1. Triplagia primordialis, n. sp. (Pl. 91, fig. 2).

Spines straight, of equal size and similar form, equidistant, three-sided prismatic, each with two pairs of opposite lateral branches, which are correspondingly parallel to the two other spines; the proximal branches twice as long as the distal branches.

Dimensions.—Length of each spine 0.2, of the basal branches 0.07.

Habitat.—Central Pacific, Station 271, depth 2425 fathoms.

2. Triplagia triradialis, n. sp.

Spines straight, of equal size and similar form, equidistant, three-sided prismatic, regularly pinnate, with six pairs of opposite pinnulæ, tapering gradually towards the distal ends; the basal pinnulæ are again branched, with straight ramules.

Dimensions.—Length of each spine 0.27, of the basal branches 0.08.

Habitat.—North Pacific, Station 256, surface.

3. Triplagia horizontalis, n. sp.

Spines curved, cylindrical, irregularly branched, with three primary and nine to twelve secondary branches, which are also slightly curved. The angles between the bases of the three spines are equal, but the size and form of the branches different.

Dimensions.—Length of the three spines 0.15 to 0.25, of the basal branches 0.05 to 0.08.

Habitat.—West Tropical Pacific, Station 226, depth 4475 fathoms.

Genus 385. Plagiacantha,^[4] Claparède, 1856, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, November 13.

Definition.—Plagonida with three radial spines, arising from one common central point and corresponding to the edges of a three-sided pyramid.

The genus Plagiacantha agrees with the preceding Triplagia in the simple structure of the triradial skeleton, composed of three diverging radial spines, which are united in a common centre at the oral pole of the central capsule. But whilst the three radial rods of Triplagia lie in one horizontal plane, here they lie in different planes and correspond to the three edges of a flat pyramid. Plagiacantha arachnoides, described in 1856 by Claparède, was the earliest known form of all Plectoidea.

1. Plagiacantha arachnoides, Claparède.

Spines straight, cylindrical, divided into three divergent straight branches of equal size; each branch two to three times as long as the simple thicker basal part.

Dimensions.—Length of the spines 0.12 to 0.2 of the branches 0.08 to 0.14.

Habitat.—North Atlantic, coast of Norway, Claparède, surface.

2. Plagiacantha furcata, n. sp.

Spines straight, cylindrical, divided into two divergent straight branches of equal size, of about the same length as the simple basal part. The spines and their branches are smooth.

Dimensions.—Length of the spines 0.24, of their fork-branches 0.12.

Habitat.—North Pacific, Station 254, surface.

3. Plagiacantha dodecantha, n. sp.

Spines straight, three-sided prismatic, in the middle with one verticil composed of three divergent straight branches of equal size, of about the same length as the simple basal part. The edges of the spines and their branches are thorny.

Dimensions.—Length of the spines 0.24, of the branches 0.1.

Habitat.—Central Pacific, Station 274, surface.

4. Plagiacantha abietina, Richard Hertwig.

Spines straight, three-sided prismatic, with three verticils of three divergent straight lateral branches, decreasing in size towards the distal end; the branches of the first verticil about twice as long as those of the second, and four times as long as those of the third verticil. All nine branches of each spine simple, straight, three-sided pyramidal.

Dimensions.—Length of the spines 0.2 to 0.3, of the basal branches 0.06 to 0.08.

Habitat.—Mediterranean, Messina (R. Hertwig), surface.

5. Plagiacantha verticillata, n. sp.

Spines curved, three-sided prismatic, with six to nine verticils of three divergent branches, decreasing in size towards the distal end; the branches of the first verticil about twice as long as those of the fourth verticil. All branches simple, slightly curved, bristle-shaped.

Dimensions.—Length of the spines 0.27, of the basal branches 0.12.

Habitat.—South Pacific, Station 296, surface.

6. Plagiacantha elatine, n. sp.

Spines straight, three-sided prismatic, with prominent wing-like edges and ten to twelve verticils of three divergent branches, decreasing in size towards the distal end; the branches of the three or four basal verticils are again ramified and much larger than the simple branches of the distal verticils.

