Chapter XI: Literature and History (25)

6. _Histiastrum boseanum_, n. sp. (Pl. 46, fig. 1).

Arms linear, twelve times as long as broad at their base (at their distal end twice as broad as at their base), with rectilinear, parallel edges. The club-shaped end thickened, dentate, with two lateral rows of strong teeth in the equatorial plane, and with one very stout, angular, terminal spine. Central disk with two to three rings, broader than the arms. Patagium incomplete, with four to five concave chamber-rows, enveloping only the basal half of the arms. I call this splendid species in honour of Dr. Graf Bose, the great friend of nature and patron of the University of Jena.

_Dimensions._--Radius of each arm 0.25, basal breadth 0.02, distal breadth 0.05.

_Habitat._--Indian Ocean, Madagascar, Rabbe.

7. _Histiastrum coronatum_, n. sp.

_Stephanastrum_ sp., Bury, 1862, Polycystins of Barbados, pl. iv. fig. 1.

Arms linear, eight times as long as broad, with rectilinear parallel edges; the thickened, nearly spherical, distal end three times as broad, with five strong conical spines, one middle (perradial) larger and two smaller on each side of it. Central disk with two to three rings, of the same breadth as the terminal knot of the arms. Patagium incomplete, square, enveloping the arms, with the exception of the knot.

_Dimensions._--Radius of each arm 0.16, basal breadth 0.02, terminal breadth 0.06.

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

8. _Histiastrum circulare_, n. sp.

_Stephanastrum_ sp., Bury, 1862, Polycystins of Barbados, pl. xxiii. fig.
1.

Arms linear in their inner half, egg-shaped and three times as broad in their outer half, with ten to twelve strong terminal spines, the middle (perradial) larger, in all three times as long as broad. Central disk with three to four rings, broader than their distal knobs. Patagium nearly complete, circular, enveloping the whole arms, with exception of the outermost end.

_Dimensions._--Radius of each arm 0.22, basal breadth 0.02, terminal breadth 0.06.

_Habitat._--Fossil in the rocks of Barbados; and living in the depths of the Central Pacific, Station 268, depth 2900 fathoms.

9. _Histiastrum pentadiscus_, n. sp. (Pl. 46, fig. 2).

Arms in their inner half linear, twice as long as broad, in their outer half circular, three times as broad, with the same structure as the central disk, exhibiting three concentric rings around one {547}central chamber. Patagium complete, spongy, with radiating beams, enveloping the whole disk, with the exception of the outermost end of the arms, which is armed with twelve to sixteen strong conical spines, the middle (perradial) spine much larger.

_Dimensions._--Radius of each arm (without terminal spine) 0.18, basal breadth 0.025, terminal breadth 0.08.

_Habitat._--South Atlantic, Station 333, surface.

Genus 236. _Tessarastrum_,[275] n. gen.

_Definition._--#Porodiscida# with four simple, undivided, chambered arms, connected by a patagium; quadrangular shell bilateral, two opposite arms of the main axis (or principal arms) different from the two others (or lateral arms).

The genus _Tessarastrum_, formerly united by me with _Histiastrum_, differs from the latter in its bilateral or symmetrical form, and bears therefore the same relation to it that _Hagiastrum_ does to _Stauralastrum_.

Subgenus 1. _Tessarastrella_, Haeckel.

_Definition._--Both longitudinal arms of equal size and form.

1. _Tessarastrum straussii_, n. sp. (Pl. 45, fig. 8).

_Histiastrum straussii_, Haeckel, 1881, Prodromus et Atlas (pl. xlv. fig.
8).

Cross not rectangular. Both principal arms of equal size and form, four times as long as broad, and twice as long as the broader lateral arms; the former with ten to eleven, the latter with five to six joints, separated by convex transverse septa. Distal ends of the arms blunt. Axes of the smaller arms not perpendicular to that of the larger arms; therefore the anterior angles between them smaller than the posterior angles. Patagium between the arms incomplete. I call this remarkable species after the great German philosopher David Strauss.

_Dimensions._--Radius of the principal arms 0.24, of the lateral arms 0.12; greatest breadth (in the middle) of the former 0.05, of the latter 0.06.

_Habitat._--North Pacific, _Hyalonema_-ground, March 5, 1875.

2. _Tessarastrum spinozæ_, n. sp.

Cross rectangular. Both principal arms of equal size and form, ten times as long as broad, and twice as long as the lateral arms, which are only five times as long as broad. All arms linear, at their distal end club-shaped, and armed with twenty to thirty very strong angular spines. Patagium incomplete, enveloping only the basal half of the arms. (Resembles _Histiastrum boseanum_, {548}Pl. 46, fig. 1, but differs in the broader arms and the unequal size of both pairs.) I call this species after the great monistic philosopher Benedictus Spinoza.

_Dimensions._--Radius of the principal arms 0.3, of the lateral arms 0.16; basal breadth 0.025, distal breadth 0.05.

_Habitat._--South Atlantic, Station 333, surface.

3. _Tessarastrum brunonis_, n. sp. (Pl. 45, fig. 9).

_Histiastrum brunonis_, Haeckel, 1881, Prodromus et Atlas (pl. xlv. fig.
9).

Cross rectangular. Both principal arms of equal size and form, three times as long as broad, each with ten joints, three times as long as the lateral arms, which are nearly square, with four joints. All arms rounded, at their truncated end little broader than at their base. No spines. Patagium complete, envelops the whole shell, and is composed of two parallel lattice-lamellæ on each side of the flat disk, which are connected by very fine perpendicular bars. This is shown clearly in fig. 9, Pl. 45, where the disk is seen from the edge. I call this species after the great Italian philosopher Giordano Bruno.

_Dimensions._--Radius of the principal arms 0.22, of the lateral arms 0.12; basal breadth 0.04, distal breadth 0.05.

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

Subgenus 2. _Tessarastromma_, Haeckel.

_Definition._--The two principal arms of different size or form.

4. _Tessarastrum democriti_, n. sp. (Pl. 45, fig. 7).

_Histiastrum democriti_, Haeckel, 1881, Prodromus et Atlas (pl. xlv. fig.
7).

Cross not rectangular; the two anterior angles smaller than the two posterior. All four arms club-shaped, twice as broad at their rounded obtuse distal end as at their base, of unequal length. Posterior principal arm one and a fourth times as long as the posterior, and one and two-thirds as long as the lateral arms. Patagium incomplete, enveloping only the basal half of the arms. I call this species after the great Greek philosopher Democritus.

_Dimensions._--Radius of the posterior arm 0.3, of the anterior 0.25, of each lateral arm 0.22; basal breadth 0.05, distal breadth 0.1.

_Habitat._--Pacific, central area, Station 271, depth 2425 fathoms.

Genus 237. _Stephanastrum_,[276] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.

_Definition._--#Porodiscida# with four simple, undivided, chambered arms, connected on the distal ends by a spongy, square or rhomboidal, patagial girdle (or a patagium with four large, interbrachial openings). Shell either regular or bilateral (with equal or unequal arms).

