Chapter XI: Literature and History (22)

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.

Subgenus 1. _Hymenacturium_, Haeckel.

_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.

{474}2. _Hymenactura pythagoræ_, Haeckel.

_Hymeniastrum pythagoræ_, Ehrenberg, 1854, Mikrogeol., Taf. xxxvi. fig.
31; Abhandl. d. k. Akad. d. Wiss. Berlin, 1875, p. 76, Taf. xxx. fig. 5.

Phacoid shell three times as broad as the medullary shell, with six 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 two-thirds as broad. In each arm eight to nine transverse rows of square chambers, each chamber with one pore on the surface. Patagium enveloping the basal half of the arms, with four to five rectilinear parallel rows of chambers, forming together a regular triangle. Differs from the preceding species mainly in the quite different structure of the patagium, from the true _Hymeniastrum pythagoræ_ in the phacoid shell of the central disk, which encloses only one simple medullary shell.

_Dimensions._--Diameter of the phacoid shell 0.11, of the medullary shell 0.04; length of the arms 0.13, basal breadth 0.07, distal breadth 0.12.

_Habitat._--Fossil in the rocks of Barbados.

3. _Hymenactura trigona_, n. sp.

_Astromma_ sp., Bury, 1862, Polycystins of Barbados, pl. xv. fig. 1.

Phacoid shell twice as broad as the medullary shell, with four to five pores on its radius. Arms slender, lanceolate, three times as long as the diameter of the phacoid shell and five times as long as broad in the middle part; distal ends blunt. Patagium enveloping only the basal half of the arms, forming a regular triangle with concave sides.

_Dimensions._--Diameter of the phacoid shell 0.08, of the medullary shell 0.035; length of the arms 0.2, greatest breadth 0.04.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms; fossil in the rocks of Barbados.

4. _Hymenactura hexagona_.

_Hymeniastrum_ sp., Bury, 1862, Polycystins of Barbados, pl. xv. fig. 3.

Phacoid shell twice as broad as the medullary shell, with four to five pores on its radius. Arms nearly equilateral triangular, three times as long as the diameter of the phacoid shell, at the truncated distal end nearly three-fourths as broad as long, at the narrow base only one-fourth as broad. Patagium complete, enveloping the arms completely, so that the whole body represents a regular hexagonal disk.

_Dimensions._--Diameter of the phacoid shell 0.08, of the medullary shell 0.04; length of the arms 0.18, basal breadth 0.04, distal breadth 0.16.

_Habitat._--Fossil in the rocks of Barbados and Nicobar.

{475}Subgenus 2. _Hymenactinium_, Haeckel.

_Definition._--Distal ends of the arms armed with terminal spines.

5. _Hymenactura ptolemæi_, n. sp.

_Astromma_ sp., Bury, 1862, Polycystins of Barbados, pl. xv. figs. 5, 6.

Phacoid shell twice as broad as the medullary shell, with six to seven pores on its radius. Arms nearly square, about as large as the phacoid shell, at the truncated distal end little broader than at the base, and armed with a strong pyramidal terminal spine. Patagium incomplete, enveloping the basal half of the arms.

_Dimensions._--Diameter of the phacoid shell 0.1, of the medullary shell 0.05; length and greatest breadth of the arms 0.08.

_Habitat._--Pacific, central area, Station 268, depth 2900 fathoms; fossil in the rocks of Barbados.

6. _Hymenactura copernici_, n. sp. (Pl. 38, fig. 9).

Phacoid shell three times as broad as the medullary shell, with six pores on its radius. Arms lanceolate, nearly twice as long as the phacoid shell, in the middle part twice as broad as the medullary shell, with a strong conical terminal spine at the distal end. In each arm about ten transverse rows of chambers. Patagium enveloping the basal half of the arms, with four convex parallel rows of chambers.

_Dimensions._--Diameter of the phacoid shell 0.1, of the medullary shell 0.035; length of the arms 0.17, greatest breadth 0.07.

_Habitat._--Pacific, central area, Station 266, depth 2750 fathoms.