Dimensions.—Length of the spines 0.36, of the basal branches 0.14.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

Subfamily 2. Tetraplagida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with four radial spines.

Genus 386. Tetraplagia,^[5] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with four equal radial spines, arising from one common central point, and corresponding to the four axes of a tetrahedron.

The genus Tetraplagia is one of the most important Plectoidea, and possibly the common ancestral form of many Nassellaria (compare above, p. 901). The skeleton is composed of four radial rods, diverging from one common centre in different directions, and corresponding to the four axes, which extend from the central point of a tetrahedron to the central points of its four faces. The whole skeleton of Tetraplagia exhibits therefore the same form, which is observed in the single quadriradiate spicula of some Beloidea (in some species of Lampoxanthium, Sphærozoum, &c.).

1. Tetraplagia geometrica, n. sp.

Spines straight, cylindrical, perfectly equal, corresponding in regular disposition exactly to the four geometrical axes of a regular tetrahedron; in the middle arise from each spine two opposite equal lateral branches of half the length of the spine.

Dimensions.—Length of the spines 0.12, of the branches 0.06.

Habitat.—Central Pacific, Station 266, surface.

2. Tetraplagia phænaxonia, n. sp. (Pl. 91, fig. 3).

Spines straight or slightly curved, three-sided prismatic, with irregular short branches arising from the three edges; the branches are thorny, tapering towards the apex.

Dimensions.—Length of the spines 0.15, of the basal branches 0.03.

Habitat.—Tropical Atlantic, Station 332, surface.

3. Tetraplagia abietina, Haeckel.

Spines straight, three-sided prismatic, verticillate, with six to eight verticils of three simple straight branches; the branches of each edge are parallel, tapering towards the distal end. R. Hertwig regards this species only as a four-spined variety of his three-spined Plagiacantha abietina; but a specimen, observed by me in Corfu, exhibited all the characters of Tetraplagia.

Dimensions.—Length of the spines 0.2, of the basal branches 0.07.

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

Genus 387. Plagoniscus,^[6] n. gen.

Definition.—Plagonida with four unequal radial spines, arising from one common central point; one vertical or apical spine opposed to three divergent or basal spines.

The genus Plagoniscus agrees with the preceding Tetraplagia (its probable ancestral form) in the possession of four radial spines, diverging from one common central point. But whilst in this latter all four spines are equal, corresponding exactly to the four axes of a tetrahedron, here in Plagiocarpa an important difference exists between one vertical or apical spine and three other divergent basal spines; these latter corresponding probably to the three "feet," the former to the single "apical horn" of the majority of Nassellaria. Perhaps we find here one of the oldest and simplest types of their "triradial or cortinar structure" (compare above, p. 902).

1. Plagoniscus tripodiscus, n. sp. (Pl. 91, fig. 4).

Spines three-sided prismatic, thorny. Apical spine nearly straight, verticillate, with four to five verticils of three thorny branches, tapering towards the apex. Three basal spines somewhat shorter, curved, with three thorny edges.

Dimensions.—Length of the apical spine 0.2, of the basal spines 0.15.

Habitat.—Central Pacific, Station 263, surface.

2. Plagoniscus euscenium, n. sp.

Spines three-sided prismatic, thorny, with dentate edges, and three to six verticils of three short branches. Apical spine straight, with six verticils, nearly twice as long as the three curved basal spines, each of which bears three verticils; the basal verticils larger and ramified. Resembles somewhat Euscenium eucolpium, Pl. 53, fig. 12, but has no latticed shell.

Dimensions.—Length of the apical spine 0.3, of the basal spines 0.16.

Habitat.—North Pacific, Station 247, surface,

3. Plagoniscus cortinaris, n. sp.

Spines three-sided prismatic, straight, verticillate. Apical spine with twelve verticils, one and a half times as long as the three basal spines, each of which bears eight verticils of three branches. The branches are also prismatic, straight, on each edge parallel, tapering towards the apex, in the three basal spines forked, in the apical spine more branched. Similar to the cortinar skeleton of Clathrocorys (Pl. 64, figs. 8-10).

