part 0.004, in the distal part 0.008.

_Habitat._--Mediterranean (Messina, Portofino), surface.

6. _Acanthometron conicum_, n. sp.

Spines short and thick, conical, gradually thinner from the thick conical base towards the simple apex. Central part of the base with a large fulcral pyramid, but without leaf-cross. Central capsule transparent, colourless (?).

_Dimensions._--Length of the spines 0.08 to 0.12, basal breadth 0.02.

_Habitat._--North Pacific, Station 244, surface.

{744}7. _Acanthometron pellucidum_, J. Müller.

_Acanthometra pellucida_, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 47, Taf. xi. figs. 1-3.

_Acanthometra pellucida_, Haeckel, 1862, Monogr. d. Radiol., p. 378.

Spines cylindrical, very thin and long, needle-shaped, nearly of equal thickness in their whole length. Central base with a small fulcral pyramid, without leaf-cross. Distal apex bifid or bifurcate, with two thin parallel teeth. Central capsule pellucid, colourless, with yellow pigment-bodies. (Differs from _Acanthometron elasticum_ mainly in the bifid apex.)

_Dimensions._--Length of the spine 0.1 to 0.3, breadth 0.002 to 0.004.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

8. _Acanthometron wageneri_, Haeckel.

_Acanthometra wageneri_, Haeckel, 1862, Monogr. d. Radiol., p. 378.

Spines cylindrical, in the thickened outer half about twice as broad as in the thin inner half. Central base a little thickened, with large fulcral pyramid, but without leaf-cross. Distal apex bifid, with two divergent, often denticulated teeth. Central capsule pellucid, with yellow pigment-bodies. (Differs from _Acanthometron dolichoscion_ mainly in the bifid apex.)

_Dimensions._--Length of the spines 0.2 to 0.5, breadth in the inner part 0.003, in the outer 0.006.

_Habitat._--Adriatic Sea; Triest, Wagner; Corfu, Haeckel, surface.

Subgenus 2. _Phyllostaurus_, Haeckel, 1862, Monogr. d. Radiol., p. 381.

_Definition._--Spines at the central base with a broad leaf-cross, composed of four prominent triangular lamellæ; the meeting edges of the neighbouring lamellæ are propped one upon another in such a manner that there are formed twenty-two hollow pyramidal spaces or compartments (compare p. 721).

9. _Acanthometron siculum_, Haeckel.

_Acanthometra sicula_, Haeckel, 1862, Monogr. d. Radiol., p. 382, Taf. xvii. figs. 1, 2; Taf. xviii. fig. 8.

Spines elongate, conical, tapering gradually from the thick base towards the simple distal apex. Conical circular base supported by a basal leaf-cross of double the breadth. Central capsule yellowish-brown, opaque.

_Dimensions._--Length of the spines 0.3 to 0.4, basal breadth 0.008 to 0.012, leaf-cross 0.024.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

{745}10. _Acanthometron catervatum_, Haeckel.

_Acanthometra brevispina_, Haeckel, 1862, Monogr. d. Radiol., p. 382, Taf. xv. fig. 5, Taf. xviii. fig. 9.

Spines cylindrical, nearly of equal breadth throughout their whole length. Apex either simple, conical, or bifid. Base with a large leaf-cross, four to six times as broad as the spine itself. Central capsule transparent, yellow. The Atlantic specimens have much longer spines than those figured from the Mediterranean, but are otherwise not different. Therefore I have changed the inconvenient name _brevispinum_ into _catervatum_.

_Dimensions._--Length of the spines 0.1 to 0.3, breadth 0.005 to 0.008; basal leaf-cross 0.02 to 0.032.

_Habitat._--Mediterranean (Messina); North Atlantic, Station 354, Gulf Stream (Færöe Channel) in great abundance, John Murray, surface.

Subgenus 3. _Astrolithium_, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 810.

_Definition._--Spines in the basal part grown perfectly together, so that the whole skeleton forms a single piece of acanthin; a star with twenty equal rays.

11. _Acanthometron bulbiferum_, n. sp. (Pl. 130, fig. 1).

_Astrolithium bulbiferum_, Haeckel, 1881, Prodromus, p. 466.

