All rings of the disk convoluted in a simple regular spiral, of nearly equal breadth. Pores regular, circular, two on the breadth of each ring. Very numerous (twenty to thirty or more) piercing radial beams, prolonged into bristle-shaped marginal spines, about as long as the diameter of the disk.

Dimensions.—Diameter of the disk (with six rings) 0.12; breadth of each ring 0.01; pores 0.004.

Habitat.—Mediterranean (Messina), Haeckel.

14. Stylodictya echinastrum, Ehrenberg.

All rings of the disk convoluted in a double spiral, of nearly equal breadth. Pores irregular, roundish, two to three on the breadth of each ring. Marginal spines numerous, twenty to thirty, of very different size, the largest conical, strong, about as long as the diameter of the disk, and on the base as broad as one ring.

Dimensions.—Diameter of the disk (with five rings) 0.12; breadth of each ring 0.01; pores 0.002 to 0.006.

Habitat.—Fossil in the rocks of Barbados.

15. Stylodictya clavata, Ehrenberg.

All rings of the disk convoluted in a half spiral, of nearly equal breadth; each ring by four zigzag beams (crossed in two perpendicular diameters) divided into four equal quarters; the spiral line of each quarter ring halving both neighbouring quarters. Pores regular, circular, two on the breadth of each ring. Eight marginal spines short, conical, with thin pedicle; four perradial (as prolongations of the four internal beams) alternating with four interradial spines arising from the margin of the disk.

Dimensions.—Diameter of the disk (with five rings) 0.13; breadth of each ring 0.014; pores 0.003.

Habitat.—Fossil in the rocks of Barbados.

Genus 222. Stylochlamydium,^[261] Haeckel, 1881, Prodromus, p. 460.

Definition.—Porodiscida with numerous (five or more, commonly eight to twelve) solid radial spines, regularly or irregularly disposed on the margin of the circular or polygonal disk; margin of the disk surrounded by a thin, porous (but not chambered), equatorial girdle.

The genus Stylochlamydium is intermediate between Perichlamydium (with which it was formerly united) and Stylodictya. It deals with the former in the peculiar equatorial girdle, with the latter in the radial spines of the disk margin, which pierce the girdle. To both these genera it shows slow transitions, and can hardly be subjected to a sharp definition.

Subgenus 1. Stylochlamys, Haeckel.

Definition.—All rings of the disk concentric, circular (or somewhat polygonal).

1. Stylochlamydium asteriscus, n. sp. (Pl. 41, fig. 10).

All rings of the disk concentric, circular, or polygonal, with increasing breadth from the centre; the fifth ring twice as broad as the second. Pores irregular, roundish, three to four on the breadth of each ring; in the rings two to six times as large as in the equatorial girdle, which is half as broad or two-thirds as broad as the radius of the disk. Twelve bristle-shaped radial spines are connected by the girdle near to the points; four crossed spines arising from the central chamber; two others between these in each quadrant arising from the first ring.

Dimensions.—Diameter of the disk (with five rings) 0.15, with the girdle 0.25; breadth of the second ring 0.01, of the fifth ring 0.02; pores 0.001 to 0.005.

Habitat.—Pacific, central area, Station 272, depth 2600 fathoms.

2. Stylochlamydium limbatum, Haeckel.

All rings of the disk concentric, circular, of equal breadth. Pores regular, circular, two on the breadth of each ring; twice to three times as large as the fine pores of the equatorial girdle, which is about half as broad as the radius of the disk. Twelve (or eleven) bristle-shaped radial spines are connected by the girdle near to the points, irregularly disposed.

Dimensions.—Diameter of the disk (with five rings) 0.12, with the girdle 0.2, breadth of each ring 0.01; pores 0.001 to 0.003.

Habitat.—Fossil in the Tertiary rocks of Sicily, Caltanisetta, Grotte.

3. Stylochlamydium venustum, Haeckel.

All rings of the disk concentric, circular, of equal breadth. Pores regular, circular, everywhere of nearly equal size, three on the breadth of each ring, a little smaller in the equatorial girdle, which is about as broad as the radius of the disk. Twenty to twenty-four bristle-shaped radial spines, irregularly disposed, proceed with their free points over the margin.