Dimensions.—Length of the apical spine 0.24, of the basal spines 0.16.

Habitat.—South Atlantic, Station 338, surface.

4. Plagoniscus nassellaris, n. sp.

Spines cylindrical, curved, irregularly branched. Apical spine half as long as, and less branched than the three basal spines, which are sigmoidal, nearly horizontally expanded in the proximal half, descending in the distal half.

Dimensions.—Length of the apical spine 0.1, of the three basal spines 0.18.

Habitat.—West Tropical Pacific, Station 224, surface.

Genus 388. Plagonidium,^[7] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with four equal radial spines, arising in pairs from the two poles of a common central rod.

The genus Plagonidium, and the following Plagiocarpa possess four radial spines, like the two preceding genera. But whilst the four rods in these latter arise from a common centre, here they arise in two pairs from the two poles of a common middle rod; they have therefore exactly the same form which we find in the single spicula of some Beloidea (e.g., Thalassoxanthium furcatum, Sphærozoum furcatum, &c.). Probably the middle rod is horizontal and serves as supporting base for the central capsule, whilst two opposite spines are directed upwards, two other downwards.

1. Plagonidium bigeminum, n. sp.

Spines straight, three-sided prismatic, four to six times as long as the common central rod, pinnate, with four to five pairs of opposite pinnulæ, the distal of which are simple, the proximal again branched.

Dimensions.—Length of the spines 0.16, of the middle rod 0.032.

Habitat.—Indian Ocean, Sunda Strait (Rabbe), surface.

2. Plagonidium quadrigeminum, n. sp.

Spines cylindrical, curved, eight to ten times as long as the common central rod, in the distal half forked; the fork-branches curved, somewhat longer than the basal part.

Dimensions.—Length of the spines 0.18, of the middle rod 0.02.

Habitat.—Antarctic Ocean, Kerguelen Island, Station 159, surface.

Genus 389. Plagiocarpa,^[8] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with four unequal radial spines, arising in pairs from the two poles of a common central rod; one ascending apical spine opposed to three descending basal spines.

The genus Plagiocarpa agrees with the preceding Plagonidium in the possession of a common horizontal middle rod, the two poles of which bear two pairs of divergent spines; but whilst in the preceding all four spines are equal, here they are differentiated in the same manner as in Plagoniscus, which differs only in the absence of the middle rod. The two observed and closely allied species of this genus are of peculiar interest, since they belong possibly to the common ancestral forms of the Nassellaria; the basal middle rod corresponds perhaps to the basal part of a sagittal ring, the apical spine to its dorsal part, the three other spines to the basal feet (compare above, p. 902).

1. Plagiocarpa procortina, n. sp. (Pl. 91, fig. 5).

Spines cylindrical, curved, thorny, three to four times as long as the common middle rod. Apical spine and the meeting caudal spine (or the posterior basal spine) somewhat longer and with more numerous thorns than the two paired pectoral (or anterior) spines. From the common base of the latter arises an anterior prolongation of the horizontal middle rod, which in the sagittal plane is curved upwards and corresponds to the sternal foot of many Cyrtellaria. An ascending branch of this spine is opposed to a descending branch of the apical spine, both together forming an incomplete sagittal ring.

Dimensions.—Length of the two larger spines 0.15, of the two smaller 0.12.

Habitat.—Mediterranean (Portofino near Genoa), surface.

2. Plagiocarpa procyrtella, n. sp.

Spines of form and arrangement similar to those of the preceding species, but longer and more branched, six to eight times as long as the shorter common middle rod. The two characteristic opposed branches (the ascending branch of the basal rod and the descending branch of the apical spine), which in the preceding species nearly compose a sagittal ring, are here absent.

Dimensions.—Length of the two larger 0.27, of the two smaller 0.21.

Habitat.—North Atlantic, Iceland (Steenstrup), surface.

Subfamily 3. Hexaplagida, Haeckel.

Definition.—Plagonida with six radial spines.