{549}The genus _Stephanastrum_, founded (1847) by Ehrenberg for the very peculiar _Stephanastrum rhombus_, differs from the nearly allied foregoing genera in the imperfect development of the peculiar patagium, connecting only the distal ends of the four arms, while it is absent at their base. Two new species, different from _Stephanastrum rhombus_ by the regular square form, were found in the Challenger collection.

Subgenus 1. _Stephanastrella_, Haeckel.

_Definition._--All four arms of the cross have the same size.

1. _Stephanastrum quadratum_, n. sp. (Pl. 46, fig. 5).

All four arms of the same size, six times as long as broad at their base, ending with a strong, short, four-sided pyramidal spine. In the outer half of each arm are two opposite lateral spongy wings, which form an equilateral triangle, and from union of the bases of the four triangles arises the peculiar patagium, which forms a square with four large interbrachial openings.

_Dimensions._--Radius of each arm 0.25, basal breadth 0.035; length of the sides of the square patagium 0.3.

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

2. _Stephanastrum capitatum,_ n. sp. (Pl. 44, fig. 1).

All four arms of the same size, five times as long as broad at their base, at their distal end with a spongy, nearly spherical capitulum of twice their breadth, provided with a very strong, angular, pyramidal, terminal spine (half as long as the arm). All four arms connected by a square patagium, arising immediately below the capitula, and perforated by four large interbrachial openings.

_Dimensions._--Radius of each arm (without the terminal spine) 0.25, basal breadth 0.05; length of the sides of the square patagium 0.3.

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

Subgenus 2. _Stephanastromma_, Haeckel.

_Definition._--Two opposite arms of the cross larger than the two others.

3. _Stephanastrum rhombus,_ Ehrenberg.

_Stephanastrum rhombus_, Ehrenberg, 1854, Mikrogeol., Taf. xxxvi. fig.
33; Abhandl. d. k. Akad. d. Wiss. Berlin, 1875, Taf. xxv. fig. 1.

Two arms of the longitudinal axis one and a third times as long as two arms of the transverse axis. All four arms linear, about eight times as long as broad, at their distal end somewhat {550}thickened, club-shaped, with a pyramidal terminal spine. The ends of the arms are connected by a riband-shaped, straight, spongy patagium of the same breadth as the arms. Between the rhomboidal patagium and the arms remain four large rectangular triangles as interbrachial openings.

_Dimensions._--Radius of the longer arms 0.2, of the shorter 0.15; basal breadth 0.02; length of the sides of the rhombic patagium 0.25.

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

Genus 238. _Dicranastrum_,[277] Haeckel, 1881, Prodromus, p. 460.

_Definition._--#Porodiscida# with four forked, spongy, or chambered arms, without a patagium; shell regular (not bilateral), with four equal arms crossed at right angles.

The genus _Dicranastrum_ comprises a number of very remarkable, hitherto unknown, Euchitonida, which are rather common in the Pacific (mainly on the surface), and characterised by the bifurcation of the cross-arms of the regular square shell. It bears therefore to its probable ancestral form, _Stauralastrum_, the same relation that in the triradiate Euchitonida _Chitonastrum_ does to _Dictyastrum_. The arms are commonly of very delicate structure, more or less spongy.

Subgenus 1. _Dicranaster_, Haeckel.

_Definition._--Each cross-arm with two simple branches.

1. _Dicranastrum furcatum_, n. sp. (Pl. 47, fig. 2).

Arms simply forked, three times as long as broad at their base, with eight blunt ends of the fork-branches. The simple proximal half of each arm about the same size as each branch of the dichotomous distal part, twice as long as broad. Edges of the arms ragged.

_Dimensions._--Radius of each arm 0.38, basal breadth 0.12; breadth of the forked part 0.3.

_Habitat._--Pacific, central area, Station 271, surface.

2. _Dicranastrum dichotomum_, n. sp.

Arms simply forked, four times as long as broad at their base; each arm with two blunt branches. The simple proximal part of each arm is three times as long and twice as broad as each branch of the dichotomous distal part. Ends of the arms blunt, truncated.

_Dimensions._--Radius of each arm 0.35, basal breadth 0.08; breadth of the forked part 0.2.

_Habitat._--South Pacific, Station 281, surface.

{551}3. _Dicranastrum cornutum_, n. sp. (Pl. 45, fig. 2).

_Hagiastrum cornutum_, Haeckel, 1879, Atlas (pl. xlv. fig. 2).

Four arms simply forked, four times as long as broad; each arm with two triangular diverging branches ending in strong conical spines. The simple basal part of each arm about of the same length as each branch of the distal part, twice as long as broad. Distance between the terminal spines of each arm nearly as great as its length.

_Dimensions._--Radius of the arm 0.24, basal breadth 0.05; breadth across the bifurcation 0.12.

_Habitat._--South Atlantic, Station 325, surface.

4. _Dicranastrum antilope_, n. sp.

Four arms simply forked, three times as long as broad; each arm with two lanceolate diverging branches, ending in strong angular spines. The simple basal part of each arm is twice as long as each branch of the distal part. Distance of the two terminal spines of each arm scarcely half as great as its length.

_Dimensions._--Radius of the arm 0.32, basal breadth 0.11; breadth of the branches 0.07.

_Habitat._--Tropical Atlantic (off Ascension), Station 342, surface.

Subgenus 2. _Tricranastrum_, Haeckel, 1881, Prodromus, p. 460.

_Definition._--Each cross arm with three terminal branches, one middle (perradial) and two lateral (adradial) branches.

5. _Dicranastrum wyvillei_, n. sp. (Pl. 47, fig. 3).

_Tricranastrum wyvillei_, Haeckel, 1879, Natürl. Schöpfungsgesch., p.
705, Taf. xvi. fig. 5.

Arms trifid, one and a half times as long as broad; each arm with three blunt terminal branches of equal size. The simple basal part of each arm twice as long as the trifid distal part. (The central capsule depicted in fig. 3, Pl. 47, has the same form as the skeleton, and is only a little smaller.)

_Dimensions._--Radius of the arm 0.4, basal breadth 0.12, greatest breadth (in the distal part) 0.24.

_Habitat._--Pacific, central area, Station 271, surface.

6. _Dicranastrum tricuspis_, n. sp.

Arms trifid, twice as long as broad; each arm with three pointed terminal branches, ending in strong conical spines, the middle branch somewhat larger than the other two. The simple basal part of each arm three times as long as the trifid distal part.

_Dimensions._--Radius of the arm 0.3, basal breadth 0.06, greatest breadth (in their distal part) 0.15.

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

{552}7. _Dicranastrum trifarium_, n. sp.

Arms trifid, three times as long as broad at their base; each arm with three pointed terminal branches, ending in strong sulcate spines; the middle branch twice as large as the two others. The simple basal part of each arm two and a half times as long as the trifid distal part.

_Dimensions._--Radius of the arm 0.35, basal breadth 0.03, greatest breadth (in the distal part) 0.05.

_Habitat._--South Pacific, Station 298, surface.

Subgenus 3. _Tetracranastrum_, Haeckel.

_Definition._--Each cross-arm with four terminal branches, the two fork-branches being again bifurcated.

