4. Stauralastrum clavigerum, n. sp.
Arms twice as long as broad, in their distal half lenticular, nearly circular, twice as broad as in their square proximal half; their distal breadth equals the diameter of the central disk, which exhibits three to four rings. Edges of the arms concave.
Dimensions.—Radius of each arm 0.2, basal breadth 0.04, distal breadth 0.08.
Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.
5. Stauralastrum dilatatum, n. sp.
Arms of nearly equal length and breadth, at their convex distal end three times as broad as at their narrow base; their distal breadth three times as large as the radius of the central disk, which exhibits three to four rings. (Resembles Histiastrum quadrigatum, Pl. 46, fig. 3, but has no patagium.) Edges of the arms concave.
Dimensions.—Radius of each arm 0.15, basal breadth 0.04, terminal breadth 0.12.
Habitat.—South Pacific, Station 300, depth 1375 fathoms.
Definition.—Ends of the arms with one or more terminal spines.
6. Stauralastrum rhopalophorum, n. sp. (Pl. 45, fig. 1).
Hagiastrum rhopalophorum, Haeckel, 1881, Prodromus, p. 460.
Arms cylindrical, eight times as long as broad at their base, at their distal end club-shaped, three times as broad as at their base; their distal breadth twice as large as the diameter of the central disk, which exhibits two to three rings. Surface thorny, with larger spines towards the end, and one radial, very strong, angular terminal spine. Edges of the arms parallel.
Dimensions.—Radius of each arm (without terminal spine) 0.32, basal breadth 0.03, terminal breadth 0.1.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
7. Stauralastrum antiquum, n. sp.
Rhopalastrum sp. Bury, 1862, Polycystins of Barbados, pl. xiv. fig. 5.
Arms six times as long as broad at their base, in their distal half nearly spherical, three times as broad as in their cylindrical basal half; their distal breadth nearly equals the diameter of the central disk which exhibits three to four rings. On the end of each arm one strong, angular, terminal spine. (Differs from the preceding species by larger central disk and stouter arms, also by less developed spines.) Edges of the arms parallel.
Dimensions.—Radius of each arm 0.25, basal breadth 0.04, terminal breadth 0.12.
Habitat.—Fossil in the Barbados rocks; and living in the depths of the Central Pacific, Station 266, depth 2750 fathoms.
8. Stauralastrum staurolonche, n. sp.
Arms four times as long as broad at their base, gradually increasing towards their truncated end, which is one and a half times as broad as their base; their distal breadth equals the radius of the central disk, which exhibits four to five rings. At the end of each arm is a very strong conical terminal spine. (Resembles Histiastrum quaternarium, Abhandl. k. Akad. Wiss. Berlin, 1875, Taf. xxiv. fig. 3, but has no patagium.) Edges of the arms rectilinear, divergent.
Dimensions.—Radius of each arm 0.25, basal breadth 0.045, distal breadth 0.07.
Habitat.—Fossil in the Barbados rocks; and living in the depth of the Equatorial Atlantic, Station 348, depth (2450) fathoms.
9. Stauralastrum horridum, n. sp.
Arms three times as long as broad at their base, gradually increasing towards their rounded end, which is twice as broad as their base, their distal breadth equals the diameter of the central disk, which exhibits four to five rings. Surface thorny, at the distal end of each arm is a group of twenty to twenty-five smaller and five to six larger, straight, conical spines. Edges of the arms rectilinear, divergent.
Dimensions.—Radius of each arm 0.15, basal breadth 0.05, distal breadth 0.1.
Habitat.—Pacific, central area, Station 270, depth 2925 fathoms.
Definition.—Porodiscida with four simple, undivided, chambered arms, without 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 Hagiastrum, as here defined, was formerly united by me with the foregoing Stauralastrum, but differs from it by the bilateral or symmetrical form. Whilst in the latter all four arms and the four angles between them are equal, they are here differentiated into pairs.
Definition.—Both longitudinal arms of equal size and form.
1. Hagiastrum buddhae, n. sp. (Pl. 45, fig. 5).
Cross rectangular. Both longitudinal arms of equal size, twice as long as the transverse arms; all arms smooth, club-shaped, twice as broad at their globose distal part as at their base, each with three large conical terminal spines.
Dimensions.—Radius of the principal arms 0.4, of the lateral arms 0.2; basal breadth 0.06, distal breadth 0.12.
Habitat.—Indian Ocean, Belligemma, Ceylon, surface, Haeckel.
2. Hagiastrum bramae, n. sp.
Cross rectangular. Both longitudinal arms of equal size, one and a half times as long as the transverse arms; all arms thorny, club-shaped, at their pear-shaped distal part three times as broad as at their base, provided with numerous conical spines, one larger terminal spine at their distal point. (Resembles Stauralastrum rhopalophorum, Pl. 45, fig. 1, but is distinguished by the different size of the arm-pairs, and by the stronger spines.)
Dimensions.—Radius of the principal arms 0.3, of the lateral arms 0.2; basal breadth 0.04, distal breadth 0.12.
Habitat.—Indian Ocean, off Maldive Islands, surface, Haeckel.
Definition.—The two longitudinal arms different in size or form.
3. Hagiastrum mosis, n. sp. (Pl. 45, fig. 3).
Cross rectangular. All four arms nearly isosceles, triangular, at their narrow base half as broad as at their truncated, concavely fluted, distal end. The posterior principal arm with twelve to thirteen joints, twice as long as the anterior arm, which has six to seven joints and is one and a half times as long as the two lateral arms (with four to five joints).
Dimensions.—Radius of the posterior arm 0.3, of the anterior 0.15, of the lateral arms 0.1; basal breadth 0.05, terminal breadth 0.1.
Habitat.—Mediterranean (Smyrna), surface, Haeckel.