Genus 208. _Astractura_,[247] Haeckel, 1881, Prodromus, p. 459.

_Definition._--#Coccodiscida# with four chambered arms on the margin of the circular or quadrangular disk, crossed in two equatorial diameters, without a connecting patagium.

The genus _Astractura_ has the form of a regular cross, four radial arms being opposite in two equatorial diameters perpendicular one to another. In the Porodiscida the same form is repeated by _Stauralastrum_, in the Spongodiscida by _Spongasteriscus_. The oldest known species of the genus is _Astromma aristotelis_ of Ehrenberg, in which genus this author confounded triradial and four-radial forms.

{476}Subgenus 1. _Astracturium_, Haeckel.

_Definition._--Distal end of the arms blunt or truncated, without terminal spines.

1. _Astractura ordinata_, n. sp.

? _Astromma_ sp., Bury, 1862, Polycystins of Barbados, pl. xiv. fig. 3.

Phacoid shell three times as broad as the medullary shell, with six pores on its radius, without a completely developed chambered ring. Arms trapezoidal, somewhat longer than the radius of the disk, at the truncated distal end as broad as long, at the base one-third smaller.

_Dimensions._--Diameter of the phacoid shell 0.08, of the medullary shell 0.03; length of the arms 0.05, distal breadth 0.05, basal breadth 0.035.

_Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms; also fossil in Barbados.

2. _Astractura clavigera_, n. sp.

_Astromma aristotelis_, Bury, 1862, Polycystins of Barbados, pl. iv. fig.
2.

Phacoid shell twice as broad as the medullary shell, with ten pores on its radius, surrounded by one perfect chambered ring. Arms club-shaped, about as long as the radius of the central disk, at the rounded distal end two-thirds, at the base one-third as broad as long.

_Dimensions._--Diameter of the phacoid shell 0.1, of the medullary shell 0.05; length of the arms 0.06, basal breadth 0.02, distal breadth 0.04.

_Habitat._--Pacific, central area, Stations 270 to 274, depths 2350 to 2925 fathoms; fossil in the rocks of Barbados.

Subgenus 2. _Astractinium_, Haeckel.

_Definition._--Distal end of the arms furnished with a radial spine.

3. _Astractura aristotelis_, Haeckel.

_Astromma aristotelis_, Ehrenberg, 1856, Microgeol., Taf. xxxvi. fig. 32;
Abhandl. d. k. Akad. d. Wiss. Berlin, 1875, p. 66, Taf. xxx. fig. 4.

_Astromma aristotelis_, Haeckel, 1862, Monogr. d. Radiol., p. 489.

? _Astromma aristotelis_, Bury, 1862, Polycystins of Barbados, pl. xiv.
fig. 4.

Phacoid shell three times as broad as the medullary shell, with six to eight pores on its radius, surrounded by one perfect chambered ring. Arms nearly trapezoidal, about as long as the diameter of the central disk, at the base half as broad, at the truncated distal end nearly as broad as long, and armed with a large pyramidal terminal spine. Each arm with about eight transverse and longitudinal rows of chambers.

{477}_Dimensions._--Diameter of the phacoid shell 0.11, of the medullary shell 0.04; length of the arms 0.1, basal breadth 0.05, distal breadth 0.09.

_Habitat._--Cosmopolitan; Atlantic, Indian, Pacific, at various depths; fossil in the Tertiary rocks of Barbados and Nicobar.

4. _Astractura democriti_, n. sp.

_Astromma aristotelis_, var., Ehrenberg, 1875, Abhandl. d. k. Akad. d.
Wiss. Berlin, p. 66, Taf, xxx. fig. 3.

? _Astromma aristotelis_, var., Bury, 1862, Polycystins of Barbados, pl.
xiv. fig. 4.

Phacoid shell twice as broad as the medullary shell, with five to seven pores on its radius, without perfect chambered ring. Arms about twice as long as broad, longer than the diameter of the central disk, at the truncated distal end a little broader than at the base, and armed with a strong, pyramidal, terminal spine.