8. _Dicranastrum bifurcatum_, n. sp. (Pl. 47, figs. 1, 1_a_).

Arms doubly forked or quadripartite, six times as long as broad at their base; each arm in its proximal half simple, three times as long as broad; in its distal half doubly forked; the secondary branches with blunt, roundish ends, nearly as large as the primary branches. Central disk (fig. 1_a_) with three concentric rings around the central chamber; from its periphery radiate thin radial beams in the spongy framework of the delicate arms. The central capsule has the same form as the skeleton, and is only a little smaller.

_Dimensions._--Radius of the arm 0.45, basal breadth 0.08; breadth of the terminal branches 0.03.

_Habitat._--North Pacific, east of Japan, Station 241, surface.

Genus 239. _Myelastrum_,[278] Haeckel, 1881, Prodromus, p. 460.

_Definition._--#Porodiscida# with four forked, spongy, or chambered arms, without a patagium; shell bilateral, with paired different arms; two equal anterior arms of different shape from the two equal posterior arms.

The genus _Myelastrum_ differs from the foregoing _Dicranastrum_, the ancestral form, by the twofold differentiation of the four arms. Whilst in the latter all four arms are equal, separated by equal angles, here the two anterior arms are constantly different from the two posterior. The lateral angles between the two arm-pairs are equal, while the posterior and the anterior angle (between the two arms of each pair) are more or less different. The shell assumes, therefore, a very characteristic bilateral form, similar to the "quadricorn cross" of the grey central substance in the transverse section of the human medulla spinalis. Though the spongy shell is commonly a most delicate and {553}thin disk it nevertheless reaches unusual dimensions, its diameter in some species being more than a millimetre.

Subgenus 1. _Myelastrella_, Haeckel.

_Definition._--Posterior arms simple, undivided; anterior arms lobated or cleft, with one or more incisions at the distal end.

1. _Myelastrum medullare_, n. sp. (Pl. 47, fig. 13).

Anterior arms bifid, nearly square, with a shallow incision at their broad truncated end. Posterior arms somewhat smaller, nearly triangular, with simple blunt ends. Sagittal constriction three-fourths as large as the transverse one. Surface smooth.

_Dimensions._--Radius of the anterior arms 0.36, of the posterior 0.3; longitudinal constriction 0.24, transverse 0.36.

_Habitat._--Pacific, central area, Station 271, surface.

2. _Myelastrum spinale_, n. sp.

Anterior arms bifid, twice as long as broad, with a deep incision at their truncated end. Posterior arms slender, half as large, with simple blunt ends. Sagittal constriction one and a half times as large as the transverse. Surface spiny.

_Dimensions._--Radius of the anterior arms 0.4, of the posterior 0.2; longitudinal constriction 0.3, transverse 0.2.

_Habitat._--North Pacific, Station 248, surface.

3. _Myelastrum heteropterum_, n. sp. (Pl. 47, fig. 8).

Anterior arms trifid, about as long as broad, with two incisions at their broad truncated end. Posterior arms about half as large, simple, with rounded blunt ends. Sagittal constriction two-thirds as large as the transverse. Surface bristly.

_Dimensions._--Radius of the anterior arms 0.65, of the posterior 0.035; longitudinal constriction 0.4, transverse 0.6.

_Habitat._--South Pacific, Station 291, surface.

Subgenus 2. _Myelastromma_, Haeckel.

_Definition._--All four arms (posterior as well as anterior) lobated or cleft.

4. _Myelastrum octocorne_, n. sp. (Pl. 47, fig. 12).

All four arms with a deep incision in their distal half; anterior arms broader, but shorter than the posterior; in the anterior arms the two lobes are of the same size, in the posterior arms the {554}median lobe is longer than the lateral lobe. Sagittal constriction little smaller than the transverse. Surface of the disk rough.

_Dimensions._--Radius of the anterior arms 0.45, of the posterior 0.62; longitudinal constriction 0.25, transverse 0.3.

_Habitat._--Pacific, central area, Station 271, surface.

5. _Myelastrum farfalla_, n. sp. (Pl. 47, fig. 10).

All four arms with a shallow incision at their distal end. Anterior arms broader, but shorter than the posterior; in each arm the anterior lobe is shorter than the posterior. Sagittal constriction smaller than the transverse. Surface smooth.

_Dimensions._--Radius of the anterior arms 0.4, of the posterior 0.6; longitudinal constriction 0.5, transversal stricture 0.6.

_Habitat._--Pacific, central area, Station 274, surface.

6. _Myelastrum papilio_, n. sp. (Pl. 47, fig. 6).

All four arms with a shallow incision at their distal end. Anterior arms triangular, of the same breadth, but of the double length of the square posterior arms. Sagittal and transverse constrictions of the same length. Surface ciliated. (Resembles a butterfly.)

_Dimensions._--Radius of the anterior arms 0.7, of the posterior 0.4; longitudinal and transverse constrictions 0.4.

_Habitat._--North Pacific, near Japan, Station 241, surface.

7. _Myelastrum decaceros_, n. sp. (Pl. 47, fig. 7).

Anterior arms bifid, with one shallow incision at their distal end. Posterior arms of the same length, but much broader, trifid, with two incisions (one larger and one smaller). Sagittal and transverse constrictions of the same length. Surface spiny.

_Dimensions._--Radius of all four arms 0.4; longitudinal and transverse constrictions 0.35.

_Habitat._--Pacific, central area, Station 272, surface.

8. _Myelastrum dodecaceros_, n. sp. (Pl. 47, figs. 11, 11_a_).

Anterior arms trifid, with two shallow incisions at their distal end. Posterior arms about one-third larger, also trifid, with one deeper anterior and one shallower posterior incision. Sagittal constriction much longer than the transverse. Surface ciliated, covered with numerous thin and long radial bristles (fig. 11_a_).

_Dimensions._--Radius of the anterior arms 0.5, of the posterior 0.7; longitudinal constriction 0.5, transverse 0.35.

_Habitat._--Pacific, central area, Station 270, surface.

{555}9. _Myelastrum ciliatum_, n. sp.

Anterior arms trifid, with two shallow incisions. Posterior arms somewhat smaller, also trifid, with two deeper incisions. Sagittal constriction a little larger than the transverse. Margin of the disk ciliated, with radial bristle-shaped spines, as prolongations of the inner radial beams, arising from the central disk.

_Dimensions._--Radius of the anterior arms 0.6, of the posterior 0.5; longitudinal constriction 0.4, transverse 0.35.

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

10. _Myelastrum lobatum_, n. sp.

Anterior arms somewhat broader but shorter than the posterior arms. Each arm four-lobed, with three terminal shallow incisions of nearly equal size. Sagittal constriction smaller than the transverse. Surface of the disk bristly.

_Dimensions._--Radius of the anterior arms 0.4, of the posterior 0.5; longitudinal constriction 0.3, transverse 0.35.

_Habitat._--North Pacific, Station 253, surface.

11. _Myelastrum rotula_, n. sp.

