Chapter XI: Literature and History (19)

The #Phacodiscida# (Pls. 31-35), the second family, have the same extracapsular "phacoid shell" as the Cenodiscida, but differ from these by the possession of one or two intracapsular concentric medullary shells, which are connected with the former by radial beams, perforating the lenticular central capsule. The radial beams are commonly numerous, and arranged in two opposite bunches around the shortened main axis. But often also besides these occur other longer radial beams, situated in the equatorial plane; the number of these is commonly four, and they form a regular rectangular cross, lying opposite in pairs in two equatorial diameters, perpendicular one to another. In the simplest genera of this family (the _Sethodiscida_) the equatorial margin of the phacoid shell is simple or surrounded by a solid smooth girdle; in all other genera are developed on the margin solid radial spines lying in the equatorial plane, either regularly disposed in a somewhat constant number (two to eight, _Heliosestrida_), or irregularly disposed, in a larger and more variable number (ten to twenty or more, _Heliodiscida_).

The #Coccodiscida# (Pls. 36-38) form a third family of the #Discoidea#, directly associated with the Phacodiscida; both have the same characteristic extracapsular "phacoid shell," which is connected by radial beams with a simple or double, intracapsular medullary shell. But whilst in the foregoing family the equatorial margin of the phacoid shell is simple or only armed with radial spines, in the Coccodiscida it is surrounded by peculiar concentric chambered girdles, or rings, which resemble those of the following family, the Porodiscida. Each of these "chambered girdles" is composed of a circular ring in the equatorial plane, a variable number of radial beams dividing it into incomplete chambers, and two porous cover-plates or "sieve-plates," covering the upper and lower face of the disk. These sieve-plates may be regarded as incomplete lenticular cortical shells, which are only developed in the peripheral part of the disk, whilst their central part is represented by the only complete cortical shell, the "phacoid shell." The number of these concentric chamber-girdles amounts to from one to ten or more. The margin of the disk is either simple (Lithocyclida) or armed with radial spines (Stylocyclida), or provided with two to five chambered radial arms (Astracturida); the structure of the arms is the same as that of the girdles.

{405}The #Porodiscida# (Pls. 41-47), the largest family of all #Discoidea#, begins the series of #Cyclodiscaria#, or those #Discoidea# in which there is no "phacoid shell," but a small simple central chamber surrounded by a number of small latticed chambers of nearly the same size and form. In the Porodiscida these chambers are arranged in complete circular concentric rings or spiral convolutions; in the small family of Pylodiscida the central chamber is surrounded by three radial arm-chambers separated by three open spaces; in the third family of #Cyclodiscaria#, the Spongodiscida, all the chambers are arranged more or less irregularly, and the whole disk becomes spongy; also the surface of the disk is spongy, whilst in both former families it is covered by two regular even porous plates or "sieve-plates." Probably all #Cyclodiscaria# can be derived phylogenetically from _Archidiscus_, a very small and simple lenticular disk, which is composed of a small spherical latticed central chamber and of a single concentric chambered ring or girdle; the margin of this ring is connected with the central chamber by a variable number of radial beams. This _Archidiscus_ can be derived either from _Saturnalis_ (Pl. 13, fig. 16.) by the development of lattice-work between the equatorial ring and both polar faces of the concentric central chamber, or from _Sethodiscus_ (Pl. 33, figs. 1-3) by flattening of the lenticular shell, so that the enclosed inner medullary shell (the central chamber) meets the outer phacoid shell at both poles.

The Porodiscida are commonly flat or biconvex (rarely biconcave) disks, the central chamber of which is surrounded not by a single, but by a variable number (commonly three to six) of concentric chambered girdles or rings; they arise from _Archidiscus_ by apposition of new concentric chambered rings around the first ring, all lying in the equatorial plane. Afterwards the disk often becomes thickened by apposition of concentric chamber-rings on both flat sides also, so that two to four or more layers are stratified one over the other. The circular concentric rings often become interrupted, or spirally convoluted (wholly or partially); also the chambers sometimes become irregularly crowded. But in all cases both surfaces of the disk (upper and lower) continue to be porous plates or sieve-plates, at least in the centre, but they never become spongy.