_Dimensions._--Diameter of the phacoid shell 0.1, of the medullary shell 0.05; length of the arms 0.15, basal breadth 0.04, distal breadth 0.08.

_Habitat._--Pacific, central area, Station 266, depth 2750 fathoms; fossil in Barbados.

5. _Astractura hippocratis_, n. sp.

Phacoid shell three times as broad as the medullary shell, with seven pores on its radius, surrounded by two to three perfect chambered rings. Arms nearly square, scarcely as long and nearly as broad as the diameter of the central disk, at the rounded distal end armed with a very strong, pyramidal, terminal spine, longer than the arm itself, and at the base as broad as the medullary shell.

_Dimensions._--Diameter of the phacoid shell 0.09, of the medullary shell 0.03; length of the arms 0.08, breadth 0.1.

_Habitat._--Pacific, central area, Station 265, depth 2900 fathoms.

Genus 209. _Stauractura_,[248] Haeckel, 1881, Prodromus, p. 459.

_Definition._--#Coccodiscida# with four chambered arms on the margin of the circular or quadrangular disk, crossed in two equatorial diameters, connected by a spongy patagium.

The genus _Stauractura_ differs from the foregoing in the patagium between the arms, and bears therefore the same relation to it as _Histiastrum_ in the Porodiscida does to _Stauralastrum_. All known species of this genus form a regular square, if we connect the end points of the arm axes by lines.

Subgenus 1. _Stauracturium_, Haeckel.

_Definition._--Distal end of the arms blunt or truncated, without terminal spines.

{478}1. _Stauractura octogona_, n. sp.

Phacoid shell twice as broad as the medullary shell, with eight pores in its radius. Arms nearly square, little larger than the phacoid shell, with broad truncated distal ends. The corners of the latter are so connected by the complete patagium, that the whole shell forms a regular octagon.

_Dimensions._--Diameter of the phacoid shell 0.1, of the medullary shell 0.05; length of the arms 0.12, distal breadth 0.11.

_Habitat._--Pacific, central area, Station 266, depth 2750 fathoms.

2. _Stauractura tetragona_, n. sp.

Phacoid shell three times as broad as the medullary shell, with eleven pores on its radius. Arms club-shaped, nearly as long as the diameter of the phacoid shell, and one and a half times as long as broad at the distal part; at the distal end rounded, blunt, without terminal spine. The terminal points of the arms are so connected by a thin, complete patagium, that the whole shell becomes a regular square.

_Dimensions._--Diameter of the phacoid shell 0.13, of the medullary shell 0.045; length of the arms 0.12, greatest breadth 0.08.

_Habitat._--Pacific, central area, Station 265, depth 2900 fathoms.

Subgenus 2. _Stauractinium_, Haeckel.

_Definition._--Distal end of the arms furnished with a radial spine.

3. _Stauractura medusina_, n. sp.

Phacoid shell two and a half times as broad as the medullary shell, with nine pores on its radius. Arms club-shaped, one and a half times as long as the diameter of the phacoid shell, and in the outer third nearly as broad as the latter, at the base scarcely one-third as broad; their truncated distal end armed with a strong pyramidal terminal spine. Patagium incomplete, enveloping only the basal half of the arms, with three rectilinear parallel rows of chambers, forming a square.

_Dimensions._--Diameter of the phacoid shell 0.1, of the medullary shell 0.04; length of the arms 0.14, greatest breadth 0.08.

_Habitat._--Pacific, central area, Station 268, depth 2900 fathoms.

4. _Stauractura quadrata_, n. sp.

Phacoid shell twice as broad as the medullary shell, with seven pores on the radius. Arms nearly square, about as large as the phacoid shell, with a strong pyramidal terminal spine at the distal end. Patagium complete, between every two arms triangular, with rectilinear parallel {479}rows of chambers; it connects the end points of the arm radius in such a manner that the whole shell becomes a regular square.

_Dimensions._--Diameter of the phacoid shell 0.09, of the medullary shell 0.045; length of the arms 0.1, greatest breadth 0.08.

_Habitat._--South Pacific, Station 295, depth 1500 fathoms.