Genus 390. Hexaplagia,^[9] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with six radial spines, arising from one common central point.

The genus Hexaplagia differs from the preceding genera of Plagonida in the possession of six radial spines, diverging from one common central point. Commonly, these six spines seem to lie opposite in pairs in three different meridian planes, and in this case Hexaplagia may be regarded as a Plagiacantha, the three radial spines of which are prolonged over the basal pole of the central capsule. But in other species the six spines seem to lie in different planes. Further observations are required.

1. Hexaplagia arctica, n. sp.

Spines opposite in pairs in three diameters, crossed in the common centre, straight, three-sided prismatic, with dentate edges. The three pairs are equal, but the upper spine of each pair only half as long as the lower spine.

Dimensions.—Length of the upper spine 0.13, of the lower 0.27.

Habitat.—Arctic Ocean (Greenland), in the stomach of a Medusa (Olrik).

2. Hexaplagia antarctica, n. sp.

Spines opposite in pairs in three diameters, crossed in the common centre, slightly curved, three-sided prismatic, with verticillate branches; the three pairs are equal, but the lower spine of each pair bears three to four verticils, each of three branches, and is two to three times as long as the upper spine, which bears two verticils only.

Dimensions.—Length of the upper spine 0.12, of the lower 0.3.

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

3. Hexaplagia collaris, n. sp.

Spines in opposite pairs in three diameters, crossed in the common centre, straight, cylindrical, pinnate, with three to four pairs of opposite lateral branches or regular pinnulæ. All six spines are equidistant, of equal size and similar form, and lie with their branches nearly in one horizontal plane. Therefore they are similar to the six radial rods in the collar septum of many Spyroidea and Cyrtoidea (e.g., Calpophæna, Pl. 53, fig. 18).

Dimensions.—Length of all six spines 0.18, of their basal pinnulæ 0.04.

Habitat.—East of New Zealand, Station 169, surface.

4. Hexaplagia australis, n. sp.

Spines unequal, at unequal intervals, not opposite in pairs, cylindrical, irregularly branched and curved. In this species a definite arrangement of the six different spines could not be detected.

Dimensions.—Length of the spines 0.2 to 0.3, of their largest branches 0.1.

Habitat.—South of Australia, Station 160, surface.

Genus 391. Plagonium,^[10] Haeckel, 1881, Prodromus, p. 423.

Definition.—Plagonida with six radial spines, arising in two opposite divergent groups from the two poles of a common central rod.

The genus Plagonium differs from the preceding closely allied Hexaplagia in the remarkable peculiarity, that the six radial spines do not arise from one common central point, but from the two poles of a common horizontal middle rod; three divergent spines on each pole. The skeleton of Plagonium exhibits therefore the same remarkable form which is found in the isolated spicula of numerous Beloidea (e.g., Lampoxanthium punctatum, Sphærozoum punctatum), and bears the same relation to Hexaplagia that Plagonidium does to Tetraplagia.

1. Plagonium sphærozoum, n. sp. (Pl. 91, fig. 6).

Spines straight and stout, three-sided prismatic, about twice as long as the common middle rod, irregularly branched or nearly verticillate, with short thorny branches. Similar to the single spicula of some species of Sphærozoum and Lampoxanthium.

Dimensions.—Length of the spines 0.12, of the middle rod 0.06.

Habitat.—Equatorial Atlantic, Station 347, surface.

2. Plagonium lampoxanthium, n. sp.

Spines irregularly curved, slender, cylindrical, six to eight times as long as the common middle rod, in the proximal half smooth, in the distal half covered with short thorns. (Similar to an isolated spiculum of Lampoxanthium punctatum or of Sphærozoum variabile, Pl. 4, fig. 5.)

Dimensions.—Length of the spines 0.18, of the middle rod 0.03.

Habitat.—North Pacific, Station 240, surface.

3. Plagonium arborescens, n. sp.

Spines irregularly curved and branched, slender, cylindrical, thorny, twelve to sixteen times as long as the middle rod; the branches are large, arborescent, their ramules again ramified and very thorny.