The margin of the disk exhibits in the Porodiscida a great variety of different forms, serving for distinction of subfamilies and of genera. In the Trematodiscida the margin remains quite simple, as in the Archidiscida, or is only surrounded by a hyaline equatorial girdle. In the Ommatodiscida it is distinguished by one or two peculiar oscula, surrounded by a corona of spines. The Stylodictyida are distinguished by a number of solid radial spines, and the Euchitonida by a number of chambered, or spongy, radial arms, arising from the margin of the disk and lying in the equatorial plane. The variety of these radial marginal appendages is in the Porodiscida much greater than in the Coccodiscida.

The #Pylodiscida# (Pl. 48, figs. 12-20) represent a new, small, but very remarkable family of #Discoidea#, all triradial, and distinguished by the peculiar formation of {406}large open spaces in the latticed discoidal shell, which reappear in a similar shape among the #Larcoidea# in the Pylonida (_Tetrapyle_, &c.). We get the best understanding of this peculiar formation if we return to _Archidiscus_, probably the common ancestral form of all #Cyclodiscaria#, of the Porodiscida as well as of the Pylodiscida and Spongodiscida. In some species of _Archidiscus_ (_Archidiscus hexoniscus_, _Archidiscus pyloniscus_, &c.) the small lenticular shell is composed of a spherical latticed central chamber and of a concentric equatorial girdle composed of six such chambers, either all six equal, or alternately larger and smaller. This latter form is nearly identical with _Triodiscus_, and if we imagine the lattice-work of only three ring-chambers complete, whilst that of the three alternating chambers is reduced to the marginal ring, we get _Triopyle_, by loss of this ring _Triolene_ (a disk composed of four simple lattice-chambers, lying in one plane, three radial around one central spherule). The genera mentioned form together the subfamily of Triopylida. In the second subfamily, Hexapylida, the same formation is doubled; here three double arm-chambers are separated by three double spaces (two in each radius). Also here the three distal spaces may be either quite open (_Pylolena_), or half closed by the marginal girdle (_Hexapyle_), or quite loosely latticed (_Pylodiscus_). If the margin of this latter form become surrounded by a perfect chambered equatorial girdle, we get _Discozonium_, and if this acquire a peculiar marginal ostium (surrounded by a corona of spines) we arrive at _Discopyle_. These two latter genera form the third subfamily, the Discopylida. The eight genera of Pylodiscida represent therefore a continuous phylogenetic series.

The #Spongodiscida# are the sixth and last family of the #Discoidea#, differing from the five other families in the irregular, spongy structure of the discoidal skeleton; both surfaces of the flat disk (upper and lower) are here principally covered with a rough, spongy framework, whilst in the five other families they are covered by the flat and smooth porous plates or sieve-plates. Nevertheless there is no sharp boundary between the Spongodiscida and the closely allied Porodiscida. In these latter also the discoidal shell becomes often more or less spongy (mainly in the peripheral part, _e.g._, in _Myelastrum_, Pl. 47); but at least the central part of the disk here remains constantly covered by sieve-plates. The massive skeleton of the Spongodiscida is either of perfectly irregular structure, only composed of innumerable fine branched siliceous threads, interwoven in all possible directions; or only the outer part of the disk is composed of such spongy framework, whilst the central part is more or less distinctly composed of concentric chambered rings, as in the Porodiscida. These latter forms indeed exhibit an immediate transition to this family, and were formerly (in 1862) separated by me as Spongocyclida. Also the polymorphous shape of the disk margin in the Spongodiscida is quite analogous to that of the Porodiscida. Whilst in the first subfamily, the Spongophacida, the margin is quite simple; in the second, the Spongotrochida, it is armed with solid radial spines; and in the third, the Spongobrachida, it is provided with two, three, {407}or four spongy, radial arms--the former as well as the latter lying in the equatorial plane of the disk, either regularly or irregularly disposed.

_The Equatorial Margin_ of the lenticular disk exhibits in all six families of #Discoidea# similar characters, mainly serving for the distinction of subfamilies and genera. In the most primitive genera of all six families the margin is simple, without radial prolongations (spines or arms); it is quite simple in _Cenodiscus_, _Sethodiscus_, _Phacodiscus_, _Lithocyclia_, _Coccodiscus_, _Archidiscus_, _Porodiscus_, _Pylodiscus_, and _Spongodiscus_. In some genera the simple margin of the lenticular disk is bordered and surrounded by a thin, hyaline, equatorial girdle of silex, either quite solid or slightly porous (_Zonodiscus_, _Periphæna_, _Perizona_, _Perichlamydium_, and _Spongophacus_).