Genus 210. _Pentactura_,[249] Haeckel, 1881, Prodromus, p. 459.

_Definition._--#Coccodiscida# with five chambered arms on the margin of the circular or pentagonal disk, without a connecting patagium.

The genus _Pentactura_ possesses five free radial arms, and resembles _Pentalastrum_ among the Porodiscida. The distance of the five arms seems to be sometimes equal, at other times different in one and the same species.

1. _Pentactura pentactis_, Haeckel.

_Astromma pentactis_, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss.
Berlin, p. 66, Taf. xxx. fig. 1.

Phacoid shell two and a half times as broad as the medullary shell, with eight pores on its radius. Arms regularly or irregularly disposed, nearly square, about half as large as the phacoid shell, at the truncated distal end little broader than at the base. In the specimen figured, and very imperfectly described by Ehrenberg, (_loc. cit._), the arms were asymmetrically disposed, two opposite in one axis of the disk, two others on one side of this axis, the fifth opposite to these on the other side. Another specimen which I found in the Barbados rocks had five arms of equal size, regularly disposed, at equal intervals.

_Dimensions._--Diameter of the phacoid shell 0.1, of the medullary shell 0.04; length and breadth of the arms 0.05 to 0.06.

_Habitat._--Fossil in the rocks of Barbados.

2. _Pentactura astropecten_, n. sp.

Phacoid shell three times as broad as the medullary shell, with ten pores on its radius. Arms regularly disposed, club-shaped, nearly twice as long as the diameter of the phacoid shell, and equal to it in breadth at the rounded distal end. The latter is armed with a short conical spine, and is twice as broad as the base.

_Dimensions._--Diameter of the phacoid shell 0.09, of the medullary shell 0.03; length of the arms 0.17, basal breadth 0.05, distal breadth 0.08.

_Habitat._--Pacific, central area, Station 265, depth 2900 fathoms.

{480}Genus 211. _Echinactura_,[250] n. gen.

_Definition._--#Coccodiscida# with five chambered arms on the margin of the circular or pentagonal disk, connected by a spongy patagium.

The genus _Echinactura_ differs from the foregoing _Pentactura_ in the patagium between the arms, and bears therefore the same relation to it as _Pentinastrum_ in the Porodiscida does to _Pentalastrum_.

1. _Echinactura culcita_, n. sp.

Phacoid shell pentagonal, three times as broad as the medullary shell, with nine pores on its radius. Arms egg-shaped, about as long as the diameter of the phacoid shell, and in the distal part half as broad. The blunt ends of the arms are so connected by the complete patagium that the whole shell becomes a regular pentagonal disk.

_Dimensions._--Diameter of the phacoid shell 0.1, of the medullary shell 0.035; length of the arms 0.11, basal breadth 0.04, distal breadth 0.06.

_Habitat._--Pacific, central area, Station 266, depth 2750 fathoms.

2. _Echinactura asteriscus_, n. sp.

Phacoid shell twice as broad as the medullary shell, with seven pores on its radius. Arms lanceolate, twice as long as the diameter of the phacoid shell, in the middle part nearly as broad as the latter, at the distal end with a strong conical terminal spine. Patagium incomplete, enveloping only the basal half of the arms.

_Dimensions._--Diameter of the phacoid shell 0.09, of the medullary shell 0.045; length of the arms 0.2, greatest breadth 0.08.

_Habitat._--Pacific, central area, Station 268, depth 2900 fathoms.

3. _Echinactura goniaster_, n. sp.

Phacoid shell three times as broad as the medullary shell, with nine pores on its radius. Arms club-shaped, one and a half times as long as the diameter of the phacoid shell, in the distal part nearly as broad as the latter, in the basal part one-third as broad; at the end a strong pyramidal spine. Patagium nearly complete, with concave dentated margin between each pair of arms, therefore the pentagonal disk of the whole shell with concave sides.

_Dimensions._--Diameter of the phacoid shell 0.12, of the medullary shell 0.04; length of the arms (without terminal spine) 0.18, basal breadth 0.04, distal breadth 0.1.