Anterior arms little broader than the posterior, but of the same length. Each arm four-lobed, with three terminal deep incisions of equal size. Sagittal and transverse constrictions equal. The whole disk nearly circular, resembles a wheel with sixteen spokes. Surface smooth.

_Dimensions._--Radius of all four arms 0.5; longitudinal and transverse constrictions 0.3.

_Habitat._--Pacific, central area, Station 274, surface.

12. _Myelastrum giganteum_, n. sp.

Anterior arms somewhat broader and shorter than the posterior. Each arm four-lobed, with three shallow terminal incisions, the middle incision twice as deep as the two laterals. Sagittal constriction a little larger than the transverse. Margin ciliated, with radial bristle-shaped spines as prolongations of the inner radial beams, proceeding from the central disk (as in _Myelastrum dodecaceros_, Pl. 47, figs. 11, 11_a_).

_Dimensions._--Radius of the anterior arms 0.6, of the posterior 0.8; longitudinal constriction 0.6, transverse 0.5.

_Habitat._--Pacific, central area, Station 271, surface.

{556}13. _Myelastrum anomalum_, n. sp. (Pl. 47, fig. 9).

All four arms of different size and form; anterior arms broader, posterior longer; one anterior arm trifid, the three other arms bifid; length of the branches unequal; all four angles between the arms unequal. (This anomalous form, seen only once, may be an individual abnormality.)

_Dimensions._--Radius of the arms 0.4 to 0.7; constrictions 0.3.

_Habitat._--North Pacific, Station 237, surface.

Genus 240. _Pentalastrum_,[279] Haeckel, 1881, Prodromus, p. 461.

_Definition._--#Porodiscida# with five simple, undivided, chambered arms, without a patagium.

The genus _Pentalastrum_ opens the small series of Euchitonida, in which the shell is not provided with three or four arms, as usual, but with five. All forms of this little group are rare. Some species resemble in their external form and in the articulation of their arms certain forms of Asterida. In _Pentalastrum_, the most simple genus, the five arms are simple, not forked, and without a patagium. It can be derived from _Dictyastrum_ or _Stauralastrum_ by increase of the number of arms.

Subgenus 1. _Pentalastrella_, Haeckel.

_Definition._--All five arms equal, with equal angles between them. Shell a regular pentagon.

1. _Pentalastrum asteracanthion_, n. sp.

All five arms equal, club-shaped, at their thickened obtuse end three times as broad as at their base, twice as long as broad. Angles between the arms equal.

_Dimensions._--Radius of each arm 0.2, basal breadth 0.03, distal breadth 0.08.

_Habitat._--Pacific, central area, Station 272, depth 2600 fathoms.

2. _Pentalastrum astropecten_, n. sp.

All five arms equal, with five to six distinct, simple joints, the basal joint two-thirds as broad as the terminal joint, which bears a strong conical spine. Angles between the arms equal. (Resembles _Pentinastrum asteriscus_, Pl. 44, fig. 2, but has no patagium.)

_Dimensions._--Radius of each arm 0.14, basal breadth 0.024, distal breadth 0.036.

_Habitat._--Pacific, central area, Station 263, depth 2650 fathoms.

{557}Subgenus 2. _Pentalastromma_, Haeckel.

_Definition._--Arms of different sizes, one odd arm larger than the two others; the opposite odd angle generally different from the four other angles.

3. _Pentalastrum ophidiaster_, n. sp. (Pl. 44, fig. 3).

Arms nearly triangular, at their obtuse truncated distal end twice as broad as at their base. Four arms equal, with five joints each; the fifth arm twice as long, with seven joints. Angles between the arms nearly equal; the odd angle a little larger.

_Dimensions._--Radius of the larger odd arm 0.25, of the four smaller arms 0.15; basal breadth 0.035, distal breadth 0.07.

_Habitat._--Pacific, central area, Station 274, surface.

4. _Pentalastrum cometa_, n. sp.

Arms nearly cylindrical, at their obtuse truncated distal end one and a half times as broad as at their base. Posterior odd arm very large, with eleven joints, about three times as long as the two lateral arms (with five joints each) and four times as long as the two anterior arms (with three joints each). Angles between the paired arms different; the two lateral angles smaller than the two posterior, and these smaller than the odd anterior angle.

_Dimensions._--Radius of the odd posterior arm 0.5, of the lateral arms 0.25, of the anterior arms 0.18; basal breadth 0.05, distal breadth 0.08.

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

Genus 241. _Pentinastrum_,[280] Haeckel, 1881, Prodromus, p. 461.

_Definition._--#Porodiscida# with five simple, undivided, chambered arms, connected by a patagium.

The genus _Pentinastrum_ differs from the foregoing _Pentalastrum_ only in the development of a patagium or connecticulum between the arms, and bears therefore the same relation to it that _Histiastrum_ does to _Stauralastrum_, or _Hymeniastrum_ to _Dictyastrum_.

1. _Pentinastrum asteriscus_ , n. sp. (Pl. 44, fig. 2).

All arms equal, twice as long as broad, at their base two-thirds as broad as at their truncated distal end, which bears a strong, pyramidal, terminal spine. Each arm is divided by five transverse septa into six joints or chambers, and each of these by a radial beam into a pair of chambers. The five radial beams arise from the innermost chamber of the central disk, and end in the five terminal {558}spines. The diameter of the central disk is larger than the length of the arms. The angles between the arms are equal and filled up by an incomplete patagium, so that the whole disk forms a regular pentagon with five concave sides.

_Dimensions._--Radius of each arm (without terminal spine) 0.14; breadth at their base 0.02, at their terminal joint 0.03; radius of the central disk 0.06.

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

2. _Pentinastrum goniaster_, n. sp.

? _Stephanastrum_ sp., Bury, 1862, Polycystins of Barbados, pl. xx. fig.
1.

All five arms equal, four times as long as broad, club-shaped, at their globose distal end twice as broad as at their base, and armed with a strong conical terminal spine. Diameter of the central disk equals only one-third of the length of the arms. The articulation of the spongy arms is somewhat obscure. Patagium complete, totally fills up the interbrachial spaces, so that the whole disk forms a regular pentagon with five rectilinear sides, except that the terminal spines project at the corners.

_Dimensions._--Radius of each arm 0.18, basal breadth 0.02, distal breadth 0.04; radius of the central disk 0.05.

_Habitat._--South Pacific, Station 295, depth 1500 fathoms; also fossil in the rocks of Barbados.

Genus 242. _Pentophiastrum_,[281] n. gen.

_Definition._--#Porodiscida# with five forked chambered arms, without a patagium.

The genus _Pentophiastrum_ differs from the two preceding genera by the bifurcation of the five arms, and can be derived either from _Pentalastrum_ by the ramification of the distal ends of the arms, or from the similar _Myelastrum_ by the increase in the number of arms.

1. _Pentophiastrum dicranastrum_, n. sp.

All five arms equal, with equal angles between them. Each arm in the basal half simple, in the distal half forked; both branches of it equal, with obtuse ends. (This regular species resembles _Dicranastrum furcatum_, Pl. 47, fig. 2, but with five rays instead of four; also the form of the arms is more slender and the edges smooth.)