A quite peculiar and remarkable character of few genera is the development of one or two oscula, larger marginal openings, which are surrounded by a corona of spines, and probable are fit for the issue a peculiar bunch of pseudopodia or of a "sarcode flagellum." Such oscula occur only in two families of #Cyclodiscaria#; in the Porodiscida and Pylodiscida; in the former _Ommatodiscus_, in the latter _Discopyle_ (Pl. 48, figs. 19, 20) is distinguished by a single marginal osculum; besides this, in the former occurs _Stomatodiscus_, with two such oscula, opposite on the poles of one axis (Pl. 48, fig. 8). These oscula may be compared with the similar polar formations in some Ellipsida (_Lithomespilus_) and in many Cyrtoidea; but they do not prove a nearer affinity with the latter, and are only analogous, not homologous.

_Radial Spines_ occur on the margin in the equatorial plane of the #Discoidea# in the greatest variety of number, form, size, and disposition. If the number be low (between two and eight) they are commonly regularly disposed; if the number be larger (ten to twenty or more) their disposition becomes commonly more or less irregular. The regular disposition is of great promorphological importance, as indicating the axes in which the growth is preponderant, and introduces other peculiar radial formations. Regarding these axes we can generally distinguish two groups, _Artiacantha_ with a paired number (two, four, eight), and _Perissacantha_ with odd numbers (usually three). The section of _Artiacantha_ could be divided into three following groups:--A. Stylodiscida, with two radial spines only, lying opposite on both poles of one equatorial axis (the "first cross axis")--_Stylodiscus_, _Sethostylus_, _Stylocyclia_, _Xiphodictya_, _Spongolonche_ (Pl. 31, figs. 9-12; Pl. 38, fig. 1; Pl. 42, figs. 10-12, &c.); B. Staurodiscida, with four radial spines, lying opposite in pairs on the poles of two crossed equatorial axes, perpendicular to one another (first and second cross axes)--_Crucidiscus_, _Sethostaurus_, _Staurocyclia_, _Staurodictya_, _Spongostaurus_ (Pl. 31, figs. 1-8; Pl. 37, figs. 1-4; Pl. 42, figs. 1-6; Pl. 48, fig. 2, &c.); C. Octostylida, with eight radial spines, opposite in pairs in four axes, which are crossed at angles of 45°--_Heliosestrum_, _Astrosestrum_, &c. (Pl. 32, figs. 4, 5; Pl. 34, figs. 3, 6); in this latter case sometimes the radial symmetry is the same as in many Medusæ, four larger (perradial) {408}spines alternating with four smaller (interradial), indicating radii of first and second order. The section of _Perissacantha_ is much smaller, and commonly represented only by triradial forms, with three spines at equal distances (120°)--_Triactis_, _Tripocyclia_, _Tripodictya_, _Spongotripus_, &c. (Pl. 33, fig. 6; Pl. 37, fig. 5; Pl. 42, figs. 7-9).

_Radial Arms_ on the margin of the disk appear in similar variety of number, form, and disposition as the radial spines; but the number is here commonly limited to from two to four, rarely five to six. The arms are absent in the families Cenodiscida and Phacodiscida; in the four other families they return under similar forms. These arms are direct prolongations of the disk, and exhibit the same structure, so that they may be regarded both as centrifugal productions of certain radii, and also inversely as peripheral parts of a disk, the interjacent radii of which are reduced. The regular disposition and shape of the arms, an important character for the distinction of genera and species, is repeated in a quite analogous manner in the four above mentioned families, so that we can distinguish the following groups--A. Amphibrachida, with two radial arms, opposite on the poles of one equatorial axis (the first cross axis)--_Diplactura_, _Amphibrachium_, _Spongobrachium_ (Pl. 38, figs. 3-5; Pl. 44, figs. 6-11); B. Triobrachida, with three radial arms; the most important group (with all Pylodiscida); either all three arms are equal and disposed at equal distances (_Trigonactura_, _Dictyastrum_, _Rhopalodictyum_, Pl. 38, figs. 6-9; Pl. 43, figs. 5, 13, 16; Pl. 48, figs. 12-19), or a single odd arm differs in size and position, and is often larger than both the opposite paired arms (_Rhopalastrum_, _Euchitonia_, Pl. 43, figs. 6, 10, 15, &c.); C. Tetrabrachida, with four radial arms, opposite in pairs in two crossed axes, commonly perpendicular one to another, _Stauractura_, _Stauralastrum_, _Spongaster_, &c. (Pls. 46, 47).