Dimensions.—Diameter of the disk (with four rings) 0.1, with the girdle 0.3; breadth of each ring 0.011; pores 0.002.

Habitat.—North Pacific, Kamtschatka, Bailey.

4. Stylochlamydium æquale, Haeckel.

All rings of the disk concentric, circular, with increasing breadth from the centre; the sixth ring twice as broad as the second. Pores regular, circular, everywhere of equal size; on the breadth of the inner rings one, of the outer two, of the girdle three pores. Girdle only one-fourth as broad as the radius of the disk. About twenty bristle-shaped radial spines, irregularly disposed, are connected by the girdle near to the points.

Dimensions.—Diameter of the disk (with six rings) 0.11, with the girdle 0.17; breadth of the second ring 0.006, of the sixth 0.013; pores 0.006.

Habitat.—Fossil in Tertiary rocks of Sicily, Grotte, Stöhr.

Subgenus 2. Stylochlamyum, Haeckel.

Definition.—Rings of the disk all (or in part) not concentric, spirally convoluted or irregular.

5. Stylochlamydium perispirale, Haeckel.

Inner rings of the disk concentric, circular, outer rings convoluted spirally, all rings of equal breadth. Pores regular, circular, two on the breadth of each ring, twice as large as in the equatorial girdle, which is about half as broad as the radius of the disk. Twelve to sixteen bristle-shaped radial spines, irregularly disposed, are connected by the girdle near to the points.

Dimensions.—Diameter of the disk (with six rings) 0.12, with the girdle 0.2; breadth of each ring 0.011; pores in the central disk 0.004, in the girdle 0.002.

Habitat.—Fossil in Tertiary rocks of Sicily, Grotte, Stöhr.

6. Stylochlamydium spongiosum, Haeckel.

Rings of the disk partly concentric, partly spiral, more or less irregular and often interrupted, with increasing breadth from the centre. Central part of the disk more or less spongy and obscure. Equatorial girdle half as broad as the radius of the chambered disk, with smaller pores than the latter, pierced by twenty to thirty thin, bristle-shaped radial beams, which proceed over the margin of the disk.

Dimensions.—Diameter of the disk (with ten rings) 0.2, with the girdle 0.3; breadth of the rings 0.005 to 0.015; pores 0.001 to 0.005.

Habitat.—Pacific, central area, Station 268, depth 2900 fathoms; also fossil in the Tertiary rocks of Barbados and Sicily.

Subfamily 5. Euchitonida, Haeckel.

Definition.—Porodiscida with two or more (commonly three or four) radial chambered or spongy arms on the margin of the concentrically annulated disk, situated in its equatorial plane (with or without a connecting patagium between the arms).

Genus 223. Amphibrachium,^[262] Haeckel, 1881, Prodromus, p. 460.

Definition.—Porodiscida with two simple, undivided, chambered arms, opposite in one axis, without a patagium.

The genus Amphibrachium opens the long series of the Euchitonida, or of those Porodiscida which bear on the margin of the circular central disk a certain number of chambered arms, composed of a series of chambers which are separated by transverse septa. The first group or tribe of this subfamily is formed by the Amphibrachida, in which the disk bears only two arms opposite on the poles of one axis. The simplest form of these is Amphibrachium, in which both arms are simple, equal, and without a patagium or spongy connecticulum.

Subgenus 1. Amphibrachella, Haeckel.

Definition.—Both arms equal, of the same form and size, blunt at the distal end, without a terminal spine.

1. Amphibrachium sponguroides, n. sp.

Both opposite arms of the same form and size, nearly cylindrical, three times as long as broad, with six to eight transverse septa or joints, at the distal end rounded, blunt, without a terminal spine.

Dimensions.—Radius of each arm 0.22, breadth 0.065.

Habitat.—Antarctic Ocean, Station 154, surface.

2. Amphibrachium lanceolatum, n. sp.

Both arms equal, lanceolate, in the middle part three times as broad as at the two ends, two and a half times as long as broad, with eight to nine transverse septa, at the distal end blunt, without a terminal spine.

Dimensions.—Radius of each arm 0.3, greatest breadth 0.09.

Habitat.—Pacific, central area, Station 267, depth 2700 fathoms.