_Dimensions._--Radius of each arm 0.25, breadth 0.05.

_Habitat._--Equatorial Atlantic, Station 347, depth 2250 fathoms.

Subgenus _Pentophiastromma_, Haeckel.

_Definition._--Arms of different size, one odd arm opposite to the angle between both arm-pairs.

{559}2. _Pentophiastrum caudatum_, n. sp. (Pl. 47, fig. 5).

Arms in pairs different; four arms in the basal half simple, in the distal half forked; the fifth (posterior) odd arm simple, undivided, cylindrical; the anterior pair a little smaller than the posterior; the neighboring branches of the two pairs on each side larger than the two others. Axes of the arms and their branches straight.

_Dimensions._--Radius of the arms about 0.5, breadth 0.12.

_Habitat._--North Atlantic, Station 353, depth 2965 fathoms.

3. _Pentophiastrum forcipatum_, n. sp. (Pl. 47, fig. 4).

Arms in pairs different, all in the basal two thirds simple, in the distal third forked. Only in the posterior (odd) arm both branches are equal, in the four others unequal. The common axis of the posterior lateral pair is horizontal, perpendicular to the median line; the axes of the anterior pair are pincer-like, concavely curved towards the median line or principal axis.

_Dimensions._--Radius of the arms about 0.5, breadth 0.14.

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

Genus 243. _Hexalastrum_,[282] Haeckel, 1881, Prodromus, p. 461.

_Definition._--#Porodiscida# with six simple chambered arms, without a patagium.

The genus _Hexalastrum_, together with the following _Hexinastrum_, encloses those _Euchitonida_ in which the number of the chambered arms surrounding the central disk amounts to six. This is the highest number of these articulated marginal appendages which is reached in any #Discoidea#. Formerly (1881) in my Prodromus, p. 459, I supposed that the same number was reached also by one Coccodiscid, and called this genus _Hexactura_. Afterwards I was convinced that this form was also a _Hexalastrum_.

1. _Hexalastrum palmanthum_, n. sp.

All six arms equal, with equal angles between them. Each arm club-shaped, three times as long as broad, twice as broad at the thickened distal end as at the base, without a terminal spine.

_Dimensions._--Radius of each arm 0.2, basal breadth 0.02, distal breadth 0.06.

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

2. _Hexalastrum crinanthum_, n. sp.

All six arms equal, with equal angles between them. Each arm club-shaped, four times as long as broad, at the egg-shaped distal end three times as broad as in the linear basal part, provided {560}with numerous short conical spines and one longer terminal spine. (Resembles _Stauralastrum rhopalophorum_, Pl. 45, fig. 1, but with six rays instead of four.)

_Dimensions._--Radius of each arm 0.3, basal breadth 0.03, distal breadth 0.08.

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

Subgenus _Hexalastromma_, Haeckel.

_Definition._--Arms more or less different in size or form; shell bilateral.

3. _Hexalastrum orchidaceum_, n. sp. (Pl. 44, fig. 5).

_Hexactura orchidacea_, Haeckel, 1881, Prodromus, p. 459.

Arms different in length, so that two unequal opposite odd arms determine the main axis, and the four other arms lie on both sides of this as two different pairs. The proportion of their relative length is the following:--anterior lateral arms five, anterior odd arm six; posterior lateral arms seven, posterior odd arm eight. Each arm is club-shaped, two to three times as long as broad, and divided into six to eight joints by five to seven transverse septa; its distal end is armed with a terminal spine and twice as broad as its base.

_Dimensions._--Radius of the posterior odd arm 0.4, of the anterior odd arm 0.3; of the posterior lateral pair 0.35, of the anterior lateral pair 0.25; basal breadth 0.08, distal breadth 0.16.

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

Genus 244. _Hexinastrum_,[283] Haeckel, 1881, Prodromus, p. 461.

_Definition._--#Porodiscida# with six simple, undivided, chambered arms, connected by a patagium.

The genus _Hexinastrum_ differs from its ancestral form _Hexalastrum_ by the development of a patagium between the arms. The only observed species is regular.

1. _Hexinastrum geryonidum_, n. sp. (Pl. 44, fig. 4).

_Hexalastrum geryonidum_, Haeckel, 1879, Atlas (pl. xliv. fig. 4).

Disk quite regular with six radii; all six arms of the same size and form, at their broad, convexly rounded, smooth end five times as broad as at their narrow base, and little longer than broad. Each arm is divided by eight transverse septa into nine simple joints or chambers of the same height; the breadth of the distal chambers increases rapidly. The regular, hexagonal, central disk exhibits four concentric rings around the central chamber. Patagium between the arms incomplete, with concavely fluted edge.

_Dimensions._--Radius of each arm 0.15, basal breadth 0.016, distal breadth 0.08; radius of the central disk 0.04.

_Habitat._--Pacific, central area, Station 272, depth 2600 fathoms.

{561}Family XXII. #PYLODISCIDA#, n. fam. (Pl. 48, figs. 12-20).

_Definition._--#Discoidea# without phacoid shell, with flat discoidal shell, in which a simple spherical central chamber is surrounded by one or two concentric triradial girdles; each girdle with three gates, separated by three simple arm-chambers. Surface of the disk with three open or latticed gates on each flat side.

The family #Pylodiscida# represents a new small but interesting group of #Discoidea#, which exhibits rather complex affinities to different groups of #Sphærellaria#. In my Prodromus (1881, p. 464) I had enumerated only two genera of this family, _Triopyle_ and _Hexapyle_, and had united them with _Tetrapyle_ and allied genera in the family Pylonida. Indeed, the resemblance of skeletal structure in the two groups is very great. The most simple forms of both groups exhibit a simple spherical latticed central chamber, which is surrounded by few latticed chambers of similar size and form, separated by open gates. But in the Pylonida these chambers are opposite in pairs, and form together a complete lattice-girdle around the central chamber, whereas in the Pylodiscida the chambers are not opposite in pairs in one axis, and form therefore only latticed half girdles, which arise from the central chamber like radial arms, and may perhaps better be called "arm-chambers"; their number is constantly three. The free open spaces between these three arm-chambers form three gates, comparable to the two or four gates of _Amphipyle_, _Tetrapyle_, &c., and become afterwards closed by lattice-work in a similar way in both groups. A more important difference between them is indicated by the further mode of growth. The Pylonida build new girdles in all three dimensive planes (alternating in the transverse, lateral and sagittal planes); their geometric fundamental form is therefore the "lentellipsis" or the "triaxial ellipsoid." The Pylodiscida, however, grow only at the periphery of the discoidal shell in one single plane (the equatorial plane); their fundamental form is therefore the biconvex lens or the flat disk (a shortened cylinder). This important difference is my deciding motive, in separating the latter from the former and in regarding the Pylodiscida as true #Discoidea#, the more so as they can easily be derived from _Archidiscus_, the fundamental and ancestral form of the Porodiscida.