The arms are commonly simple, undivided, but sometimes also forked or branched (Pl. 43, figs. 15, 16; Pl. 47). Their basal parts are either free, separately inserted into the margin of the circular central disk, or they are connected by a "patagium," a peculiar connecticulum, like a web-membrane, which is composed of a chambered, commonly more or less spongy framework, different in texture from the lattice-work of the arms (Pl. 38, figs. 8, 9; Pl. 43, figs. 9-16; Pl. 46). Sometimes the patagium overgrows the whole shell. A peculiar modification of it appears in _Stephanastrum_ (Pl. 44, fig. 1), where only the distal parts of the arms are connected by the ring-shaped patagium, whilst the basal parts are free; therefore open gates rest between them, like those of the Pylodiscida (Pl. 48, figs. 12-20).

The _Central Capsule_ of the #Discoidea# is constantly discoidal, more or less lenticular; in some cases more biconvex, with vaulted faces and thin margin; in others more medal-shaped, with flat faces and thick margin. In the Cenodiscida alone the capsule lies freely inside the simple phacoid shell, and is separated from it by the jelly-veil. In the other five families the capsule encloses the central parts of the skeleton, and is enveloped by the superficial parts of it, whilst its membrane is perforated by radial beams {409}connecting the latter with the former. In the Phacodiscida and Coccodiscida the capsule encloses the simple or double medullary shell, but is itself enclosed by the cortical phacoid shell. In all #Cyclodiscaria# (in the Porodiscida, Pylodiscida, and Spongodiscida) the capsule fills out the greatest part of the chambered or spongy skeleton, and is only protected by the superficial parts of it, in the Porodiscida and Pylodiscida by the covering sieve-plates, in the Spongodiscida by the spongy cortical substance of the shell. The growth of the capsule corresponds to that of the including shell, gradually increasing on the margin in the equatorial plane. Whilst in the greater number of #Discoidea# its form continues circular, in many forms provided with radial arms it enters into the arms and assumes their form. The protoplasm of the capsule is commonly coloured by brown or red pigment, and often contains many oil-globules. The nucleus is originally enclosed by the medullary shell or the central chamber, and with increasing size enters into the surrounding parts; in the #Cyclodiscaria# it often fills out the internal concentric rings. The extracapsular jelly or the calymma is commonly thick, and envelops the greater part or the whole body.

_Synopsis of the Families of the_ #Discoidea#.

-------------------------------------------------------------------------
I. Section #Phacodiscaria#.
Discoidea with external phacoid shell (or lenticular latticed cortical
shell).
-------------------------------------------------------------------------
Phacoid shell simple, without enclosed 1. CENODISCIDA.
medullary shell

Phacoid shell with { Margin without
simple or { chambered girdles, 2. PHACODISCIDA.
double {
enclosed { Margin surrounded by
medullary shell. { by chambered
{ girdles. 3. COCCODISCIDA.
-------------------------------------------------------------------------
II. Section #Cyclodiscaria#.
Discoidea without external phacoid shell (no lenticular latticed
cortical shell).
-------------------------------------------------------------------------
Surface of the { Concentric rings around
shell covered { the central chamber
by convex or { complete (without
even porous { open spaces), 4. PORODISCIDA.
sieve-plates {
(not spongy). { Concentric rings around
{ the central chamber
{ interrupted by three
{ open spaces, 5. PYLODISCIDA.

Surface of the shell spongy, not covered
by peculiar porous sieve-plates, 6. SPONGODISCIDA.

Family XVIII. #CENODISCIDA#, n. fam. (Pl. 31, fig. 11; Pl. 48, figs. 1-3).