Spines needle-shaped, cylindrical, very thin and long, with simple apex; suddenly thickened and forming a broad regular cone at the central base; all twenty conical bulbs have their broad bases grown together and forming a central icosahedron of acanthin; from its twenty faces arise the circular bases of the cones in regular disposition.

_Dimensions._--Length of the spines 0.2 to 0.5, breadth 0.001 to 0.003; height of the basal cones 0.06, basal breadth of them 0.02.

_Habitat._--Central Pacific, Stations 265 to 274, surface.

12. _Acanthometron bifidum_, Haeckel.

_Astrolithium bifidum_, Haeckel, 1862, Monogr. d. Radiol., p. 400, Taf. xx. fig. 5.

Spines needle-shaped, cylindrical, of equal breadth in their whole length. Distal apex bifid, with two parallel straight teeth. Central bases of all twenty spines grown perfectly together and forming a central sphere of acanthin. Central capsule brown opaque.

_Dimensions._--Length of the spines 0.2, breadth 0.003.

_Habitat._--Mediterranean (Messina), Haeckel, surface.

{746}Genus 324. _Zygacantha_,[364] J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 51.

_Definition._--#Astrolonchida# with simple, compressed, and two-edged radial spines, without apophyses; their transverse section is elliptical or rhomboidal.

The genus _Zygacantha_ comprised in the original definition of J. Müller only a single species, _Zygacantha furcata_, distinguished from the other #Acanthometra# by forked spines with two long parallel teeth. It seems now advisable to unite in this genus all those Astrolonchida in which the simple spines are two-edged, compressed, or leaf-shaped. The term _Zygacantha_ may be conceived as the general expression of the important fact, that in all Icosacantha the twenty spines are opposite in pairs.

Subgenus 1. _Zygacantharium_, Haeckel.

_Definition._--Spines at the central base without leaf-cross and without hollow pyramidal compartments, united by the opposed triangular faces of their pyramidal bases, resting one upon another.

1. _Zygacantha lanceolata_, Haeckel.

_Acanthometra lanceolata_, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 48, Taf. xi. fig. 12.

_Acanthometra lanceolata_, Haeckel, 1862, Monogr. d. Radiol., p. 380.

Spines lanceolate, from the broader middle part equally thinned towards the two ends. Apex simple. Base pyramidal, without leaf-cross. Each flat lamellar spine exhibits an elevated middle rib (like a lanceolate leaf), and is therefore compressed quadrangular.

_Dimensions._--Length of the spines 0.1 to 0.15, greatest breadth (in the width) 0.03 to 0.04.

_Habitat._--Mediterranean (Saint Tropez, French shore), J. Müller; North Atlantic (Canary Islands), Haeckel, surface.

2. _Zygacantha costata_, n. sp.

Spines compressed, two-edged, linear, of nearly equal breadth in their whole length. Apex truncate. Base pyramidal, without leaf-cross. Each flat lamellar spine exhibits an elevated middle rib, which in the distal half is cleft into two divergent rods ending in the corners of the truncated apex. (Similar to _Zygacantha dicopa_, but with broader free spines, which are not grown together in the centre.)

_Dimensions._--Length of the spines 0.1 to 0.15, breadth 0.02.

_Habitat._--Central Pacific, Stations 265 to 274, surface.

{747}3. _Zygacantha compressa_, Haeckel.

_Acanthometra compressa_, Haeckel, 1862, Monogr. d. Radiol., p. 378, Taf. xviii. figs. 4_a_, 4_b_.

Spines in the proximal half compressed, broad lanceolate, four to six times as broad as in the thin cylindrical distal half. Apex simple or short bifid. Base pyramidal, thickened, without leaf-cross. No middle rib. Central capsule opaque, yellowish or reddish-brown.

_Dimensions._--Length of the spines 0.3 to 0.5, breadth of the inner half 0.015 to 0.02, of the outer half 0.002 to 0.003.

_Habitat._--Mediterranean (Messina); Atlantic (Canary Islands), Station 352, surface.

4. _Zygacantha furcata_, J. Müller.

_Zygacantha furcata_, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 51, Taf. ix. fig. 6.

_Acanthometra furcata_, J. Müller, 1856, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 499.