One single form of _Archidiscus_ seems to be of peculiar importance in this relation, viz., _Archidiscus hexoniscus_ (Pl. 48, fig. 10). In this species the simple central chamber is surrounded by a latticed ring or girdle, composed of six equal chambers of the same size and form, all lying in the same plane with the central chamber. In a nearly allied species, viz., _Archidiscus pyloniscus_, the six ring-chambers are different, three smaller (with denser network) alternating with three larger (of looser network); if we imagine the network of the latter reduced to a marginal bar we get _Triopyle_, and if also this bar disappear by reduction we get _Triolena_, the most simple form of the Pylodiscida. {562}Even _Archidiscus pyloniscus_ may possibly represent the same form among the Porodiscida as _Triodiscus lenticula_ among the Pylodiscida; this important form indicates clearly the close affinity of the two families.

If we take the latter, nearly identical form as the common starting point of both families of Cyclodiscaria, then probably _Triopyle_ and _Triolene_ must be regarded as retrograde forms, derived from _Triodiscus_ in the one case, from _Archidiscus_ in the other, by reduction of three interradial arm-chambers, whilst three perradial only remain. But it is also possible that the most simple form, _Triolene_, originated independently from some _Cenosphæra_, three simple radial chambers, like the latticed central chamber, being derived from the latter by apposition in three equidistant radii, whilst three other radii between them remained free. In this case the other genera of Pylodiscida are derived from their ancestral form _Triolene_.

Adopting this latter view, we find that all eight genera of Pylodiscida, here distinguished, may be regarded as following members of a continuous series. If the three simple arm-chambers of _Triolene_, surrounding the equal central chamber, become united at their distal ends by a concentric equatorial ring, then originates _Triopyle_; and this graduates into _Triodiscus_ by fenestration of the three open gates between the three latticed arms. Whilst these three genera form together the subfamily Triopylida, a second family, Hexapylida, is composed of three other analogous genera, in which the same process of development becomes repeated.

_Pylolena_, the most simple form of Hexapylida, arises from _Triodiscus_ by the development of three new arm-chambers (of the second order) which are apposed at the distal end of the three primary arm-chambers (of the first order) in the same radius. If the distal ends of these three secondary arm-chambers become united by a concentric latticed ring or girdle, we get _Hexapyle_ (with six open gates, two in each radius), and if its six gates become afterwards closed by loose lattice-work, we arrive at _Pylodiscus_ (a repetition of _Triodiscus_).

A third subfamily, Discopylida, is formed by the building of a chambered equatorial girdle around the margin of _Pylodiscus_. This girdle has quite the same structure as the similar chambered rings or girdles of the Porodiscida and Coccodiscida. Between the two sieve-plates of the disk surface is enclosed a variable number (twelve to twenty-four or more) of chambers, imperfectly separated by radial beams, which connect the margin of the _Pylodiscus_-shell with an outer peripheral concentric ring. In _Discozonium_ this marginal ring is perfect, whilst in _Discopyle_ it is interrupted by a peculiar large opening, a "marginal osculum" surrounded by a corona of spines, quite the same remarkable formation which we encountered in _Ommatodiscus_ among the Porodiscida.

All Pylodiscida are therefore triradial (with three perradial arms and three interradial gates between them), and many of them have a great resemblance to certain triradial Porodiscida and Spongodiscida, perhaps not only a morphological resemblance, but also {563}a true phylogenetic relation. But it is remarkable that we do not find further forms of development in this family, by multiplication either of the arm-chambers (further growth in the three perradii) or of the concentric chambered rings (in the periphery of the disk margin).

The central capsule of the Pylodiscida is constantly flat, discoidal, and enclosed between the two sieve-plates of the surface. Its form is either circular or triangular.

_Synopsis of the Genera of the Pylodiscida._

-------------------------------------------------------------------------
I. Subfamily Triopylida.
Three gates between three simple arm-chambers.
-----------------------------------------------------------------------
Three gates open, without a barring equatorial
girdle, 245. _Triolena_.

Three gates barred by a { Gate-faces simple, 246. _Triopyle_.
latticed equatorial {
girdle. { Gate-faces latticed, 247. _Triodiscus_.
-------------------------------------------------------------------------
II. Subfamily Hexapylida.
Six gates between three double arm-chambers (three inner and three
outer); no chambered marginal girdle.
-------------------------------------------------------------------------
Three outer gates open, without a barring equatorial
girdle, 248. _Pylolena_.

{ Both faces of the outer
Three outer gates barred { gates simple, 249. _Hexapyle_.
by a latticed (second) {
equatorial girdle. { Both faces of the outer
{ gates latticed, 250. _Pylodiscus_.
-------------------------------------------------------------------------
III. Subfamily Discopylida.
Six gates between three double arm-chambers.
------------------------------------------------------------------------
{ No peculiar osculum on
In the equatorial plane { the margin of the disc, 251. _Discozonium_.
on the margin of the {
Pylodiscus-shell is a { One pecuiar osculum (with
chambered equatorial { a corona of spines) on
girdle. { the margin of the disc, 252. _Discopyle_.

Subfamily 1. TRIOPYLIDA, Haeckel.

_Definition._--#Pylodiscida# with a simple, spherical or lenticular, central chamber, surrounded by three simple arm-chambers, which are separated by three notches or gates.

Genus 245. _Triolena_,[284] n. gen.

_Definition._--#Pylodiscida# with a simple, spherical or lenticular, central chamber, surrounded by three simple arm-chambers. Notches between the three arms open.

The genus _Triolena_ is the most simple form of all Pylodiscida, and must be regarded as their common ancestral form, from an ontogenetic as well as a phylogenetic point of view. The small shell is composed of a simple, spherical or lenticular, latticed, central chamber, and of three simple, surrounding equal arms, which are also simple latticed chambers, lie in the equatorial plane, and are separated by three equal angles or open gates.

{564}1. _Triolena primordialis_, n. sp. (Pl. 48, fig. 12).

Arm-chambers trapezoid, nearly square, of the same size as the circular, lenticular, primordial, central chamber. Surface of the disk smooth.

_Dimensions._--Diameter of the shell 0.045, of the central chamber 0.015, of each arm 0.015.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

2. _Triolena tribelone_, n. sp.

Arm-chambers lanceolate, of the same breadth as and twice the length of the triangular, central chamber; at the pointed end of each arm is a conical terminal spine (in the equatorial plane). Surface of the disk thorny.

_Dimensions._--Diameter of the shell 0.05, of the central chamber 0.015; length of the arms 0.02, breadth 0.016.

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

3. _Triolena trispinosa_, n. sp.

Arm-chambers ovate, in the basal half nearly as broad as the hexagonal central chamber, at the pointed distal end with a strong conical radial spine of double the length. Surface rough.

_Dimensions._--Diameter of the shell 0.055, of the central chamber 0.02.

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

4. _Triolena hexabelone_, n. sp.

Arm-chambers nearly triangular, at the base half as broad as the hexagonal central chamber, at the truncate distal end one and a half times as broad, and armed with two radial conical spines (in the equatorial plane). Surface of the disk smooth.

_Dimensions._--Diameter of the shell 0.06, of the central chamber 0.02.

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

5. _Triolena trigonalis_, n. sp.

