Distance between the paired arms one and a half times as great as their distance from the odd arm. All three arms nearly of the same form and size, about one and a half times as long as the diameter of the central disk, at the base half as broad as at the convex rounded end, without spines. Ehrenberg has only observed a fragment with two paired arms; some perfect specimens with three arms, observed by me, differed from the nearly allied Rhopalastrum pistillum (from the same locality) by the larger disk, the broader arms, and the larger unpaired angle.

Dimensions.—Radius of all three arms 0.2, basal breadth 0.05, terminal breadth 0.09.

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

5. Rhopalastrum ypsilinum, n. sp. (Pl. 43, fig. 2).

Distance between the paired arms half as large as their distance from the odd arm. All three arms at the concavely curved distal end three times as broad as at the narrow base. Odd arm twice as long and broad as the paired arms. Central disk smaller than the latter. No terminal spines.

Dimensions.—Radius of the odd arm 0.32, of the paired arms 0.2; terminal breadth of the former 0.2, of the latter 0.12.

Habitat.—Indian Ocean, Cocos Islands, Rabbe, surface.

6. Rhopalastrum clavatum, n. sp.

Distance between the paired arms half as large as their distance from the odd arm. All three arms club-shaped, at the thickened, nearly spherical, distal end three times as broad as at the narrow base. Odd arm nearly twice as long and broad as the paired arms. Central disk equal to the distal knob of the latter. No terminal spines.

Dimensions.—Radius of the odd arm 0.3, of the paired arms 0.18; distal breadth of the former 0.16, of the latter 0.08.

Habitat.—South Atlantic, Station 332, surface.

7. Rhopalastrum irregulare, n. sp. (Pl. 43, fig. 8).

Distance between all three arm-points different. All three arms cylindrical, nearly of the same length, but of different form, irregularly curved, about four times as long as broad, with blunt ends.

Dimensions.—Length of each arm about 0.2, breadth 0.05.

Habitat.—Antarctic Sea, Station 157, depth 1950 fathoms.

Subgenus 2. Rhopalastromma, Haeckel.

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

8. Rhopalastrum martellum, n. sp.

Distance between all three arm-points nearly the same; but the odd arm is one and a half times as large as both paired arms, and is perpendicular to the common axis of the latter, therefore the shell has the form of a hammer. Each arm is twice as broad at the distal end as at the base, and armed with a conical terminal spine; the latter is vertical in the odd arm, horizontal in the paired arms.

Dimensions.—Radius of the odd arm 0.3, of the paired arms 0.15; distal breadth of the former 0.12, of the latter 0.06.

Habitat.—North Atlantic, Station 354, surface.

9. Rhopalastrum triceros, n. sp. (Pl. 43, fig. 4).

Distance between the paired arms about two-thirds as large as their distance from the odd arm. All three arms club-shaped, three times as broad at the thickened distal part as at the base, and armed with one single, conical, terminal spine. Odd arm of the same breadth, but twice as long as the paired arms.

Dimensions.—Radius of the odd arm 0.35, of the paired arms 0.2; basal breadth 0.04, distal breadth 0.12.

Habitat.—Pacific, central area, Station 274, surface.

10. Rhopalastrum hexaceros, n. sp. (Pl. 43, fig. 3).

Distance between the paired arms equals four-fifths of their distance from the odd arm. All three arms nearly of the same size, about square, a little broader at the truncated distal end, which is armed at both corners with a strong, conical, radial spine.

Dimensions.—Radius of each arm 0.2, basal breadth 0.1, distal breadth 0.12.

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

11. Rhopalastrum arcticum, n. sp. (Pl. 43, fig. 6).

Distance between the paired arms half as large as their distance from the odd arm, which is a little larger. All three arms of the same form, lanceolate, twice to three times as long as broad, twice as broad in the middle as at either end. Each arm with twelve to fourteen transverse septa, at the distal end with a bunch of conical spines, and one single, very large, pyramidal, terminal spine.