_Acanthometra furcata_, Haeckel, 1862, Monogr. d. Radiol., p. 380.

Spines compressed, fork-shaped, divided by two thickened knots into three sections of nearly equal length; middle section broader than the proximal, but smaller than the distal section, which is cleft by a deep fissure into two long parallel straight branches. Base pyramidal, without leaf-cross. Central capsule purple, with yellow bodies.

_Dimensions._--Length of the spines 0.1 to 0.15, basal breadth 0.003 to 0.005, distal breadth 0.02 to 0.03.

_Habitat._--Mediterranean (Cette). J. Müller, surface.

Subgenus 2. _Zygacanthidium_, Haeckel.

_Definition._--Spines at the central base with a cross of four prominent leaves; the meeting edges of the neighbouring lamellæ so rest one upon another that twenty-two hollow pyramidal compartments are formed (compare p. 721).

5. _Zygacantha dichotoma_, Haeckel.

_Zygacantha dichotoma_, Haeckel, 1862, Monogr. d. Radiol., p. 381.

_Acanthometra dichotoma_, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 50, Taf. ix. fig. 5.

Spines compressed, pincer-shaped, cleft nearly throughout their whole length into two thin parallel straight rods or fork branches, which are united only in their middle by a narrow bridge, and at their central base by the pyramidal small central apex; above this pyramid each rod is divided into two broad triangular leaves, forming a large basal leaf-cross. Central capsule purple, with yellow bodies.

_Dimensions._--Length of the spines 0.1 to 0.15, breadth 0.01 to 0.02.

_Habitat._--Mediterranean (Nice), J. Müller, surface.

{748}6. _Zygacantha complanata_, n. sp.

Spines compressed, two-edged, linear, of equal breadth throughout their whole length. Apex bifid. Base thickened, with a large leaf-cross. (Similar to _Amphilonche complanata_, but different in the equal size of all twenty spines, which are somewhat broader.)

_Dimensions._--Length of the spines 0.12 to 0.18, breadth 0.002.

_Habitat._--North Pacific, Station 244, surface.

7. _Zygacantha semicompressa_, Haeckel.

_Acanthometra hemicompressa_, Car, 1884, Zool. Anzeiger, p. 94, with woodcut.

Spines in the proximal half compressed, two-edged, linear, three to four times as broad as in the needle-shaped cylindrical distal half. Apex simple. Base pyramidal, with a small leaf-cross. Central capsule transparent.

_Dimensions._--Length of the spines 0.1 to 0.2, breadth in the proximal half 0.02, in the distal half 0.006.

_Habitat._--Mediterranean (Genoa), Haeckel; Adriatic Sea (Trieste), Car, surface.

8. _Zygacantha foliacea_, n. sp.

Spines lanceolate, tapering from the broader middle part towards the two ends, with two dentated or serrated edges and a prominent middle rib. Apex simple. Base with a small leaf-cross. The spines like the leaves of _Agave americana_.

_Dimensions._--Length of the spines 0.15 to 0.2, middle breadth 0.02 to 0.03.

_Habitat._--Indian Ocean, near the Island of Socotra, Haeckel.

Subgenus 3. _Zygacanthonium_, Haeckel.

_Definition._--Spines in the centre grown perfectly together and forming a single star-shaped piece of acanthin.

9. _Zygacantha dicopa_, Haeckel.

_Astrolithium dicopum_, Haeckel, 1862, Monogr. d. Radiol., p. 400, Taf. xx. figs. 3, 4.

Spines compressed, two-edged, linear, of nearly equal breadth throughout their whole length, with a prominent middle rib, which in the distal third is cleft into two divergent teeth, ending in the two corners of the broad, obliquely truncated apex. All twenty spines with their central bases grown together and forming one single piece of acanthin--a star with twenty rays.

_Dimensions._--Length of the spines 0.12 to 0.2, breadth 0.01 to 0.02.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Pacific, many Stations, surface.

{749}Genus 325. _Acanthonia_,[365] Haeckel, 1881, Prodromus, p. 465.

_Definition._--#Astrolonchida# with simple, four-edged, prismatic or pyramidal radial spines, without apophyses; their transverse section is square.