_Habitat._--Pacific, central area, Station 265, depth 2900 fathoms.

{481}Family XXI. #PORODISCIDA#, Haeckel (Pls. 41-47).

_Porodiscida_, Haeckel, 1881, Prodromus, p. 459.
_Trematodiscida_ et _Discospirida_, Haeckel, 1862, Monogr. d. Radiol.,
pp. 485, 491, 513.
_Calodictya_, Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss.
Berlin, p. 53 (_partim_).

_Definition._--#Discoidea# without phacoid shell, with flat discoidal shell, in which a simple spherical central chamber is surrounded by concentric chambered rings (each ring divided by radial beams into imperfect chambers). Surface of the disk on the two flat sides covered by a porous sieve-plate.

The family #Porodiscida# is by far the largest and richest in different and common forms among all #Discoidea#; already in my Monograph (1862) nine genera and twenty-eight species have been enumerated. Their number is here increased to more than thirty genera and two hundred species. Many of these species appertain to the most common and widely distributed SPUMELLARIA, both living and fossil. But the study of their structure is not easy, and requires (as in the foregoing Coccodiscida) not only careful examination of the facial views of the disk, but also of the marginal view and of slides and sections through different planes.

In my Monograph (1862, pp. 485, 491, 513) I had constituted for these #Discoidea# two different families, the Trematodiscida and Discospirida; but the comparative study of a far greater number of different types in the Challenger collection has since convinced me that those two families are but little different, and united by transitional forms within one and the same genus, so that they must be united as Porodiscida. Of the group, which Ehrenberg formerly had called "Calodictya," many genera appertain to the Porodiscida, whilst many others are true Spongodiscida.

The Porodiscida represent the first and the most important family of the #Cyclodiscaria#, or of those #Discoidea# which are devoid of the peculiar extracapsular lenticular "phacoid shell," characteristic of the three preceding families (united therefore as #Phacodiscaria#). Probably all #Cyclodiscaria# can be derived from _Archidiscus_, from a morphological as well as a phylogenetic point of view. _Archidiscus_ seems to be the common ancestral form not only of the Porodiscida, but also of the nearly allied Pylodiscida and Spongodiscida. This important _Archidiscus_ (Pl. 48, figs. 9-11) is a small lenticular circular disk, in which a simple latticed spherical central chamber is surrounded by one single concentric ring, connected with it by a variable number of radial beams in the equatorial plane. From this typical _Archidiscus_, as from their "architype," all other #Cyclodiscaria# may be derived; the Porodiscida by regular apposition of new concentric chambered rings on the margin, the Spongodiscida by irregular apposition of a spongy framework, the Pylodiscida by a peculiar interrupted, concentric, triradial growth, three radial arm-chambers alternating with three open gates or holes, so that already the first chambered ring is not complete.

{482}_Archidiscus_ (Pl. 48, figs. 9-11) is not only the common phylogenetic ancestral form of all #Cyclodiscaria#, but also the common ontogenetic original form of all Porodiscida, or at least of the greater part of them. The numerous species of _Archidiscus_, which are distinguished in the sequel, are at the same time the embryonic forms of different Porodiscida, corresponding to the "biogenetic main law of development." The small shell of _Archidiscus_ is sometimes completely lenticular, circular, at other times more or less polygonal; commonly on the biconvex centre much thicker than on the margin, but sometimes also of nearly equal thickness (like a medal or a short cylinder). The latticed central chamber of it is probably in the majority of species spherical, but in some more or less compressed, lenticular; the number of small pores on its surface is probably commonly between ten and twenty (four to eight usually being visible on each hemisphere). The number of radial beams, which connect it with the equatorial ring, varies commonly from four to eight; but sometimes only two or three are to be found, in other cases nine to ten or more. The regular disposition of these beams (in certain equatorial axes of the disk) is probably of great importance, as determining the later development of characteristic radial appendages of the margin in the more highly developed Porodiscida. The equatorial ring itself, forming the margin of the lenticular disk, is either a simple solid ring or a broader latticed girdle; in the latter case it merges slowly into the opposite sieve-plates of the two flat disk sides, or the porous "cover-plates," covering its parallel or convex surfaces. These latter can be regarded as direct peripheral continuations of the polar regions of the spherical central chamber. The ring-chambers, surrounding the latter in a single circle, are commonly of nearly the same breadth, but often also of different irregular size. Their number varies between two and ten or more, but commonly between four and eight; each ring-chamber is covered on the upper and lower side by the sieve-plate, bounded on the inner (proximal) side by the wall of the central chamber, on the outer (distal) side by the marginal ring, on both lateral sides by the contiguous neighbouring ring-chambers.