3. Amphibrachium dilatatum, n. sp. (Pl. 44, fig. 6).

Both arms equal, trapezoidal, somewhat broader than long, on the convex distal end three times as broad as on the narrow base, with five to six transverse septa, without a terminal spine. Central disk large, somewhat irregular, with three to four rings, twice as broad as the base of the arms.

Dimensions.—Radius of each arm 0.32, basal breadth 0.15, terminal breadth 0.44.

Habitat.—South Atlantic, off Patagonia, Station 319, surface.

Subgenus 2. Amphibrachidium, Haeckel.

Definition.—Both arms equal, of the same size and form, with terminal spines.

4. Amphibrachium amphilonche, n. sp.

Both arms equal, lanceolate, in the middle part four times as broad as at the two ends, twice as long as broad; on the distal end of each arm is a long conical spine.

Dimensions.—Radius of each arm (without terminal spine) 0.2, breadth 0.08.

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

5. Amphibrachium capitatum, n. sp.

Both arms equal, club-shaped, three times as long as broad, in the outer distal half thickened, three times as broad as at the narrow base; on the distal end of each arm a strong, angular, terminal spine. (The form of the arms like that of Stephanastrum capitatum, Pl. 44, fig. 1.)

Dimensions.—Radius of each arm 0.22, basal breadth 0.02, terminal breadth 0.06.

Habitat.—Equatorial Atlantic, Station 347, surface.

6. Amphibrachium armatum, n. sp.

Both arms equal, four times as long as broad, in the thickened distal part twice as broad as at the base, thorny, with twenty to thirty larger spines on the distal end, and a very large pyramidal spine in the longitudinal axis. (The form of the arms like that of the odd arm in Euchitonia carcinus, Pl. 43, fig. 10.)

Dimensions.—Radius of each arm 0.24, basal breadth 0.03, distal breadth 0.06.

Habitat.—North Pacific, Station 244, depth 2900 fathoms.

Subgenus 3. Amphibrachoma, Haeckel.

Definition.—Both arms of different size or form, without terminal spines.

7. Amphibrachium indicum, n. sp.

Both arms club-shaped, but very different in size and form; larger arm three times as long and twice as broad as the smaller arm; the larger with nine joints slowly increasing in size, the smaller with four joints, rapidly increasing; the terminal joint three times as broad as the basal. Distal end blunt, rounded, without spines.

Dimensions.—Radius of the larger arm 0.24, of the smaller 0.08; distal breadth of the former 0.06, of the latter 0.03; basal breadth 0.015.

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

Subgenus 4. Amphibrachura, Haeckel.

Definition.—Both arms of different size or form, with terminal spines.

8. Amphibrachium clavula, n. sp.

Both arms different in size and form; larger arm club-shaped, four times as long as broad, at the distal end three times as broad as at the base, and twice as long as the smaller arm, which resembles a stalked knob, with thin basal peduncle and spherical distal part. Ends of the two arms thorny (with numerous smaller, and three to five larger spines); one very large conical terminal spine on each pole of the main axis.

Dimensions.—Radius of the larger arm 0.3, of the smaller 0.15; distal breadth of the former 0.06, of the latter 0.04; basal breath 0.02.

Habitat.—South Atlantic, Station 333, surface.

Genus 224. Amphymenium,^[263] Haeckel, 1881, Prodromus, p. 460.

Definition.—Porodiscida with two simple, undivided, chambered arms, opposite in one axis, connected by a patagium.

The genus Amphymenium differs from the preceding Amphibrachium, its ancestral form, by development of a patagium or connecticulum between both arms. This forms a latticed or more spongy envelop, which surrounds either the middle part of the shell, or the whole shell with exception of the distal ends of both arms. If the envelop become very spongy, the shell may be confounded with the cylindrical Ellipside Spongocore (nearly allied to Spongurus); possibly also Ommatogramma of Ehrenberg belongs to this genus.

Subgenus 1. Ommatogramma, Ehrenberg (?).

Definition.—Both opposite arms of the same size and form, blunt, without terminal spines.

1. Amphymenium pupula, n. sp. (Pl. 44, fig. 8).

Both arms equal, twice as long as broad, three-jointed; the terminal joint egg-shaped, as large as both other joints together; distal end rounded, blunt. Patagium nearly complete, enveloping the arms with exception of the distal end. Perimeter nearly spindle-shaped.