Dimensions.—Radius of each arm (without spine) 0.17, greatest breadth of it 0.05 to 0.06.

Habitat.—Arctic Ocean, lat. 83° 19' N., North Polar expedition of the "Alert."

Genus 229. Hymeniastrum,^[268] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.

Definition.—Porodiscida with three simple, undivided, chambered arms, connected by a patagium; triangular shell regular, with three equal arms and three equal angles.

The genus Hymeniastrum was founded by Ehrenberg (1847) with a very incomplete diagnosis, and hitherto known only by one single species, figured by him as Hymeniastrum pythagoræ (Mikrogeol., 1854, Taf. xxxvi. fig. 31). This form occurs in two different states, externally quite identical; in one state the central disk (as figured, loc. cit.), is a simple lens or hollow disk, containing a medullary shell or "central chamber"; in the other state the central disk is composed of two concentric rings surrounding the "central chamber." We retain here the name Hymeniastrum for this latter state, expressed in the diagnosis given above, and call the former state (the Coccodiscid) Hymenactura (compare above, p. 473). One practical advantage, obtained in this way, is that all genera of Discoidea ending with "-astrum" belong to the Porodiscida. Hymeniastrum differs from Dictyastrum by the possession of a patagium, and from Euchitonia by the equal size of the angles and the arms.

Subgenus 1. Hymenastrella, Haeckel.

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

1. Hymeniastrum leydigii, Haeckel.

Arms twice as long as broad, two-thirds as broad at the base as at the blunt, nearly truncated distal end. Patagium nearly complete, perfectly filling out the interbrachial spaces, with six to seven regular, concave chamber-rows; only the truncate terminal faces of the arms free.

Dimensions.—Radius of the arms 0.15 to 0.18, breadth on their base 0.04 to 0.05, on their broadest distal end 0.06 to 0.07.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface. Very common and variable.

2. Hymeniastrum köllikeri, Haeckel.

Arms nearly lanceolate, twice as long as broad, half as broad at the base as at the broadest middle part, pointed at the distal end. Patagium complete, enveloping the whole triangular disk, with irregular chamber-rows.

Dimensions.—Radius of the arms 0.2, breadth on their base 0.04, on the broadest part 0.08.

Habitat.—Mediterranean (Messina), surface.

3. Hymeniastrum gümbelii, Haeckel.

Arms twice as long as broad, nearly twice as broad in the circular distal half as in the square proximal half. Patagium incomplete, circular, with eight to nine convex chamber-rows, enveloping only the proximal square halves of the arms; the lenticular distal halves remain free.

Dimensions.—Radius of the arms 0.14; breadth at the base 0.04, at the distal lenticular part 0.07.

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

4. Hymeniastrum euclidis, n. sp. (Pl. 43, fig. 13).

Arms one and a half times as long as broad, club-shaped, three times as broad in the oval distal part as in the narrow square proximal part. Patagium complete, enveloping the whole triangular disk, and also the convex ends of the arms, with eight to nine convex chamber-rows.

Dimensions.—Radius of the arms 0.18; breadth at the base 0.025, at the broadest distal part 0.07.

Habitat.—Pacific, central area, Stations 270 to 274, surface.

5. Hymeniastrum pythagoræ, Ehrenberg.

Arms nearly as broad as long, two-thirds as broad at the base as at the truncated distal end. Patagium incomplete, enveloping only the basal half of the arms, with four to five rectilinear parallel chamber-rows. (This form has the greatest resemblance to the figure given by Ehrenberg, loc. cit., but differs by the central disk, which is composed of two concentric rings surrounding the small central chamber; compare above Hymenactura pythagoræ, p. 474.)

Dimensions.—Radius of the arms 0.18; breadth at the base 0.08, at the truncated end 0.12.

Habitat.—Equatorial Atlantic, Station 347, depth 2250 fathoms.