The important question as to the phylogenetic origin of _Archidiscus_ can be answered in a twofold way. The most simple form of _Archidiscus_ (_Archidiscus dioniscus_) can be derived immediately from the Stylosphærida, _Saturnalis_ (Pl. 13, fig. 16), only by the development of lattice-work between the equatorial ring and the two polar faces of the concentric central chamber (on the surface of the biconvex jelly-mantle). But on the other hand _Archidiscus_ may also be derived from the simplest Phacodiscida, _Sethodiscus_ (Pl. 33, figs. 1-3), by the stronger compression of the biconvex lenticular shell, so that the enclosed medullary shell on the two poles runs together with the lenticular phacoid shell, of which only the peripheral part remains free, and thus forms the chambered ring; this latter explanation seems the more natural in many cases, as often in the Porodiscida the central chamber is enclosed in one or two concentric spherical or lenticular lattice-shells.

{483}The second subfamily of the Porodiscida are the Trematodiscida, which are derived from the preceding Archidiscida by concentric growth in the equatorial plane. In the same way in which the simple chambered ring of _Archidiscus_ is connected by radial beams with the central chamber, so in _Porodiscus_ a variable number of concentric rings is connected with that first ring. The number of these concentric rings varies between two and ten or more, but commonly amounts to between three and five. The radial beams connecting them are either piercing or interrupted; their number increases gradually from the centre towards the periphery. The chambers between them are sometimes more regular, at other times more irregular in size and form. Their upper and lower wall is formed by the two covering "sieve-plates," or the porous cover-plates, which are continued from the central disk to the margin. If these two sieve-plates continue being parallel, the disk becomes medal-shaped or a short cylinder; if the two plates become more or less concavely vaulted one against the other, the disk becomes biconvex lenticular, the middle part thicker than the marginal part. Rarely the contrary is the case, the margin thicker than the centre, and then the disk biconcave.

In my Monograph I had separated as two different subfamilies the true Trematodiscida (with circular concentric rings) from the Discospirida (with spirally convoluted rings). But the enormous mass of specimens, which I afterwards examined in the Challenger collection, has convinced me that this separation was not natural. For in one and the same genus of most nearly allied forms we find on one hand quite regular concentric circular forms (_Trematodiscus_), on the other hand spirally convoluted forms (_Discospira_), and connecting between them such forms as are in the central part concentric, in the marginal part spiral (_Perispira_)--or conversely, these in the centre spiral, on the margin concentric (_Centrospira_)--and frequently also more or less irregular forms with interrupted rings (_Atactodiscus_); therefore, all those genera (Prodromus, 1881, Nos. 448-452) have only the value of subgenera of _Porodiscus_. But a distinct genus is _Perichlamydium_, in which the two sieve-plates run on the margin of the lens and form a broad hyaline porous or solid girdle. More important is the distinction of the Ommatodiscida, in which the margin of the disk exhibits one larger osculum, armed with a corona of spines (_Ommatodiscus_), or two oscula, opposite on the poles of one axis (_Stomatodiscus_). Whilst in many Porodiscida all chambers of the concentric rings lie in one and the same (equatorial) plane, in many others with further growth they become stratified in floors, and the whole disk is therefore composed of two to four or more parallel disks, each with a system of concentric chambered rings or girdles, quite as in the majority of the Coccodiscida (p. 457). Often the central part of the lenticular disk becomes thickened by apposition of such floors or strata, whilst the marginal part remains simple, with one single stratum. The communication between the chambers of the different strata seems to be the same as in the similar Coccodiscida.