Dimensions.—Radius of each arm 0.17, greatest breadth 0.06; transverse breadth of the patagium 0.13.

Habitat.—Pacific, central area, Station 273, depth 2350 fathoms.

2. Amphymenium naviculare, Haeckel.

Both arms equal, three times as long as broad, spongy, not jointed; distal end a little club-shaped, blunt. Patagium nearly complete, enveloping the arms with exception of the distal end. Perimeter nearly lanceolate. The imperfect diagnosis and figure of Ehrenberg make it doubtful whether this species belongs to the Porodiscida (Amphymenium) or to the Spongodiscida (Spongobrachium) or perhaps to the Spongurida (Spongocore).

Dimensions.—Radius of each arm 0.1, greatest breadth 0.03; transverse breadth of the patagium 0.05.

Habitat.—North Pacific, Californian Sea, depth 2600 fathoms, Ehrenberg.

3. Amphymenium zygartus, n. sp. (Pl. 44, fig. 7).

Both arms equal, four times as long as broad, with seven to eight joints, slowly decreasing in size towards the blunt end. Patagium incomplete, protecting only the middle part of the shell on both sides; on each side two parallel lattice-plates, connected by transverse radial beams, perpendicular to the surface. Perimeter nearly rectilinear. (Resembles much certain forms of Zygartus, Pl. 40, but is a true Discoid, no Prunoid.) Compare also Pl. 45, fig. 8.

Dimensions.—Radius of each arm 0.2, greatest breadth 0.05; transverse breadth of the patagium 0.11.

Habitat.—Pacific, central area, Station 271, surface.

4. Amphymenium monstrosum, n. sp. (Pl. 44, fig. 11).

Both arms equal, little longer than broad, with six to seven convex joints. The axis of both arms is not common and straight, as in all other species of this genus, but broken, therefore the incomplete patagium, which envelops only two to three joints of the arms, is on one side convex, on the other side concave; it is formed by a simple lattice-plate, connected with the arms by numerous radial beams. This anomalous form, seen only once, may perhaps be a monstrosity of Euchitonia.

Dimensions.—Radius of each arm 0.12, greatest breadth 0.06; transverse breadth of the patagium 0.13.

Habitat.—South Pacific, Station 290, surface.

Subgenus 2. Ommathymenium, Haeckel.

Definition.—Both opposite arms of the same size and form, armed at the distal end with terminal spines.

5. Amphymenium amphistylium, n. sp. (Pl. 44, fig. 9).

Both arms equal, three times as long as broad, thickened towards the truncated distal end, and armed with a strong pyramidal terminal spine. Each arm with seven joints, separated by convex, transverse septa, and halved by a radial beam lying in the longitudinal axis. Patagium incomplete, cylindrical, enveloping only the middle part of the shell.

Dimensions.—Radius of each arm 0.18, greatest breadth 0.06; transverse breadth of the patagium 0.08.

Habitat.—North Atlantic, Station 354, surface.

6. Amphymenium fusiforme, n. sp.

Both arms equal, lanceolate, three times as long as broad in the width, with seven to eight joints. Distal end pointed, armed with a strong conical terminal spine. Patagium complete, enveloping the whole shell with exception of the terminal spines. Whole form spindle-shaped.

Dimensions.—Radius of each arm 0.2, greatest breadth 0.07; transverse breadth of the patagium 0.15.

Habitat.—South Atlantic, Station 330, surface.

Genus 225. Amphirrhopalum,^[264] Haeckel, 1881, Prodromus, p. 460.

Definition.—Porodiscida with two chambered arms, opposite in one axis, without a patagium; one arm or both forked at the distal end.

The genus Amphirrhopalum differs from Amphibrachium, its ancestral form, by bifurcation of the distal ends of the arms, which may affect either both arms, or only one of them.

Subgenus 1. Amphirrhopalium, Haeckel.

Definition.—Both opposite arms of similar size and form, with blunt branches, without terminal spines.

1. Amphirrhopalum ximorphum, n. sp.

Both arms equal, in the proximal half simple, in the distal half forked, with six to seven transverse septa; distal end of each branch blunt, without terminal spine, somewhat broader than the base of the whole arm. Axis of the branches concavely curved. (Resembles Amphicraspedum maclaganium, Pl. 45, fig. 11, but wants the patagium.)