6. Hymeniastrum archimedis, n. sp.

Arms nearly triangular, at the truncated, slightly convex end three times as broad as at the narrow base and one and a third times as broad as long; each arm with six simple chambers. Patagium incomplete, enveloping only the basal half of the arms with three to four convex chamber-rows. (Differs from Rhopalastrum hexagonum, Pl. 43, fig. 7, only by the patagium.)

Dimensions.—Radius of the arms 0.2; breadth at the base 0.07, at the truncated end 0.2.

Habitat.—Pacific, central area, Station 274, surface.

Subgenus 2. Hymenastromma, Haeckel.

Definition.—Arms at the distal end provided with radial spines.

7. Hymeniastrum ternarium, Haeckel.

Arms two and a half times as long as broad, three times as broad at the truncated end as at the narrow base, with a strong, conical, radial spine at the end. Patagium incomplete, enveloping only the basal half of the arms, with three to four convex chamber-rows.

Dimensions.—Radius of the arms (without terminal spine) 0.02; breadth at the base 0.025, at the broadest terminal part 0.08.

Habitat.—Fossil in the rocks of Barbados.

8. Hymeniastrum trigonarium, n. sp.

Arms one and a half times as long as broad, a little broader at the rounded end than at the base, with three strong conical radial spines, one larger (in the radius of each arm) and two smaller on both sides of this. Patagium complete, with four to five rectilinear parallel chamber-rows, enveloping the whole arms (with exception of the terminal spines) and forming a perfect equilateral triangle.

Dimensions.—Radius of the arms (without terminal spines) 0.18; breadth at the base 0.1, at the distal part 0.12.

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

Genus 230. Euchitonia,^[269] Ehrenberg, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 831 (sensu emendato).

Definition.—Porodiscida with three simple, undivided, chambered arms, connected by a patagium; triangular shell bilateral, one odd arm opposite to the odd angle between the two paired arms.

The genus Euchitonia, quite insufficiently characterised by Ehrenberg, was founded by him (1860) for one single species, afterwards (1872) described and figured as Euchitonia furcata. Retaining this species correctly as the type of this genus, I give to it here the above diagnosis. In my Monograph (1862, p. 503) I described seven Mediterranean species of Euchitonia. Three of these have in common the characters according to the present diagnosis: Euchitonia mülleri, Euchitonia virchowii, Euchitonia beckmanni; two others appertain (on account of the regular, not bilateral form) to Hymeniastrum, and two others (on account of the forked, not simple arms) to Trigonastrum. Afterwards (1880) three true fossil species of Euchitonia were described by Stöhr (Euchitonia cruciata, Euchitonia zittelii, and Euchitonia acuta). Some species of this genus are cosmopolitan, and appertain to the most common and everywhere represented Discoidea.

Subgenus 1. Stylactis, Ehrenberg, 1872 (loc. cit.).

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

1. Euchitonia furcata, Ehrenberg.

Distance between the paired arms about half as large as their distance from the odd arm. All three arms nearly of the same size and form, about twice as long as broad, one and a half times at the blunt convex end as broad as at the base. Patagium incomplete, with concave chamber-rows and irregular network, only enveloping the basal half of the arms.

Dimensions.—Radius of all three arms 0.15, basal breadth of each arm 0.04, terminal breadth 0.06.

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

2. Euchitonia mülleri, Haeckel.

Distance between the paired arms about two-thirds as large as their distance from the odd arm, which is somewhat larger. Length of the arms equals two and a half times the breadth of the blunt convex end or five times the breadth of the base. Patagium with concave chamber-rows, nearly complete, enveloping the arms with exception of the terminal face. (This common species is very variable; compare my Monograph.)

Dimensions.—Radius of all three arms 0.16 to 0.22, basal breadth 0.03 to 0.05, terminal breadth 0.06 to 0.08.

Habitat.—Cosmopolitan; one of the most common Discoidea in all seas, on the surface as well as at different depths; also fossil in the Tertiary rocks of Barbados and Sicily.