_Definition._--#Discoidea# with simple extracapsular phacoid shell (or lenticular latticed cortical shell), without medullary shell and without chambered equatorial girdles.

The new family #Cenodiscida# opens the long series #Discoidea#, as their most simple and primitive form. The circular lenticular central capsule is enclosed by a {410}simple latticed shell of the same form, only separated from it by a thinner or thicker jelly-veil. The lenticular or discoidal fenestrated shell is therefore an extracapsular or "cortical shell," without an enclosed medullary shell.

The few genera of the Cenodiscida differ only in the shape of the equatorial margin of the lenticular disk. In the first subfamily, Zonodiscida, the margin is either quite simple (_Cenodiscus_) or surrounded by a smooth, solid equatorial girdle (_Zonodiscus_). In the second subfamily, Trochodiscida, the margin is armed with solid radial spines, lying in the equatorial plane. According to the number and disposition of these marginal spines, we distinguish _Stylodiscus_ (with two spines, opposite in one equatorial axis), _Crucidiscus_ (with four spines, opposite in pairs in two equatorial axes, perpendicular one to another), _Theodiscus_ (with three marginal spines), and _Trochodiscus_ (with numerous, commonly twenty to thirty, irregularly disposed spines). The spines are constantly simple, not branched; sometimes more conical or cylindrical, at other times more angular or pyramidal.

The two convex faces of the lenticular shell are constantly of similar shape, commonly smooth, sometimes more or less thorny, or armed with bristle-shaped radial spines. The pores are commonly more or less regular, circular, and disposed in series, which are occasionally more radial, at other times more concentric. If the wall of the hollow lens be rather thick, the difference in the shape of the central and peripheral pores is often striking. The central pores perforating the thick wall perpendicularly are short cylindrical tubes; the marginal pores perforating it in an oblique direction are longer conical tubes. The bars between the central pores are often somewhat smaller.

_The Central Capsule_ of the Cenodiscida is in all cases a perfect, circular, biconvex lens, the equatorial diameter of which is commonly between two-thirds and three-fourths of the enclosing lattice-shell. The interval between the two is filled up by the jelly-veil, or the hyaline "calymma," which is perforated by the numerous pseudopodia that pass through the shell-pores.

As the Cenodiscida possess the most simple shell-form of all #Discoidea#, we may regard _Cenodiscus_ as the common ancestral form of this large section, in the same manner as _Cenosphæra_ is the ancestral form of #Sphæroidea#, _Cenellipsis_ of the Prunoidea, Cenolarcus of the Larcoidea. But it is also possible that a part of Cenodiscida (or all?) arises from the Phacodiscida by reduction and loss of the medullary shell. For in some cases we find arising from the inside of the shell centripetal radial beams, which end at a certain equal distance from the hollow centre (Pl. 31, fig. 11; Pl. 38, fig. 2). Cenodiscus itself can be derived either from _Cenosphæra_ by compression of the spheroidal shell in one axis, or from _Sethodiscus_ by loss of the intracapsular medullary shell, or from _Actidiscus_ (the lenticular Actissa) by formation of a cortical shell around the lenticular central capsule.

{411}_Synopsis of the Genera of the Cenodiscida._

I. Subfamily {Margin quite simple,
Zonodiscida. { without equatorial girdle, 175. _Cenodiscus_.
Margin of the disk {
without radial {Margin surrounded by a solid
spines. { siliceous equatorial girdle, 176. _Zonodiscus_.

II. Subfamily {Two spines opposite in one
Trochodiscida. { equatorial axis, 177. _Stylodiscus_.
Margin of the disk {
armed with radial {Three spines on the margin of
spines (lying in { the disk, 178. _Theodiscus_.
the equatorial {
plane). {Four spines opposite in pairs
{ in two perpendicularly
{ crossed equatorial axes, 179. _Crucidiscus_.
{
{Ten to twenty or more radial
{ spines (variable in number
{ and commonly irregular
{ in disposition), 180. _Trochodiscus_.

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, 1_a_).

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.

{412}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.

{413}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).

_Sethostylus endostylus_, Haeckel, 1881, Prodromus et Atlas (pl. xxxi,
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.

{414}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.