Arm-chambers nearly triangular, at the base half as broad as the circular central chamber, at the concave lunulate distal end twice as broad, and armed with four conical radial spines (two on each side of the equatorial plane). Surface thorny.

_Dimensions._--Diameter of the shell 0.05, of the central chamber 0.015.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

{565}Genus 246. _Triopyle_,[285] Haeckel, 1881, Prodromus, p. 464.

_Definition._--#Pylodiscida# with a simple, spherical or lenticular, central chamber, surrounded by three simple arm-chambers. Notches between the three arms transformed into gates by a connecting equatorial girdle.

The genus _Triopyle_ differs from the preceding _Triolene_ in the development of a simple ring or latticed equatorial girdle, which connects the distal ends of the three arm-chambers, and transforms the open notches between them into three gates. The ring may be circular, triangular, or hexagonal.

1. _Triopyle circulus_, n. sp.

Disk circular, three times as broad as the hexagonal central chamber. Three arm-chambers trapezoidal, at the convex distal end as broad, at the base half as broad as the three circular gates between them. Surface smooth. No marginal spines on the girdle.

_Dimensions._--Diameter of the disk 0.045, of the gates 0.015.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

2. _Triopyle hexagona_, n. sp. (Pl. 48, fig. 13).

Disk hexagonal, three times as broad as the circular central chamber. Three arm-chambers trapezoidal, at the truncated distal end as broad, at the base half as broad as the three triangular gates between them. Surface smooth. No marginal spines.

_Dimensions._--Diameter of the disk 0.05, of the gates 0.02.

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

3. _Triopyle trigona_, n. sp.

Disk triangular, four times as broad as the circular central chamber. Three arm-chambers nearly triangular, at the narrow base half as broad, at the distal end twice as broad as the circular or roundish gates. Surface smooth. On the margin (in the equatorial plane) three strong pyramidal spines (at the end of the arms).

_Dimensions._--Diameter of the disk 0.04, of the gates 0.015.

_Habitat._--Central Pacific, Station 274, depth 2750 fathoms.

4. _Triopyle cordigera_, n. sp.

Disk hexagonal, four times as broad as the hexagonal central chamber. Three arm-chambers triangular, at the base half as broad as at the truncated distal end, about the same size as the {566}three heart-shaped gates. On the six corners of the margin (which forms a regular hexagon) are six pyramidal radial spines, as prolongations of the arm-edges.

_Dimensions._--Diameter of the disk 0.05, of the gates 0.02.

_Habitat._--Indian Ocean, Ceylon, surface, Haeckel.

5. _Triopyle renigera_, n. sp.

Disk hexagonal, five times as broad as the circular central chamber. Three arm-chambers trapezoidal, at the base one-third, at the distal end two-thirds as broad as the three kidney-shaped gates. On the six corners of the margin (which forms an irregular hexagon) six conical radial spines, as prolongations of the arm-edges.

_Dimensions._--Diameter of the disk 0.06, of the gates 0.02.

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

6. _Triopyle spinigera_, n. sp.

Disk roundish, triangular, four times as broad as the triangular central chamber, which is armed with three radial spines between the arms. Arm-chambers club-shaped, at the narrow base one-fourth, at the distal end half as broad as the square gates. On the margin twelve large conical spines, two opposite on each face of the distal end of each arm-edge. Three smaller radial spines on the three corners of the girdle (in the same interradial meridian planes as the three spines of the central chamber). Compare _Triodiscus spinosus_.

_Dimensions._--Diameter of the disk 0.05, of the gates 0.02.

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

Genus 247. _Triodiscus_,[286] n. sp.

_Definition._--#Pylodiscida# with a simple, spherical or lenticular, central chamber, surrounded by three simple arm-chambers. Notches between the three arms closed by lattice-work and by an equatorial girdle.

The genus _Triodiscus_ differs from the preceding _Triopyle_ in the development of loose lattice-work on both sides of the discoidal shell. This network closes the gates and transforms the whole shell into a fenestrated lens. The singular species of _Triodiscus_ correspond to certain species of _Triopyle_.

1. _Triodiscus lenticula_, n. sp.

Disk circular, lenticular, three times as broad as the central chamber. Three arm-chambers trapezoidal, of the same size and form as the three gates between them, which are closed by a loose {567}delicate network (differs from _Triopyle circulus_ by the production of the two convex latticed plates, which envelop the whole lens). Margin smooth.

_Dimensions._--Diameter of the disk 0.045, of the gates 0.015.

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

2. _Triodiscus trigonus_, n. sp.

Disk triangular, four times as broad as the central chamber. Three arm-chambers at the base half as broad, at the distal end twice as broad as the roundish gates. Surface smooth. On the three corners of the margin (in the arm-radius) three strong spines. (Differs from _Triopyle trigona_ only in the loose framework closing the gates.)

_Dimensions._--Diameter of the disk 0.04, of the gates 0.015.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

3. _Triodiscus spinosus_, n. sp. (Pl. 48, fig. 14).

Disk subcircular, four times as broad as the triangular central chamber. Three arm-chambers club-shaped, at the base one-third, at the distal end two-thirds as broad as the semicircular gates. Surface thorny. On the margin fifteen larger radial spines, three on the corners of the disk (in the radius of the gates), twelve on the two faces of the arms ends (two opposite on the edge of each end.) (Differs from _Triopyle spinigera_ mainly by the delicate hexagonal network closing the gates.)

_Dimensions._--Diameter of the disk 0.05, of the gates 0.02.

_Habitat._--North Atlantic, Canary Islands (Lanzerote, Haeckel).

Subfamily 2. HEXAPYLIDA, Haeckel.

_Definition._--#Pylodiscida# with triopyle-shaped medullary shell, surrounded by three distal arm-chambers, which are separated by three open notches or gates.

Genus 248. _Pylolena_,[287] n. gen.

_Definition._--#Pylodiscida# with triopyle-shaped medullary shell, surrounded by three distal arm-chambers. Notches between the three arms open.

The genus _Pylolena_ opens the series of the Hexapylida, or of those Pylodiscida in which the centre of the shell is formed by a tri-radiated medullary shell like _Triopyle_. In the equatorial plane of this triopyle-shaped disk are developed on its margin three distal arm chambers, as prolongations of the three arms of _Triopyle_, but much larger. In _Pylolene_ the three angles or notches between the distal arms remain open, repeating the form of _Triolene_.

{568}1. _Pylolena inermis_, n. sp.

Arms club-shaped, at the base half as broad, at the rounded distal end as broad as the triopyle-shaped medullary shell. Surface and margin smooth.

_Dimensions._--Diameter of the cortical shell 0.15, of the medullary shell 0.05.

_Habitat._--Equatorial Atlantic, Station 347, surface; Central Pacific, Station 272, surface.

2. _Pylolena armata_, n. sp. (Pl. 48, fig. 15).

Arms trapezoid, at the base half as broad, at the distal end twice as broad as the triopyle-shaped medullary shell. Surface and margin thorny. Twelve strong conical spines at the distal end of the three arms, two opposite on both faces of the arm-edges.

