habitat.—Mediterranean (Messina), surface.
16. Porodiscus bilix, Haeckel.
Discospira bilix, Stöhr, 1880, Palæontogr., vol. xxvi. p. 113, Taf. vi. fig. 3.
All rings of the disk spirally convoluted around the large elliptical central chamber. Spiral convolutions of the inner rings simple, of the outer rings double; the latter twice as broad as the former. Pores regular, circular, in the inner rings one pore on the breadth, in the outer rings two to three pores. Radial beams piercing, numerous, on the margin prominent.
Dimensions.—Diameter of the disk (with six rings) 0.14; breadth of the inner rings 0.007, of the outer 0.013; pores 0.0017.
Habitat.—Fossil in the Tertiary rocks of Sicily, Grotte, Stöhr.
17. Porodiscus bispiralis, Haeckel.
Stylodictya bispiralis, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 160. Taf. xxiv. fig. 1.
All rings of the disk spirally convoluted, in a perfect double spiral, with increasing breadth of the rings; the fourth ring twice as broad as the first. Pores regular, circular; in the inner rings one pore on the breadth, in the outer rings two to three pores. Radial beams piercing, numerous, prominent on the margin.
Dimensions.—Diameter of the disk (with four rings) 0.1; breadth of the inner rings 0.006, of the outer 0.012; pores 0.0015.
Habitat.—Fossil in the rocks of Barbados; living in the depths of the Central Pacific, Station 268, depth 2900 fathoms.
18. Porodiscus duplex, Haeckel.
Discospira duplex, Stöhr, 1880, Palæontogr., vol. xxvi. p. 114, Taf. vi. fig. 4.
All rings of the disk spirally convoluted, in a perfect double spiral, with increasing breadth of the broad rings. Pores regular, circular; in the inner rings two to three, in the outer four to five on the breadth. Radial beams interrupted. Margin of the rings thorny.
Dimensions.—Diameter of the disk (with three rings) 0.15; breadth of the rings 0.02 to 0.04; pores 0.007.
Habitat.—Fossil in Tertiary rocks of Sicily, Grotte, Stöhr.
19. Porodiscus semispiralis, n. sp. (Pl. 41, fig. 4).
All rings of the disk spirally convoluted, divided by four radial, perpendicularly crossed and zigzag shaped beams into four quadrants; the quarter rings of each quadrant halving the rings of each adjacent quarter. Half spiral line often irregular or partly interrupted. Breadth of all rings nearly equal. Pores irregular, roundish, two on the breadth of each ring. Margin of the disk dentated.
Dimensions.—Diameter of the disk (with nine rings) 0.22; breadth of each ring 0.012; pores 0.002 to 0.006.
Habitat.—Pacific, central area, Station 272, depth 2600 fathoms.
Definition.—Rings of the disk more or less irregular, partly concentric, partly spirally convoluted, often interrupted.
20. Porodiscus deformis, Haeckel.
Discospira deformis, Stöhr, 1880, Palæontogr., vol. xxvi. p. 114, Taf. vi. fig. 6.
Rings of the disk irregular, partly concentric, partly spiral, often interrupted, increasing in breadth from the centre. Radial beams not piercing; pores regular, circular, two to six on the breadth of the different rings. (Very variable and irregular, sometimes more spiral, at other times more concentric, but always with equal pores.)
Dimensions.—Diameter of the disk (with four rings) 0.18; breadth of the first ring 0.014, of the second 0.03, of the fourth 0.06; pores 0.005.
Habitat.—Fossil in the Tertiary rocks of Barbados and Sicily; living in the Central Pacific, Stations 266 to 268, depths 2700 to 2900 fathoms.
21. Porodiscus irregularis, n. sp. (Pl. 41, fig. 7).
Atactodiscus irregularis, Haeckel, 1881, Prodromus, p. 459.
Perispongidium irregulare, Haeckel, 1878, MS. et Atlas (pl. xli. fig. 7).
Rings of the disk irregular, partly concentric, partly spiral, often interrupted, with nearly equal breadth. Radial beams not piercing; pores irregular, roundish, two to four on the breadth of each ring; network in the periphery of the disk spongy. (Very variable and irregular, sometimes more spiral, at other times more concentric; disk in the peripheral part often more or less spongy. Differs from the preceding by the equal breadth of the rings and the different size of the pores.)
Dimensions.—Diameter of the disk (with six rings) 0.15; breadth of each ring 0.011; pores 0.002 to 0.004.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface; also fossil in Barbados.
Definition.—Porodiscida with a simple circular disk (without radial spines and chambered arms), surrounded on the margin by a thin porous (but not chambered) equatorial girdle.
The genus Perichlamydium differs from Porodiscus only in the development of a thin, porous, equatorial girdle, which surrounds the circular margin of the chambered disk. This girdle lies in the equatorial plane of the lenticular disk, and represents a very delicate siliceous plate, perforated by numerous small pores. Sometimes the proximal part of the girdle is ribbed by thin radial beams, the distal prolongations of the radial rods of the central disk. If these ribs reach the margin of the girdle and are prominent over it, Perichlamydium passes over into Stylochlamydium.
1. Perichlamydium praetextum, Ehrenberg.
Perichlamydium praetextum, Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 43; Mikrogeol., 1854, Taf. xxii. fig. 21 (non 20).
Perichlamydium praetextum, Haeckel, 1862, Monogr. d. Radiol., p. 495.
All rings of the disk (three to four) concentric, circular, of equal breadth, with interrupted (not piercing) radial beams. Equatorial girdle without radial beams, nearly as broad as the disk; its circular pores of the same size as those of the disk; about two pores on the breadth of each ring.
Dimensions.—Diameter of the disk (without the girdle, with four rings) 0.11; breadth of each ring 0.012; breadth of the girdle 0.06 to 0.1; pores 0.004.
Habitat.—Cosmopolitan; Atlantic, Indian, Pacific, surface; also fossil in the Tertiary rocks of Barbados and Sicily.
2. Perichlamydium saturnus, n. sp. (Pl. 41, fig. 5).
All rings of the disk (three to four) concentric, circular (sometimes partly concentric, circular, partly spiral, or irregular), with interrupted (not piercing) radial beams. Equatorial girdle without radial beams, about half as broad as the disk; its circular pores very small, scarcely half as large as those of the disk; about three pores on the breadth of each ring. (Very variable in the ring-form, differs from the preceding in the small pores of the girdle.)
Dimensions.—Diameter of the disk (without the girdle, with three rings) 0.11; breadth of each ring 0.02, pores 0.004, breadth of the girdle 0.05, pores 0.002.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
3. Perichlamydium accrescens, Haeckel.