3. Euchitonia triangulum, Haeckel.

Distance between the paired arms about two-thirds as large as their distance from the odd arm. All three arms nearly of the same size and form, about one and a half times as long as broad, nearly as broad at the base as at the blunt rounded end. Patagium incomplete, with convex chamber-rows, enveloping about two-thirds of the arms.

Dimensions.—Radius of all three arms 0.15, basal breadth 0.05, terminal breadth 0.06.

Habitat.—Pacific and Atlantic, Stations 253, 272, 354, surface; also fossil in the Tertiary rocks of Barbados and Sicily.

4. Euchitonia cruciata, Stöhr.

Distance between the paired arms about one and a half times as large as their distance from the odd arm, which is a little larger. Length of the arm nearly equals twice the breadth, which is the same at the base and at the rounded blunt ends. Patagium incomplete, with concave chamber-rows, enveloping only the base of the arms, and forms between them three other smaller arms; therefore the whole shell forms six angles with six alternating arms.

Dimensions.—Radius of all three arms 0.15, basal breadth 0.05.

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

5. Euchitonia lanceolata, n. sp. (Pl. 43, fig. 9).

Distance between the paired arms about one and a third times as large as their distance from the odd arm. All three arms nearly of the same size and form, three times as long as broad, lanceolate, much broader in the middle part than at both ends; distal end cuspidate, but not spiny. Patagium incomplete, with convex chamber-rows, envelops about two-thirds of the arms.

Dimensions.—Radius of all three arms 0.18, greatest breadth (in the width) 0.06.

Habitat.—North Atlantic, Station 353, depth 2965 fathoms.

6. Euchitonia zittelii, Haeckel.

Distance between the paired arms about one-fifth as large as their distance from the odd arm. All three arms nearly of the same size and form, in the proximal half thinner and nearly square, in the distal half thicker and circular; the latter half twice to three times as broad as the former. Patagium incomplete, with convex chamber-rows, envelops only the proximal square half of the arms.

Dimensions.—Radius of all three arms 0.15, basal breadth 0.03, terminal breadth 0.08.

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

7. Euchitonia stöhrii, n. sp. (Pl. 43, fig. 12).

Distance between the paired arms about four-fifths as large as their distance from the odd arm, which is one and a half times as long as the former. Arms about three times as long as broad, twice as broad in the distal half as in the proximal half, with rounded blunt ends. Patagium complete, with concave chamber-rows, enveloping the whole shell, also the ends of the arms.

Dimensions.—Radius of the paired arms 0.2, of the odd arm 0.25; basal breadth 0.03, distal breadth 0.06.

Habitat.—Fossil in the rocks of Barbados and of Nicobar Islands, Haeckel. Indian Ocean, Cocos Islands (Rabbe). Also in the Central Pacific, Station 267.

8. Euchitonia beckmannii, Haeckel.

Distance between the paired arms scarcely half as large as their distance from the odd arm, which is somewhat larger. Each arm with six simple broad chambers (without radial septa), the terminal chambers semilunar, convex, blunt, four times as broad as the first (basal) chamber. Patagium incomplete, with convex chamber-rows, enveloping the arms with exception of the broad blunt terminal face.

Dimensions.—Radius of the paired arms 0.15, of the odd arm 0.18; basal breadth 0.02 to 0.03, terminal breadth 0.1 to 0.15.

Habitat.—Mediterranean (Messina), Haeckel, surface.

9. Euchitonia virchowii, Haeckel.

Distance between the paired arms about half as large as their distance from the odd arm, which is somewhat larger. Each arm with six broad chambers, bisected by a radial septum; the terminal chamber convex, blunt, twice as broad as the basal chamber. Patagium incomplete, with concave chamber-rows, enveloping the arms with exception of the broad blunt terminal face.

Dimensions.—Radius of the paired arms 0.15, of the odd arm 0.16; basal breadth 0.05, terminal breadth 0.1 to 0.12.