_Dimensions._--Diameter of the cortical shell 0.2, of the medullary shell 0.06.

_Habitat._--South Atlantic, Station 325, surface.

Genus 249. _Hexapyle_,[288] Haeckel, 1881, Prodromus, p. 464.

_Definition._--#Pylodiscida# with triopyle-shaped medullary shell, surrounded by three distal arm-chambers. Notches between the three arms transformed into gates by a connecting equatorial girdle.

The genus _Hexapyle_ differs from the preceding _Pylolene_ in the development of an equatorial ring or latticed girdle, which connects the free extremities of the three distal ends and transforms the open notches between them into gates. It simulates therefore the formation of _Triopyle_, from which it differs by duplication of the arm-joints and of the gates (in each radius occur one proximal and one distal gate).

1. _Hexapyle triangula_, n. sp.

Cortical shell triangular, with three rounded corners, three times as broad as the triangular triopyle-shaped medullary shell. Surface smooth or rough, but not spiny. Three arms two-thirds as broad as the three egg-shaped gates of each side.

_Dimensions._--Diameter of the cortical shell (or length of one side of the triangle) 0.15, of the medullary shell 0.05; breadth of the gates 0.06, of the bridges between them 0.04.

_Habitat._--Pacific, central area, Station 272, depth 2600 fathoms.

2. _Hexapyle sexangula_, n. sp.

Cortical shell hexagonal, with six equal sides, four times as broad as the triangular triopyle-shaped medullary shell. Surface rough, but not spiny. Three arms (on their smallest part) half as broad as the three triangular gates (on their broadest part).

{569}_Dimensions._--Diameter of the cortical shell 0.16, of the medullary shell 0.04; breadth of the gates 0.06, of the bridges 0.03.

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

3. _Hexapyle circularis_, n. sp.

Cortical shell circular, four times as broad as the triangular, Triopyle-shaped medullary shell. Surface smooth. Three arms half as broad as the three kidney-shaped gates on each side of the shell.

_Dimensions._--Diameter of the cortical shell 0.2, of the medullary shell 0.05; breadth of the gates 0.08, of the bridges between them 0.04.

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

4. _Hexapyle triacantha_, n. sp.

Cortical shell triangular, thorny, with three pointed corners, prolonged into three strong conical radial spines; its diameter four times as great as that of the medullary shell. Three arms of the same breadth as the three egg-shaped gates of each side.

_Dimensions._--Diameter of the cortical shell (or length of one side of the triangle, without spines) 0.2, of the medullary shell 0.05; breadth of the gates and of the bridges between them 0.06.

_Habitat._--Indian Ocean, surface; Madagascar, Rabbe.

5. _Hexapyle hexacantha_, n. sp.

Cortical shell hexagonal, thorny, three times as broad as the medullary shell. Six stronger conical radial spines on the six corners at equal distances, lying in the equatorial plane of the disk. Three half girdles half as broad as the three triangular gates of each side.

_Dimensions._--Diameter of the cortical shell 0.15, of the medullary shell 0.05; breadth of the gates 0.05, of the bridges 0.025.

_Habitat._--North Pacific, Station 253, depth 3125 fathoms.

6. _Hexapyle dodecantha_, n. sp. (Pl. 48, fig. 16).

Cortical shell triangular, thorny, with rounded corners, three and a half times as broad as the medullary shell. Twelve stronger radial spines, six opposite in pairs on each side of the discoidal shell, as prolongations of the lateral edges of the triangular gates, which are about the same breadth as the half girdles between them.

_Dimensions._--Diameter of the cortical shell 0.21, of the medullary shell 0.06; breadth of the gates and of the bridges 0.06.

_Habitat._--Pacific, central area, Station 270, surface.

{570}7. _Hexapyle polyacantha_, n. sp.

Cortical shell circular, thorny, with a circle of numerous conical radial spines in the periphery; its diameter nearly three times as great as that of the medullary shell. Three half-girdles half as broad as the three kidney-shaped gates on each side of the discoidal shell.

_Dimensions._--Diameter of the cortical shell 0.11, of the medullary shell 0.04; breadth of the gates 0.04, of the bridges between them 0.02.

_Habitat._--Pacific, central area, Station 271, depth 2425 fathoms.

Genus 250. _Pylodiscus_,[289] n. gen.

_Definition._--#Pylodiscida# with _Triopyle_-shaped medullary shell, surrounded by three distal arm-chambers. Notches between the three arms closed by lattice-work and by an equatorial girdle.

The genus _Pylodiscus_ differs from _Hexapyle_ in the development of two convex plates of lattice-work, which close the six open gates and transform the disk into a biconvex fenestrated lens. _Pylodiscus_ simulates therefore the characteristic form of _Triodiscus_; but whilst in the latter we find only three arm-joints and three simple gates, their number is doubled in the former.

1. _Pylodiscus triangularis_, n. sp. (Pl. 48, fig. 17).

Cortical shell triangular, with rounded corners, three times as broad as the triangular, _Triopyle_-shaped medullary shell. Three arms about as broad as the three egg-shaped gates between them. (Differs from _Hexapyle triangula_ only by the delicate network of both triangular convex covering plates, which close the open gates.) Six larger and many smaller spines on the margin of the disk.

_Dimensions._--Diameter of the disk 0.15, of the gates 0.05.

_Habitat._--Pacific, central area, Station 270, depth 2925 fathoms.

2. _Pylodiscus sexangularis_, n. sp.

Cortical shell hexangular, equilateral, four times as broad as the circular, _Triopyle_-shaped medullary shell. Surface spiny. Three arms nearly of the same form and size as the three triangular gates between them. (Similar to _Hexapyle sexangularis_, but differing in the irregular loose network closing the gates.)

_Dimensions._--Diameter of the disk 0.18, of the gates 0.06.

_Habitat._--North Pacific, south of Japan, Station 237, surface

{571}3. _Pylodiscus cardiopylus_, n. sp.

Cortical shell hexangular, equilateral, five times as broad as the triangular, _Triopyle_-shaped medullary shell. Surface smooth. Three arms triangular, at the distal end as broad as the three heart-shaped gates. On the margin six strong pyramidal spines in the equatorial plane (adradial, as prolongations of the lateral arm-edges).

_Dimensions._--Diameter of the disk 0.15, of the gates 0.05.

_Habitat._--Indian Ocean, Sunda Strait, Rabbe.

4. _Pylodiscus nephropylus_, n. sp.

Cortical shell circular, three times as broad as the circular, _Triopyle_-shaped medullary shell. Surface spiny. Three arms trapezoidal, half as broad as the three kidney-shaped gates. On the margin twelve stronger, conical, radial spines, opposite in pairs on both sides of the disk, disposed in six pairs at the distal end of the arms.

_Dimensions._--Diameter of the disk 0.2, of the medullary shell 0.06.

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

Subfamily 3. DISCOPYLIDA, Haeckel.

_Definition._--#Pylodiscida# with _Triopyle_-shaped medullary shell and _Pylodiscus_-shaped cortical shell, which is surrounded by a marginal chambered equatorial girdle.

Genus 251. _Discozonium_,[290] n. gen.