Dimensions.—Radius of the arms 0.18, basal breadth 0.065; terminal breadth of each branch 0.075.

Habitat.—North Pacific, off Japan, Station 240, surface.

2. Amphirrhopalum amphidicranum, n. sp.

Both arms equal, in the proximal half simple, in the distal half forked, with irregular septa; distal end of each arm blunt, without a terminal spine, smaller than the basal breadth of the arm. Axis of the branches straight. (Resembles Dicranastrum furcatum, Pl. 47, fig. 2, but without lateral arms.)

Dimensions.—Radius of each arm 0.15, basal breadth 0.03; terminal breadth of each branch 0.02.

Habitat.—Pacific, central area, Station 263, depth 2650 fathoms.

Subgenus 2. Amphirrhopella, Haeckel.

Definition.—Both opposite arms of similar size and form, with terminal spines.

3. Amphirrhopalum bigeminum, n. sp.

Both arms equal, in the proximal larger half simple, in the distal smaller half forked; each branch triangular, with a strong conical terminal spine. Axis of the branches straight. (Resembles Dicranastrum cornutum, Pl. 45, fig. 2, but without lateral arms.)

Dimensions.—Radius of the arms (without spines) 0.15, basal breadth 0.03; breadth of the bifurcation 0.08.

Habitat.—South Pacific, Station 284, surface.

4. Amphirrhopalum echinatum, n. sp. (Pl. 45, fig. 10).

Both arms equal, in the proximal smaller half simple, nearly square, in the distal larger half forked; the branches thorny, armed at the end with numerous spines, one larger on the terminal pole of the concavely curved arm-axis.

Dimensions.—Radius of the arms (without spines) 0.15, basal breadth 0.05; breadth of branches 0.03.

Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.

Subgenus 3. Amphirrhopoma, Haeckel.

Definition.—Both opposite arms of different size or form, without terminal spines.

5. Amphirrhopalum ypsilon, n. sp.

Both arms very different. Larger arm simple, egg-shaped, twice as long as broad; smaller arm in the basal half simple, nearly square, in the distal half forked; both branches egg-shaped, blunt. (Resembles Amphicraspedum wyvilleanum, Pl. 45, fig. 12, but wants the patagium.)

Dimensions.—Radius of the larger simple arm 0.18, breadth 0.09; radius of the smaller forked arm 0.15, breadth of its branches 0.05.

Habitat.—Indian Ocean, Madagascar, Rabbe, surface.

Genus 226. Amphicraspedum,^[265] Haeckel, 1881, Prodromus, p. 460.

Definition.—Porodiscida with two chambered arms, opposite in one axis, connected by a lateral patagium; one arm or both forked at the distal end.

The genus Amphicraspedum exhibits the same bifurcation of the arms as does Amphirrhopalum, but differs from this ancestral form in the development of a patagium, an external connecticulum between the arms, which envelops the disk totally or partially.

Subgenus 1. Amphicraspedon, Haeckel.

Definition.—Both arms of equal size and form, without terminal spines of the branches.

1. Amphicraspedum maclaganium, n. sp. (Pl. 45, fig. 11).

Both arms equal, in the proximal half simple, in the distal half forked, with six to seven transverse septa; distal end of each branch rounded, blunt, somewhat broader than the base of the whole arm. Divergent axes of both branches concavely curved. Patagium incomplete, with elliptical perimeter, enveloping only the middle part of the shell. I call this interesting species in honour of Miss Nellie Maclagan, the learned translator of several zoological papers from German into English.

Dimensions.—Radius of each arm 0.25, basal breadth 0.07; distal breadth of each branch 0.08; equatorial breadth of the patagium 0.25.

Habitat.—North Atlantic, off Halifax, Station 50, surface.

Subgenus 2. Amphicraspedina, Haeckel.

Definition.—Both arms of different size or form, without terminal spines on the branches.

2. Amphicraspedum wyvilleanum, n. sp. (Pl. 45, fig. 12).

Both arms different. Larger arm simple, egg-shaped, with eleven convex joints, one and a half times as long as broad; smaller arm in the basal half simple, triangular, with six cap-like joints, in the distal half forked; both branches egg-shaped, with five joints and blunt ends. Patagium nearly complete with four to five concave chamber-rows. Called in honour of Sir C. Wyville Thomson.