{415}_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).

{416}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 {417}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.

{418}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.

{419}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 {420}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, 4_a_, 7, 8_a_; 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 {421}(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._

-------------------------------------------------------------------------
I. Subfamily Sethodiscida.
Margin of the disk without radial spines.
-------------------------------------------------------------------------
Margin simple, without {Medullary shell simple, 181. _Sethodiscus_.
equatorial girdle. {
{Medullary shell double, 182. _Phacodiscus_.

Margin surrounded by a {Medullary shell simple, 183. _Periphæna_.
hyaline equatorial girdle. {
{Medullary shell double, 184. _Perizona_.
-------------------------------------------------------------------------
II. Subfamily Heliosestrida.
Margin of the disk with two to eight solid radial spines, usually
quite regularly disposed. (Number usually constant.)
-------------------------------------------------------------------------
Two radial spines {Medullary shell simple, 185. _Sethostylus_.
(opposite in one axis). {
{Medullary shell double, 186. _Phacostylus_.

Three radial spines. Medullary shell simple, 187. _Triactiscus_.

Four radial spines (in {Medullary shell simple, 188. _Sethostaurus_.
cross form). {
{Medullary shell double, 189. _Phacostaurus_.

Six radial spines. Medullary shell simple, 190. _Distriactis_.

Eight radial spines. {Medullary shell simple, 191. _Heliosestrum_.
{
{Medullary shell double, 192. _Astrosestrum_.
-------------------------------------------------------------------------
III. Subfamily Heliodiscida.
Margin of the disk with numerous (ten to twenty or more) radial
spines, usually irregularly disposed. (Number variable.)
-------------------------------------------------------------------------
Radial spines all simple, {Medullary shell simple, 193. _Heliodiscus_.
not branched. {
{Medullary shell double, 194. _Astrophacus_.

Radial spines all or }Medullary shell simple, 195. _Heliodrymus_.
partly branched. }

{422}Subfamily 1. SETHODISCIDA, Haeckel, 1881, Prodromus, p. 457.

_Definition._--#Phacodiscida# without radial spines on the margin of the disk.

Genus 181. _Sethodiscus_,[220] Haeckel, 1881, Prodromus, p. 457.

_Definition._--#Phacodiscida# with simple medullary shell and simple margin of the circular disk, without surrounding equatorial girdle and without radial spines.

The genus _Sethodiscus_ is the most simple and primitive form of all Phacodiscida, and may be regarded as the common ancestral form of this family. The simple spherical medullary shell is connected by a variable number of radial beams with the lenticular or discoidal cortical shell (or "phacoid shell"). The margin of this latter is quite simple, circular, without solid equatorial girdle or radial spines. From the nearly allied genus _Carposphæra_ of the #Sphæroidea#, its probable ancestral form, _Sethodiscus_ can be derived simply by lenticular compression of the spheroidal cortical shell.

Subgenus 1. _Sethodiscinus_, Haeckel.

_Definition._--Surface of the disk smooth, without radial ribs or spines.

1. _Sethodiscus phacoides_, n. sp.

Disk with smooth surface, three times as broad as the medullary shell. Pores regular, circular; fourteen to fifteen on the radius of the disk. (Very similar to _Periphæna cincta_, Pl. 33, fig. 4, but without the girdle of the margin.)

_Dimensions._--Diameter of the disk 0.2, of the medullary shell 0.07, of the pores 0.005.

_Habitat._--Pacific, central area, Stations 270 to 274, in various depths.

2. _Sethodiscus macroporus_, n. sp.

Disk with smooth surface, twice as broad as the medullary shell. Pores regular, circular, very large; five to six on the radius of the disk. (Remarkable for the extraordinary size of the pores, which reaches half the radius of the medullary shell.)

_Dimensions._--Diameter of the disk 0.1, of the medullary shell 0.05, of the pores 0.012.

_Habitat._--North Atlantic, Gulf Stream, Færöe Channel, John Murray.

3. _Sethodiscus microporus_, n. sp.

Disk with smooth surface, four times as broad as the medullary shell. Pores regular, circular, very small; twenty-two to twenty-four on the radius of the disk. (The small pores are scarcely half as broad as the thick bars between them.)