4. Hagiastrum mohammedis, n. sp.
Cross with unequal angles, the anterior little smaller than the posterior. All four arms club-shaped, thorny, three times as broad at their globose distal end as at their narrow base, and furnished with ten to twelve conical spines. Posterior principal arm twice as long as the anterior, and four times as long as the rudimentary lateral arms.
Dimensions.—Radius of the posterior arm 0.4, of the anterior 0.2, of the lateral arms 0.1; basal breadth 0.02 to 0.03, distal breadth 0.06 to 0.1.
Habitat.—Philippine Sea, Samboangan, Station 200, surface.
5. Hagiastrum christi, n. sp.
Cross with unequal angles, the anterior somewhat smaller than the posterior. All four arms of similar form, lanceolate, in their middle twice as broad as at either obtuse end. The posterior principal arm with twelve joints, one and a half times as long as the anterior (with nine joints) and twice as long as the two lateral arms (each with six joints). The form and structure of the arms in this species are nearly the same as in Tessarastrum straussi (Pl. 45, fig. 8); but the arms are broader in the middle, and are not connected by a patagium.
Dimensions.—Radius of the principal posterior arm 0.2, of the anterior 0.15, of each lateral arm 0.1; greatest breadth (in the width) 0.01, basal breadth 0.03.
Habitat.—North Atlantic, Færöe Channel, Gulf Stream, surface, John Murray.
Definition.—Porodiscida with four simple, undivided, chambered arms, connected by a patagium; square shell a regular cross, with four equal arms and four right angles between them.
The genus Histiastrum, quite insufficiently characterised by Ehrenberg (1847), was afterwards (1875) illustrated by the figures of two different fossil species. One of these, Histiastrum ternarium, with three arms, belongs to Hymeniastrum; the other, Histiastrum quaternarium, is here retained as the true, typical representative species of the genus. It differs from its ancestral form Stauralastrum, by the possession of a patagium, from Tessarastrum by the regular square form of the shell.
Definition.—Distal ends of the arms blunt, without terminal spines.
1. Histiastrum quadrigatum, n. sp. (Pl. 46, fig. 3).
Arms at their distal end nearly as broad as long, and four times as broad as at their narrow base; their lateral edges concave, their terminal edge convex, without spines. Each arm is divided by seven to eight convex transverse septa into eight to nine simple, broad chambers. Central disk with three to four rings, about as broad as the fifth chamber. Patagium complete, connecting all the lateral edges of the arms.
Dimensions.—Radius of each arm 0.15, basal breadth 0.03, terminal breadth 0.12.
Habitat.—Equatorial Atlantic, Station 347, surface.
2. Histiastrum excisum, n. sp.
Arms four times as long as broad at their base, and twice as broad at their rounded blunt distal end as at their base; their lateral edges rectilinear, divergent. Central disk with three to four rings, somewhat broader than their distal end. Patagium incomplete, connecting only the basal half of the arms, with three to four concave chamber-rows, on the margin concave. (May be regarded as Euchitonia mülleri, with four arms.)
Dimensions.—Radius of each arm 0.2, basal breadth 0.05, distal breadth 0.1.
Habitat.—Atlantic, surface, Canary Islands.
3. Histiastrum velatum, n. sp. (Pl. 46, fig. 4).
Dictyastrum velatum, Haeckel, 1879, Atlas (pl. xlvi. fig. 4).
Arms pear-shaped, rapidly increasing from their narrow base, nearly circular, little longer than broad; each with eight to nine transverse chamber-rows; their lateral edges at their base concave, at their end circular. Central disk with three to four rings, somewhat smaller than one arm. Patagium complete, with six to seven radial beams, filling out perfectly the intervals between the arms. A peculiar girdle of finer network and equal breadth surrounds the whole equatorial periphery of the disk, and gives it the appearance of a square with rounded corners.
Dimensions.—Radius of each arm 0.2, basal breadth 0.03, distal breadth 0.12.
Habitat.—South Atlantic, Station 330, surface.
Definition.—Distal ends of the arms spiny, furnished with one or more terminal spines.
4. Histiastrum quaternarium, Ehrenberg.
Histiastrum quaternarium, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 74, Taf. xxiv. figs. 3, 4.
Arms six times as long as broad at their base, with rectilinear, little divergent edges; at their truncated distal end a little broader, with one single, very strong, conical, terminal spine. Central disk with four to five rings, somewhat broader than the arm. Patagium incomplete, enveloping only the basal half of the arms.
Dimensions.—Radius of each arm 0.18, basal breadth 0.03, distal breadth 0.05.
Habitat.—Fossil in the rocks of Barbados.
5. Histiastrum gladiatum, n. sp.
Astromma sp., Bury, 1862, Polycystins of Barbados, pl. v. fig. 1.
Arms triangular, eight times as long as broad at their base; at their distal end three times as broad as at their base, with rectilinear, divergent edges; their truncated end with a large conical, terminal spine. Central disk with four to five rings, broader than the arms. Patagium incomplete, enveloping only the basal half of the arms.
Dimensions.—Radius of each arm 0.12, basal breadth 0.013, distal breadth 0.04.
Habitat.—Fossil in the rocks of Barbados; and living in the depth of the Central Pacific, Station 268, depth 2900 fathoms.
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 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.
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.
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, 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.
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.
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).
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.
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.
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 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.
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.
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.
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.
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.
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.
Definition.—Each cross-arm with four terminal branches, the two fork-branches being again bifurcated.
8. Dicranastrum bifurcatum, n. sp. (Pl. 47, figs. 1, 1a).
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. 1a) 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.
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 thin disk it nevertheless reaches unusual dimensions, its diameter in some species being more than a millimetre.
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.
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 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, 11a).
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. 11a).
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.