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

Subgenus 2. Pteractis, Ehrenberg, 1872 (loc. cit.).

Definition.—Arms provided with radial spines at the distal end.

10. Euchitonia elegans, Haeckel.

Distance between the paired arms half as large as their distance from the odd arm. This latter is straight, while both the former are concavely curved towards the middle line. Arms five times as long as broad, at the distal end pointed and armed with a short conical terminal spine. Patagium nearly complete, enveloping four-fifths of the arms, with four to five concave chamber-rows.

Dimensions.—Radius of the arms 0.2, breadth of them 0.03.

Habitat.—Tropical Pacific, Philippine Sea, depth 3300 fathoms (Ehrenberg).

11. Euchitonia carcinus, n. sp. (Pl. 43, fig. 10).

Distance between the paired arms scarcely one-fourth as great as their distance from the odd arm. This latter is straight, twice as long as the former, which are concavely curved towards the middle line. The odd arm is three times as broad at the distal end as at the narrow base. The end of each arm is furnished with a strong triangular radial spine and a group of smaller spines. Patagium incomplete, with two to three concave chamber-rows, enveloping only the basal half of the arms.

Dimensions.—Radius of the paired arms 0.15, breadth 0.03; radius of the odd arm 0.3, breadth on its base 0.02, on its distal end 0.07.

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

12. Euchitonia acuta, Stöhr.

Distance between the paired arms two-thirds as great as their distance from the odd arm. All three arms nearly of the same size and form, two and a half times as long as broad, nearly lanceolate, at their broadest part one and a half times as broad as at their base. Patagium complete, enveloping the whole triangular disk, with five to six concave chamber-rows. In the figure of Stöhr the ends of the arms are simply pointed, while I find in the same fossil form a short terminal conical spine.

Dimensions.—Radius of all three arms 0.14; breadth at the base 0.04, at the broadest part 0.06.

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

13. Euchitonia ypsiloides, Haeckel.

Distance between the paired arms two-thirds as great as their distance from the odd arm, which is somewhat larger. Length of the arms equals five times the breadth of the narrow base, which is half that of the distal end; this latter is armed with three to five short conical spines. Patagium complete, with six to seven concave chamber-rows, enveloping the whole arms with the exception of the terminal spines. (Differs from Euchitonia mülleri almost solely by the possession of terminal spines.)

Dimensions.—Radius of the arms 0.18 to 0.2; breadth at the base 0.04, at the distal end 0.08.

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

14. Euchitonia echinata, n. sp. (Pl. 43, fig. 11).

Distance between paired arms three-fourths as great as their distance from the odd arm, which is somewhat larger. Arms one and a half times as long as broad, somewhat constricted in the middle part, armed at the rounded ends with numerous (thirty to forty) strong, conical spines. Patagium complete, with four or five rectilinear parallel chamber-rows, enveloping the whole of the arms with the exception of the spiny ends.

Dimensions.—Radius of the arms (without spines) 0.2, breadth 0.06 to 0.09.

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

Genus 231. Chitonastrum,^[270] Haeckel, 1881, Prodromus, p. 460.

Definition.—Porodiscida with three forked, chambered arms, without a patagium. (Arms and angles between them either equal or unequal.)

The genus Chitonastrum differs from its ancestral form, Dictyastrum, by the bifurcation of the distal ends of the arms. The few species of this genus are partly regular (like Dictyastrum), partly bilateral (like Rhopalastrum). If the number of species increases much, these two subgenera may be separated into two genera: Chitonastrella corresponding to the former, Chitonastromma to the latter.

Subgenus 1. Chitonastrella, Haeckel.

Definition.—All three arms of the same size and form, equidistant; fundamental form of the shell therefore an equilateral triangle.