{423}_Dimensions._--Diameter of the disk 0.25, of the medullary shell 0.06, of the pores 0.002.

_Habitat._--South Pacific, Station 288, surface.

4. _Sethodiscus lenticula_, n. sp. (Pl. 33, figs. 1, 2).

Disk with smooth surface, four times as broad as the medullary shell. Pores irregular, polygonal; eight to nine on the radius of the disk. (The pores of the medullary shell, fig. 2, are also irregular, polygonal, or roundish.)

_Dimensions._--Diameter of the disk 0.17, of the medullary shell 0.04, of the pores 0.01.

_Habitat._--Pacific, central area, Station 274, depth 2750 fathoms.

5. _Sethodiscus macrococcus_, n. sp. (Pl. 33, fig. 3).

Disk with smooth surface, two and a half times as broad as the medullary shell. Pores irregular, polygonal; eleven to twelve on the radius of the disk. (The pores of the medullary shell, fig. 3, are regular, circular, with elevated hexagonal frames between them. The figured specimen is a young one, both halves of the biconvex disk being not yet united in the equatorial plane.)

_Dimensions._--Diameter of the disk 0.16, of the medullary shell 0.065, of the pores 0.01.

_Habitat._--North Pacific, Station 244, depth 2900 fathoms.

6. _Sethodiscus micrococcus_, n. sp.

Disk with smooth surface, five times as broad as the medullary shell. Pores irregular, roundish, very small; twenty-six to twenty-eight on the radius of the disk. (The pores of the small medullary shell are regularly circular, of the same size as those of the disk.)

_Dimensions._--Diameter of the disk 0.2, of the medullary shell 0.04, of the pores 0.002.

_Habitat._--South Atlantic, Station 335, depth 1425 fathoms.

Subgenus 2. _Sethodisculus_, Haeckel.

_Definition._--Surface of the disk with elevated radial ribs or spines.

7. _Sethodiscus radiatus_, Haeckel.

_Haliomma radians_, Ehrenberg, 1854, Mikrogeol., Taf. xix. fig. 50.

_Haliomma radiatum_, Ehrenberg, 1854, Mikrogeol., Taf. xxi. fig. 54.

Disk with radiated surface, four times as broad as the medullary shell. Pores regularly circular; nine to ten on the radius of the disk, arranged in radial series (about forty), which are separated by smooth crests, not prominent on the smooth margin.

_Dimensions._--Diameter of the disk 0.12, of the medullary shell 0.03, of the pores 0.003.

_Habitat._--Fossil in Tertiary deposits of the Mediterranean, Greece, Sicily, Oran, &c.

{424}8. _Sethodiscus echinatus_, Haeckel.

_Haliomma echinatum_, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss.
Berlin, p. 74, Taf. xxvii. fig. 2.

Disk with radiated thorny surface, three times as broad as the medullary shell. Pores regular, circular; eleven to twelve on the radius of the disk, arranged in radial series (about sixty), which are separated by thorny crests, prominent a little on the dentated margin.

_Dimensions._--Diameter of the disk 0.18, of the medullary shell 0.06, of the pores 0.01.

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

9. _Sethodiscus cristatus_, n. sp.

Disk with radiated surface, four times as broad as the medullary shell. Pores regularly circular; sixteen to seventeen on the radius of the disk, arranged in radial series (about eighty), which are separated in the distal half by smooth elevated crests, strongly prominent on the dentated margin. (Very similar to _Phacodiscus cristatus_, Pl. 35, fig. 6, but with simple medullary shell.)

_Dimensions._--Diameter of the disk 0.2, of the medullary shell 0.05, of the pores 0.005.

_Habitat._--South Pacific, Station 285, depth 2375 fathoms.

Genus 182. _Phacodiscus_,[221] Haeckel, 1881, Prodromus, p. 457.

_Definition._--#Phacodiscida# with double medullary shell and simple margin of the circular disk, without surrounding equatorial girdle and without radial spines.

The genus _Phacodiscus_ differs from the ancestral genus _Sethodiscus_ only in the duplication of the medullary shell, and has therefore the same relation to it that _Thecosphæra_ in the #Sphæroidea# exhibits to _Carposphæra_. Both former discoidal genera differ from the two latter spheroidal by the lenticular flattening of the cortical shell.