Dimensions.—Radius of the larger simple arm 0.18, breadth 0.08; radius of the smaller forked arm 0.16; breadth of the branches 0.05; transverse breadth of the patagium 0.2.

Habitat.—South Atlantic, Station 333, surface.

Subgenus 3. Amphicraspedula, Haeckel.

Definition.—Both arms of different size or form, with terminal spines of the branches.

3. Amphicraspedum murrayanum, n. sp. (Pl. 44, fig. 10).

Both arms different in size; the larger one and a half times as long and broad as the smaller. Both arms triangular, forked at the broader distal end, with two very strong, conical, divergent, straight terminal spines. Patagium broad, incomplete, with circular perimeter. Called in honour of my friend Dr. John Murray.

Dimensions.—Radius of the larger arm (including the spines) 0.24, of the smaller 0.16; distance of the terminal points of the former 0.18, of the latter 0.09; diameter of the patagium 0.2.

Habitat.—North Atlantic, Færöe Channel, Gulf Stream, surface, John Murray.

Genus 227. Dictyastrum,^[266] Ehrenberg, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 830.

Definition.—Porodiscida with three simple, undivided, chambered arms, without a patagium; triangular shell regular, with three equal arms and three equal angles.

The genus Dictyastrum is the simplest form of the Trigonastrida, or of the Porodiscida, in which the margin of the central disk is furnished with three chambered arms. In Dictyastrum these are quite simple and regular, without a patagium, separated by equal angles, so that the whole shell represents a regular, equilateral triangle, if we connect the distal points of the arms by lines. The genus Dictyastrum, founded by Ehrenberg in 1860, differs from his Rhopalodictyum—after his own diagnosis—only by an insignificant difference in the form of the simple arms, which is scarcely a specific character. I therefore apply this name here in the above amended sense, seeing that the only figured species of Ehrenberg (Dictyastrum angulatum) occurs in two different, but externally very similar forms: one of these is a true Porodiscid (Dictyastrum) with two porous covering-plates and concentric rings; the other is a true Spongodiscid (Rhopalodictyum) with quite spongy, irregular network, and is probably identical with the Rhopalodictyum truncatum of Ehrenberg.

Subgenus 1. Dictyastrella, Haeckel.

Definition.—Arms with blunt ends, without terminal spines.

1. Dictyastrum angulatum, Ehrenberg.

Arms nearly square with straight edges, towards the truncated end a little broader, about the same diameter as the triangular central disk. The figure of Ehrenberg seems to represent a Spongodiscid (Rhopalodictyum angulatum), but in the same locality (Philippine Sea) occurs also a true Dictyastrum of quite the same form, but with three to four concentric rings of the central disk, and with jointed arms.

Dimensions.—Radius of each arm (length from the centre to the distal end) 0.13; breadth of the truncated end 0.1.

Habitat.—Tropical Pacific, Philippine Sea, Station 200, depth 250 fathoms.

2. Dictyastrum bandaicum, Haeckel.

Arms nearly square, with convex edges, in the middle a little broader than at both ends, about half the diameter of the central disk. Differs from the nearly allied preceding species by the half size of the arms and the convex edges.

Dimensions.—Radius of each arm 0.12, its greatest breadth 0.07.

Habitat.—Tropical Pacific, Banda Sea, Harting.

3. Dictyastrum hexagonum, n. sp. (Pl. 43, fig. 7).

Arms nearly triangular, one and a third times as broad at the distal end as long, and three times as broad as at the base. Central disk about the same diameter. In each arm six simple broad chambers. If we connect the six corners of the truncated distal ends by straight lines, we get a regular hexagon.

Dimensions.—Radius of each arm 0.17, basal breadth 0.06, terminal breadth 0.17.

Habitat.—Equatorial Atlantic, Station 347, surface.

4. Dictyastrum trirrhopalum, n. sp.

Arms club-shaped, five times as long as broad at the base, at the thickened end three times as broad as at the base. Diameter of the central disk equals half the length of the arms. (Similar to Rhopalastrum malleus, Pl. 43, fig. 1, but with three equal angles and much smaller disk.)