1. Chitonastrum triglochin, n. sp.

All three arms equal and equidistant. Each arm has the form of an isosceles triangle, twice as high as broad; the truncated apex of the triangle is inserted into the large central disk, whilst its distal base (four times as broad) is divided by a deep incision (half as long as the arm). Each arm with ten to twelve joints, simple in its basal half, double in its distal half. Axes of the six branches straight. (Resembles Trigonastrum regulare, Pl. 43, fig. 16, but differs in the absence of a patagium.)

Dimensions.—Radius of each arm 0.24, greatest breadth 0.11, basal breadth 0.03.

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

2. Chitonastrum bathybium, n. sp.

All three arms equal and equidistant, in the basal two-thirds simple, rectilinear, three times as long as broad, in the distal third forked, both branches equal, straight, blunt, half as broad as the basal part.

Dimensions.—Radius of each arm 0.18, basal breadth 0.04, breadth of the branches 0.02.

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

Subgenus 2. Chitonastromma, Haeckel.

Definition.—One odd arm different in size or form from the two other arms, which are paired; distance between them different; fundamental form of the shell therefore a bilateral, isosceles triangle.

3. Chitonastrum jugatum, n. sp. (Pl. 43, fig. 14).

Arms very different; odd arm club-shaped, twice as long as broad, at the blunt distal end twice as broad as at the base; its axis is perpendicular to the common axis of both paired arms, which are only two-thirds as long, not so broad, and in the distal half divided into two branches; the anterior branch is straight, nearly horizontal, the posterior shorter and curved backwards.

Dimensions.—Radius of the odd arm 0.2, of the paired arms 0.15; distal breadth of the former 0.08, basal breadth 0.04; breadth of the paired arms 0.04.

Habitat.—South Atlantic, Station 325, surface.

4. Chitonastrum dicranoides, n. sp.

All three arms in the basal half simple, nearly square, in the distal half forked; branches straight, blunt. Odd arm twice as large as the paired arms; angle between the latter larger than the angles between them and the odd arm. (The form of the arms resembles Dicranastrum furcatum, Pl. 47, fig. 2.)

Dimensions.—Radius of the odd arm 0.24, of the paired arms 0.12; basal breadth 0.06.

Habitat.—North Atlantic, Station 353, surface.

5. Chitonastrum lyra, n. sp. (Pl. 43, fig. 15).

All three arms forked and nearly of the same size, but different in form and position. The distance between the branches of the two paired arms is only one-fourth of the distance between them and the odd arm. Each arm in the basal two-thirds is simple, with eleven to twelve transverse septa, in the distal third forked, each branch with four to five transverse septa. The branches of each arm are curved convexly one to another, ending obtusely. The axis of the simple proximal part is straight in the odd arm, in the paired arms curved concavely towards the middle line. In the figured specimen, which I observed living in Portofino (in September 1880), the central chamber of the central disk and the first surrounding ring were filled with the nucleus of the cell; both external rings were filled (like all chambers of the arms) with pink oil-globules of the red central capsule. From the mantle, enveloping the shell, radiated innumerable fine pseudopodia (much too short in the figure), and between the two paired arms a long "sarcode-flagellum."

Dimensions.—Radius of each arm 0.16; greatest breadth of the odd arm 0.04; basal breadth of the paired arms 0.02; distance of both branches of each arm 0.08.

Habitat.—Mediterranean, Portofino, near Genoa, Haeckel.

Genus 232. Trigonastrum,^[271] n. gen.

Definition.—Porodiscida with three forked, chambered arms, connected by a patagium. (Arms and angles between them either equal or unequal.)

The genus Trigonastrum differs from the preceding Chitonastrum, its ancestral form, in the development of a patagium between the arms. It bears therefore to the latter the same relation that Euchitonia does to Rhopalastrum.

Subgenus 1. Trigonastrella, Haeckel.

Definition.—All three arms of the same size and form, equidistant; fundamental form of the shell therefore a regular, equilateral triangle.