Subgenus 1. _Phacodiscinus_, Haeckel.

_Definition._--Surface of the disk smooth, without radial ribs or spines.

1. _Phacodiscus rotula_, n. sp. (Pl. 35, fig. 7).

Disk with smooth surface, four and a half times as broad as the outer and fourteen times as broad as the inner medullary shell. Pores regularly circular; sixteen to eighteen on the radius of the disk. Margin of the lens very thick, truncated, nearly as broad as the outer medullary shell.

_Dimensions._--Diameter of the disk 0.21, of the outer medullary shell 0.045, of the inner 0.015; pores 0.008.

_Habitat._--North Pacific, Station 224, depth 1850 fathoms.

{425}2. _Phacodiscus lentiformis_, n. sp. (Pl. 35, fig. 8).

Disk with smooth surface, three and a half times as broad as the outer, and ten times as broad as the inner medullary shell. Pores regularly circular; twelve to thirteen on the radius of the disk. Margin of the disk sharp, as in a biconvex lens.

_Dimensions._--Diameter of the disk 0.25, of the outer medullary shell 0.07, of the inner 0.025; pores 0.01.

_Habitat._--Central Pacific, Station 265, depth 2900 fathoms; fossil in Barbados.

3. _Phacodiscus grandis_, n. sp.

Disk with thorny surface, five times as broad as the outer, and fifteen times as broad as the inner medullary shell. Pores regularly circular; twenty-two to twenty-four on the radius of the disk. Margin of the disk thick, rounded.

_Dimensions._--Diameter of the disk 0.45, of the outer medullary shell 0.09, of the inner 0.03; pores 0.01.

_Habitat._--Western Tropical Pacific, Station 225, depth 4475 fathoms.

Subgenus 2. _Phacodisculus_, Haeckel.

_Definition._--Surface of the disk with elevated radial ribs or spines.

4. _Phacodiscus echiniscus_, n. sp.

Disk with spiny surface, four times as broad as the outer, and ten times as broad as the inner medullary shell. Pores irregularly roundish; fourteen to fifteen on the radius of the disk. Margin of the disk sharp, as in a biconvex lens.

_Dimensions._--Diameter of the disk 0.3, of the outer medullary shell 0.75, of the inner 0.03; pores 0.007.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms.

5. _Phacodiscus clypeus_, n. sp. (Pl. 35, figs. 6, 9).

Disk with radiated surface, four times as broad as the outer, and twelve times as broad as the inner medullary shell. Pores regularly circular; sixteen to eighteen on the radius of the disk; in the distal half disposed in sixty to seventy radial series, which are separated by prominent crests; the sharp margin therefore a little jagged.

_Dimensions._--Diameter of the disk 0.2, of the outer medullary shell 0.05, of the inner 0.017; pores 0.006.

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

{426}Genus 183. _Periphæna_,[222] Ehrenberg, 1873 Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 246.

_Definition._--#Phacodiscida# with simple medullary shell, without radial spines, but with a solid equatorial girdle around the margin of the lenticular disk.

The genus _Periphæna_, founded by Ehrenberg in 1873 for the fossil _Periphæna decora_ of Barbados, differs from its ancestral form _Sethodiscus_ in the development of a very thin siliceous solid girdle around the margin of the lenticular disk; this girdle lies in the equatorial plane of the shell, and reappears in similar form in _Perichlamydium_ among the Porodiscida, in _Spongophacus_ among the Spongodiscida, and in _Zonodiscus_ among the Cenodiscida.

1. _Periphæna cincta_, n. sp. (Pl. 33, fig. 4).

Disk with smooth surface, four times as broad as the medullary shell. Pores regularly circular; fourteen to sixteen on the radius of the disk. Girdle of the margin about half as broad as the radius of the medullary shell, in the distal half structureless, in the proximal half with seventy to eighty short radial ribs.

_Dimensions._--Diameter of the disk 0.2, of the medullary shell 0.05, of the pores 0.005.

_Habitat._--Central Pacific, Station 268, depth 2900 fathoms.

2. _Periphæna decora_, Ehrenberg.

_Periphæna decora_, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss.
Berlin, p. 80, Taf. xxviii. fig. 6.