Dimensions.—Radius of each arm 0.25, basal breadth 0.04, distal breadth 0.12.

Habitat.—Pacific, central area, Station 273, surface.

Subgenus 2. Dictyastromma, Haeckel.

Definition.—Arms on the distal end provided with terminal spines.

5. Dictyastrum trispinosum, n. sp. (Pl. 43, fig. 5).

Arms trapezoid, at the rounded distal end twice as broad as at the base, with a strong and short, conical, terminal spine. Diameter of the circular central disk about equal to the length and the greatest breadth of the arms.

Dimensions.—Radius of each arm 0.15, basal breadth 0.06, distal breadth 0.11.

Habitat.—Equatorial Atlantic, Station 347, surface.

6. Dictyastrum triactis, Ehrenberg.

Arms rectilinear, four times as long as broad, with parallel edges, pointed at the distal end, with a short terminal spine. Diameter of the circular central disk equal to double the breadth of the arms.

Dimensions.—Radius of each arm 0.2, breadth 0.04.

Habitat.—Pacific, Philippine Sea, Station 206, depth 2100 fathoms.

7. Dictyastrum aculeatum, n. sp.

Arms lanceolate, three times as long as broad, twice as broad in the middle as at either end, with thorny surface and numerous conical terminal spines, one very large in the radius. Central disk triangular, about as broad as the arms. (Resembles Rhopalastrum arcticum, Pl. 43, fig. 6, but differs by the equal angles and the triangular disk.)

Dimensions.—Radius of each arm 0.2, breadth 0.06.

Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.

Genus 228. Rhopalastrum,^[267] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.

Definition.—Porodiscida with three simple undivided, chambered arms, without a patagium; triangular shell bilateral, one odd arm opposite to the odd angle between two paired arms.

The genus Rhopalastrum, founded by Ehrenberg (1847) with a very insufficient diagnosis, is here retained for those Trigonastrida that agree in the generic characters with the only species figured by him, viz., Rhopalastrum lagenosum (compare my Monograph, 1862, p. 500). It comprises, therefore, such Euchitonida as agree with the preceding Dictyastrum in the simple form of the three arms and the absence of a patagium, but differ from it in the different size of the three angles, and often also in the divergent form and size of the three arms; one odd arm is opposite to the odd angle between the two paired arms.

Subgenus 1. Rhopalastrella, Haeckel.

Definition.—Arms with blunt ends, without terminal spines.

1. Rhopalastrum truncatum, Haeckel.

Distance of both paired arms about half as large as their distance from the odd arm. All three arms nearly of the same form and size, very short and broad; their breadth nearly equals that of the roundish central disk, whilst their length reaches only one-fourth of it. End of the arms convex rounded, without spines.

Dimensions.—Radius of each arm 0.1, breadth 0.15.

Habitat.—Mediterranean (Messina), Atlantic (Canary Islands), surface, Haeckel.

2. Rhopalastrum pistillum, Stöhr.

Distance between the paired arms about two-thirds as large as their distance from the odd arm. All three arms nearly of the same form and size, about three times as long as the diameter of the central disk, at the base one-third as broad as at the convex rounded end, without spines. Stöhr has only observed a fragment with one arm; some perfect specimens, which I found in the Caltanisetta-rock, exhibited nearly the same form as Rhopalastrum malleus (Pl. 43, fig. 1), but differ from this by the smaller disk, the broader arms, and the smaller angle between the paired arms.

Dimensions.—Radius of all three arms 0.2; basal breadth of each arm 0.045, terminal breadth 0.13.

Habitat.—Fossil in Tertiary rocks of Sicily, Grotte (Stöhr), Caltanisetta (Haeckel).

3. Rhopalastrum malleus, n. sp. (Pl. 43, fig. 1).

Distance between the paired arms one and a third times as large as their distance from the odd arm. All three arms nearly of the same form and size, hammer-shaped, three times as broad at the truncated distal end as at the base. Central disk broader than the arms.

Dimensions.—Radius of each arm 0.25, basal breadth 0.05, distal breadth 0.15.

Habitat.—South Atlantic, Station 325, surface.

4. Rhopalastrum lagenosum, Ehrenberg.