1. Trigonastrum regulare, n. sp. (Pl. 43, fig. 16).

All three arms equal and equidistant. Each arm has the form of an isosceles triangle, twice as high as broad, the truncated apex of which is inserted into the large circular central disk, whilst its distal base (four times as broad) is divided by a deep incision (half as long as the arm). Each arm with ten to twelve joints. Patagium between the arms nearly complete, spongy. (Differs from Chitonastrum triglochin mainly in the possession of a patagium.)

Dimensions.—Radius of each arm 0.24, greatest breadth 0.11, basal breadth 0.03; length of the sides of the regular triangle 0.45.

Habitat.—Pacific, central area, Station 274, surface.

Subgenus 2. Trigonastromma, Haeckel.

Definition.—One odd arm different in form or size from the other two arms, which are paired; distance between them different; fundamental form of the shell therefore an isosceles triangle.

2. Trigonastrum krohnii, Haeckel.

All three arms different; distance between the two paired arms smaller than their distance from the odd arm. Odd arm with six joints. Each paired arm with seven joints, increasing in breadth towards the distal end. Odd arm and one paired arm forked at the end, the other paired arm simple. Patagium nearly complete. (The asymmetry in this form may perhaps be an individual anomaly, as also in Myelastrum anomalum, Pl. 47, fig. 9.)

Dimensions.—Radius of the arms about 0.13 to 0.16, breadth 0.05 to 0.08.

Habitat.—North Atlantic, Funchal, Madeira, Krohn, surface.

3. Trigonastrum gegenbauri, Haeckel.

Arms different; distance between the paired arms larger than their distance from the odd arm, which is one-third shorter. Odd arm egg-shaped, simple, with seven joints, undivided. Both paired arms equal, with ten joints, in the distal third forked. Patagium nearly complete.

Dimensions.—Radius of the odd arm 0.14, greatest breadth 0.08; radius of the paired arms 0.2.

Habitat.—Mediterranean (Messina), Haeckel, surface.

Genus 233. Stauralastrum,^[272] n. gen.

Definition.—Porodiscida with four simple, undivided, chambered arms, without a patagium; quadrangular shell a regular cross, with four equal arms placed at right angles.

The genus Stauralastrum is the most simple form of the Tessarastrida, or of those Porodiscida in which the margin of the central disk is armed with four chambered arms. In Stauralastrum these four arms are quite simple and equal, without a patagium, separated by four right angles, so that the whole shell represents a regular rectangular cross. If we connect the distal points of the arms by lines, we get a complete square. (In my Prodromus, 1881, the species of this genus were united with Hagiastrum, which genus I now retain for the simple bilateral Tessarastrida.)

Subgenus 1. Stauralastrella, Haeckel.

Definition.—Ends of the arms blunt, without terminal spines.

1. Stauralastrum cruciforme, n. sp. (Pl. 45, fig. 6).

Arms very thin, nearly linear, four to five times as long as broad, of equal breadth at the base and at the truncated distal end; their breadth equals one-third of the radius of the central disk. Edges of the arms parallel.

Dimensions.—Radius of each arm 0.8, breadth 0.016.

Habitat.—South Pacific, Station 293, surface.

2. Stauralastrum lanceolatum, n. sp.

Arms lanceolate, three times as long as broad, in their middle part three times as broad as at both ends; their greatest breadth nearly equals the diameter of the central disk. (The arms have the same form as in Euchitonia lanceolata, Pl. 43, fig. 9.) Edges of the arms convex.

Dimensions.—Radius of each arm 0.3, greatest breadth (in the middle part) 0.08.

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

3. Stauralastrum ordo, n. sp.

Arms trapezoid, about as long as broad, twice as broad at their truncated distal end as at the base; their basal breadth equals the radius of the central disk, which exhibits two to three rings. (The arms have nearly the same form as those in Hagiastrum mosis, Pl. 45, fig. 3.) Edges of the arms rectilinear, divergent towards the ends.

Dimensions.—Radius of each arm 0.12, basal breadth 0.04, distal breadth 0.08.

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