Dimensions.—Length of the spines 0.2 to 0.25, of the middle rod 0.02.

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

4. Plagonium trigeminum, n. sp.

Spines straight, slender, cylindrical, four to six times as long as the middle rod, in the basal half smooth, in the distal half arborescent, with three to four irregular verticils of ramified branches. (Similar to a single spiculum of Sphærozoum verticillatum, Pl. 4, fig. 7.)

Dimensions.—Length of the spines 0.15 to 0.2, of the middle rod 0.05.

Habitat.—Central Pacific, Station 265, depth 2900 fathoms.

5. Plagonium distriactis, n. sp.

Spines straight and stout, three-sided prismatic, smooth, ten to twelve times as long as the middle rod; each on the distal end cleft into three divergent straight branches, which are again trichotomous.

Dimensions.—Length of the spines 0.15 to 0.18, of the middle rod 0.015.

Habitat.—South Pacific, Station 288, surface.

Subfamily 4. Polyplagida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with numerous (seven to nine or more) radial spines.

Genus 392. Polyplagia,^[11] n. gen.

Definition.—Plagonida with numerous (seven to nine or more) radial spines, arising from a common centre and lying in different planes.

The genus Polyplagia alone represents the small subfamily of Polyplagida, distinguished from the other Plagonida by the multiplication of the radial spines, the number of which amounts to seven to nine or more. This increased number is commonly the result of an intercalation of new spines between the three or four primary spines; it is sometimes also effected by stronger development of branches of the latter, which become independent. The following five species of this genus are very different, require further investigation, and perhaps represent different genera:—

1. Polyplagia septenaria, n. sp.

Seven radial spines, straight, three-sided prismatic, verticillate, of different sizes; four larger spines correspond to the four axes of a tetrahedron (running from the centre to the four corners), each with five to six verticils of three simple slender branches; one of these four main spines seems to be the apical, the three others the basal spines of Plagiocarpa; in the three meridian planes between the latter and the former lie the three smaller spines, diverging upwards, each with two to three verticils. (Similar to Polyplecta heptacantha, Pl. 91, fig. 12, but without connection between the branches.)

Dimensions.—Length of the four major spines 0.26, of the three minor 0.11.

Habitat.—Central Pacific, Station 274, surface.

2. Polyplagia octonaria, n. sp.

Eight radial spines, straight, three-sided prismatic, of equal size, arising in two opposite groups from the two poles of a short common middle rod (as in Sphærozoum arborescens, Pl. 4, fig. 8, and in other Beloidea). The four spines of each group are divergent, six to eight times as long as the middle rod, each spine armed with three to four verticils of thorny branches.

Dimensions.—Length of the spines 0.15, of the middle rod 0.022.

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

3. Polyplagia novenaria, n. sp.

Nine radial spines of equal size, straight, cylindrical, lying nearly in a horizontal plane, or forming a very flat pyramid. Near the common central point every three spines arise from a short common rod, so that the centre of the skeleton is triradial. Each spine bears towards the apex two divergent straight lateral branches. This species may have been derived from Plagiacantha arachnoides by shortening and reduction of the basal parts of the three original branched primary spines.

Dimensions.—Length of the nine spines 0.24, of the three basal rods 0.02.

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

4. Polyplagia duodenaria, n. sp.

Twelve radial spines of equal size, arising from a common central point, and diverging in different directions. The twelve spines are very large, opposite in six pairs, cylindrical, longitudinally striped (the expression of concentric lamellæ), and with spinulate surface, covered with innumerable small thorns. The basal quarter of each spine is straight and simple, the second quarter twice forked, and these four fork-branches are again in the outer half of the spine richly forked or ramified, with diverging, slightly curved thin branches; each of the twelve spines with about sixty to eighty terminal branches, the ends of which seem to fall into a spherical face. The position of this remarkable species in this family is doubtful.

Dimensions.—Length of the spines 0.12 to 0.16, of the simple basal part 0.04.

Habitat.—South Pacific (off Juan Fernandez), Station 299, surface.