Also the margin of the disk exhibits in the Porodiscida the same characteristic {484}differences as in the foregoing family. In the subfamily of Stylodictyida it bears a certain number of solid radial spines, often regularly disposed (as in the Stylocyclida). In the subfamily of Euchitonida the margin is distinguished by the possession of two to six or more (commonly three or four) chambered arms, also situated in the plane of the disk, and of the same structure (sometimes more or less irregular, spongy). These arms are very variable in size, form, and structure, exhibit the same peculiarities as in the Astracturida, and are sometimes free, at other times connected by a "patagium" or an interbrachial spongy skeleton of different structure, like a web membrane (compare above, p. 458). In some genera the arms become forked or branched on the distal end. Sometimes their distal end bears a terminal radial spine.

_The Central Capsule_ of the Porodiscida assumes generally the form of the including shell, with or without arms, but is constantly somewhat smaller, as it remains enclosed by the sieve-plates of the disk surface. Often the capsule is filled with many coloured oil-globules, disposed regularly in the chamber rows. The nucleus of it is enclosed by the central chamber, and in many cases by this and the innermost concentric rings.

_Synopsis of the Genera of the Porodiscida._

-------------------------------------------------------------------------
I. Section of the Porodiscida--Archidiscaria.
Central chamber of the disk surrounded only by one single chambered
ring.
--------------------
1. Subfamily Archidiscida.
Only one single chambered girdle surrounds the central chamber; margin
smooth or spiny.
-------------------------------------------------------------------------
Margin smooth, without spines, 212. _Archidiscus_.

Margin armed with radial spines, 213. _Axodiscus_.

II. Section of the Porodiscida--Astrodiscaria.

Central chamber of the disk surrounded by two or more (commonly three to
six) concentric chambered rings or radiated girdles.
-------------------------------------------------------------------------
II. Section of the Porodiscida--Astrodiscaria.
Central chamber of the disk surrounded by two or more (commonly three
to six) concentric chambered rings or radiated girdles.
--------------------
2. Subfamily Trematodiscida.
Margin of the disk quite simple, without radial appendages (spines or
chambered arms), without peculiar oscula.
-------------------------------------------------------------------------
Margin simple, without an equatorial girdle, 214. _Porodiscus_.

Margin with a thin porous equatorial girdle, 215. _Perichlamydium_.
-------------------------------------------------------------------------
3. Subfamily Ommatodiscida. Margin of the disk without chambered arms but
distinguished by one or two large oscula, or wide openings armed with a
crown of spines.
-------------------------------------------------------------------------
Disk with a single marginal osculum, 216. _Ommatodiscus_.

Disk with two opposite marginal oscula, 217. _Stomatodiscus_.
-------------------------------------------------------------------------
4. Subfamily Stylodictyida.
Margin of the disk without peculiar oscular openings and without
chambered arms, but armed with solid radial spines.
-------------------------------------------------------------------------
Radial spines of the { Two opposite spines, 218. _Xiphodictya_.
margin two, three, or{
four, usually quite { Three equidistant
regularly disposed. { spines, 219. _Tripodictya_.
{
{ Four crossed spines, 220. _Staurodictya_.

Radial spines of the { Margin simple, without
margin five or more, { an equatorial girdle, 221. _Stylodictya_.
commonly irregularly {
disposed (generally {Margin with a thin porous
ten to twelve). { equatorial girdle, 222. _Stylochlamydium_.
-------------------------------------------------------------------------
5. Subfamily Euchitonida.{485}
Margin of the disk with radial chambered arms (or hollow chambered
appendages), on the distal end of the arms with or without solid
radial spines.
-------------------------------------------------------------------------
{ Two arms { Without a patagium, 223. _Amphibrachium_.
{ simple, {
Two arms, { undivided. { With a patagium, 224. _Amphymenium_.
opposite {
in one { Two arms { Without a patagium, 225. _Amphirrhopalum_.
main axis.{ forked (or {
{ one simple, {
{ other { With a patagium, 226. _Amphicraspedum_.
{ forked). {