The genus _Acanthonia_ comprises all those Astrolonchida (formerly united with _Acanthometron_) in which the simple spines either in their whole length or in the greatest part of it are four-edged, with square transverse section. They are sometimes more prismatic (with equal breadth), at other times more pyramidal (with decreasing breadth towards the distal apex). If _Acanthometron_ be the common simple ancestral form of the Acanthonida, then the two-edged _Zygacantha_, and the four-edged _Acanthonia_ may be regarded as two divergent main lines arising from it; the former leading to the Phractacanthida and Diporaspida, the latter leading to the Stauracanthida and Tessaraspida.

Subgenus 1. _Acanthonarium_, Haeckel.

_Definition._--Spines at the central base, without leaf-cross and without hollow pyramidal compartments, united by the triangular faces of their pyramidal bases, resting one upon another.

1. _Acanthonia tetracopa_, Haeckel (Pl. 129, figs. 9-11).

_Acanthometra tetracopa_, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 47, Taf. vii. figs. 3 to 5, Taf. xi. fig. 5.

_Acanthometra tetracopa_, Haeckel, 1862, Monogr. d. Radiol., p. 379, Taf. xviii. fig. 5.

Spines quadrangular prismatic, with prominent lamellar edges, of equal breadth in their whole length. Base pyramidal, without leaf-cross. Apex truncated, or with four short teeth (sometimes only two teeth). Central capsule opaque, brown or yellow.

_Dimensions._--Length of the spines 0.15 to 0.2, breadth 0.01 to 0.012.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, many Stations, surface.

2. _Acanthonia prismatica_, n. sp.

Spines quadrangular prismatic, without prominent edges, with four plane lateral faces, of equal breadth throughout their whole length. Base pyramidal, without leaf-cross. Apex truncate, with square apical face.

_Dimensions._--Length of the spines 0.1 to 0.16, breadth 0.005 to 0.008.

_Habitat._--Antarctic Ocean, Station 154, surface.

{750}3. _Acanthonia denticulata_, n. sp.

Spines quadrangular prismatic, with prominent elegantly denticulated edges, of equal breadth throughout their whole length. Both ends pyramidal, without leaf-cross. (Similar to _Amphilonche denticulata_, but with the twenty spines all equal, and with spherical central capsule.)

_Dimensions._--Length of the spines 0.12 to 0.16, breadth 0.008.

_Habitat._--Central Pacific, Stations 266 to 274, surface.

4. _Acanthonia mülleri_, Haeckel.

_Acanthometra mülleri_, Haeckel, 1862, Monogr. d. Radiol., p. 379, Taf. xv. fig. 3, Taf. xviii. fig. 6.

Spines quadrangular, nearly prismatic, but tapering gradually from the pyramidal base towards the distal bifid apex, which bears two thin, parallel, or little divergent teeth; four edges elegantly denticulated; base without leaf-cross. Central capsule yellowish or reddish.

_Dimensions._--Length of the spines 0.12 to 0.18, basal breadth 0.005.

_Habitat._--Mediterranean (Messina); Atlantic, Stations 348 to 352, surface.

5. _Acanthonia fragilis_, Haeckel.

_Acanthometra fragilis_, Haeckel, 1862, Monogr. d. Radiol., p. 380, Taf. xv. fig. 4, Taf. xviii. fig. 7.

Spines quadrangular, nearly prismatic, but gradually thickened from the pyramidal base towards the distal truncated end; four edges regularly denticulated; base without leaf-cross. Central capsule opaque.

_Dimensions._--Length of the spines 0.5 and more, basal breadth 0.002 to 0.003, distal breadth 0.008 to 0.016 or more.

_Habitat._--Mediterranean (Messina); North Atlantic, Station 354, surface.

6. _Acanthonia convexa_, n. sp.

Spines quadrangular, with four broad lamellar, convex, prominent edges, which from the broader middle part are thinned towards the two emarginated ends; each end with a small quadrangular pyramid, without leaf-cross. All the twenty spines of this species exhibit nearly the same form, which is seen in the caudal or posterior (geotomical) spine of _Amphilonche anomala_ (Monogr. d. Radiol., Taf. xviii. fig. 23, _b_).

_Dimensions._--Length of the spines 0.12 to 0.16, breadth in the middle