{ { Regular, 227. _Dictyastrum_.
{ { Without a {
Three { Three arms { patagium. { Bilateral, 228. _Rhopalastrum_.
arms { simple, {
(either { undivided. {
regular, { { { Regular, 229. _Hymeniastrum_.
equal, or { { With a {
bilateral,{ { patagium. { Bilateral, 230. _Euchitonia_.
with { { {
unequal {
arms). { { Without a
{ { Either { patagium, 231. _Chitonastrum_.
{ Three arms {regular or {
{ forked. {bilateral. { With a
{ { { patagium, 232. _Trigonastrum_.

Four arms { { { Regular, 233. _Stauralastrum_.
(either { { Without a {
regular, { Four arms { patagium. { Bilateral, 234. _Hagiastrum_.
with four { simple, {
equal { undivided. { { Regular, 235. _Histiastrum_.
crossed { { With a {
arms, or { { patagium. { Bilateral, 236. _Tessarastrum_.
bilateral,{ {
with { { With a terminal
paired { { patagial girdle, 237. _Stephanastrum_
arms). {
{ { Regular, 238. _Dicranastrum_.
{ Four arms { Without a {
{ forked. { patagium. { Bilateral, 239. _Myelastrum_.

{ Five arms { Without a patagium, 240. _Pentalastrum_.
{ simple, {
Five arms { undivided. { With a patagium, 241. _Pentinastrum_.
(equal or {
unequal). { Five arms { Without a patagium, 242. _Pentophiastrum_.
{ forked. {

Six arms { Six arms { Without a patagium, 243. _Hexalastrum_.
(equal or { simple, {
unequal). { undivided. { With a patagium, 244. _Hexinastrum_.

Subfamily 1. ARCHIDISCIDA, Haeckel.

_Definition._--#Porodiscida# with a simple spherical or lenticular latticed central chamber, surrounded by a single concentric latticed ring, which is divided by radial beams into two to six or more radial chambers.

{486}Genus 212. _Archidiscus_,[251] n. gen.

_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.

Subgenus 1. _Dioniscus_, Haeckel.

_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.

Subgenus 2. _Trioniscus_, Haeckel.

_Definition._--Ring with three chambers, separated by three radial beams.

{487}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.

Subgenus 3. _Tetroniscus_, Haeckel.

_Definition._--Ring with four chambers, separated by four radial beams.

5. _Archidiscus stauroniscus_, n. sp. (Pl. 48, figs. 9, 9_a_).

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 {488}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.

Subgenus 4. _Pentoniscus_, Haeckel.

_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.

Subgenus 5. _Hexoniscus_, Haeckel.

_Definition._--Ring with six chambers, separated by six radial beams.

9. _Archidiscus hexoniscus_, n. sp. (Pl. 48, figs. 10, 10_a_).

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, 11_a_).

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.

{489}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.

Subgenus 6. _Circoniscus_, Haeckel.

_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.

Genus 213. _Axodiscus_,[252] n. sp.

_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, {490}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.

{491}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.

Subfamily 2. TREMATODISCIDA, Haeckel, 1862, Monogr. d. Radiol., p. 491 (_sensu emendato et restricto_).

_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.

Genus 214. _Porodiscus_,[253] Haeckel, 1881, Prodromus, p. 459.

_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 {492}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."

Subgenus 1. _Trematodiscus_, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 841.

_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. 5_a_, 5_b_.

{493}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.

{494}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.

Subgenus 2. _Perispira_, Haeckel, 1881, Prodromus, p. 459.

_Definition._--The inner rings of the disk concentric (commonly circular); the outer rings spirally convoluted.

{495}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.

Subgenus 3. _Centrospira_, Haeckel, 1881, Prodromus, p. 459.

_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.

Subgenus 4. _Discospira_, Haeckel, 1862, Monogr. d. Radiol., p. 513.

_Definition._--All rings of the disk spirally convoluted, forming parts of a simple or double spiral turning.

{496}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 {497}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.