5. Polyplagia viminaria, n. sp.

Numerous (sixteen to twenty or more) radial spines of about equal size, arising from a common central point and diverging in different directions, richly and more or less irregularly branched. The ends of the numerous small branches seem to fall into a spherical face. The large spines of this species have the same form and structure as in the preceding, nearly allied species, but are more numerous and more irregularly branched and disposed.

Dimensions.—Length of the spines 0.2 to 0.25, of the simple basal part 0.05.

Habitat.—North Pacific, Station 241, surface.

Family XLVII. Plectanida, Haeckel.

Definition.—Plectoidea with a wattled skeleton, composed of the meeting and united branches of radial spines, which arise from a common central point or central rod, and protect the partly enclosed central capsule.

The family Plectanida comprises those Nassellaria in which the skeleton is composed of radial spines, arising from a common centre, and of a loose wickerwork, produced by concrescence of the meeting branches of those spines. This rudimentary wattled skeleton is either quite irregular or only slightly regular, but it never assumes the form of a complete lattice-shell, as in the Cyrtellaria (the Spyroidea, Botryodea, and Cyrtoidea), nor does it exhibit a ring (as in the Stephoidea). The central capsule is partly or wholly protected, and often entirely enclosed by the wattled skeleton.

Three species only of Plectanida have been hitherto known. The first described form is Plectophora arachnoides, which its discoverer Claparède observed in a living state in 1855 on the western coast of Norway, and considered as a mere variety of his Plagiacantha arachnoides. Two other species were afterwards observed in the Mediterranean, Polyplecta dumetum, 1856, by Johannes Müller (united by him with Acanthodesmia) and Polyplecta polybrocha by myself in 1864. Many new forms are found in the collection of the Challenger, so that we may here distinguish eight genera and twenty-seven species.

The family Plectanida may be divided into four different subfamilies, according to the number of the radial spines which compose the skeleton. Those four subfamilies, as also the genera contained in them, correspond exactly to the four subfamilies of Plagonida, from which they have originated: Triplectida with three, Tetraplectida with four, Hexaplectida with six, and Polyplectida with numerous (seven to nine or more) radial spines. These spines are usually united (as in the corresponding ancestral Plagonida) in one common central point, upon which rests the basal pole of the central capsule, with the porochora. More rarely (in the genera Periplecta and Plectanium) the spines arise in two opposite groups (each of two or three spines) from the two poles of a common horizontal central rod, which supports the overlying porochora of the central capsule.

The different genera of Plectanida, which are enumerated in the following synopsis, correspond so exactly to the different genera of Plagonida described above, that probably each of the former has arisen from a corresponding genus of the latter. The only difference between the two is, that in all Plagonida the branches and ramules of the radial spines end freely, without growing together, whilst in all Plectanida the meeting ends of the neighbouring spines grow together and so produce a loose and irregular wickerwork. Therefore the latter constantly possess meshes, which are missing in the former. As soon as any form of Plagonida begins to produce meshes by concrescence of meeting branches of the spines, it passes over into a corresponding form of Plectanida. The loose wickerwork or wattlework so produced is sometimes very irregular, at other times more or less regular; but it never assumes the regular form of a complete lattice-shell, as in the Cyrtellaria (Spyroidea, Botryodea, and Cyrtoidea). The general differences which this wickerwork exhibits in the different genera of Plectanida have been already described above (compare pp. 900-904).

The Central Capsule of the Plectanida agrees perfectly with that of the ancestral Plagonida, as well in general form and structure as in the peculiar topographical relation to the radial spines (compare above, p. 905). A slight difference between the two families is effected by the higher development of the skeleton in the Plectanida. Since the branches of the radial spines in this family become united and form a loose wickerwork, the central capsule becomes more or less enclosed by the latter, and more perfectly protected, than in the Plagonida, where the branches remain free. In this respect the Plectanida approach more nearly to the Cyrtoidea with which they are connected by such transitional forms as Plectaniscus and Periplecta on the one hand, Cladoscenium and Pteroscenium on the other.

Synopsis of the Genera of Plectanida.

Subfamily 1. Triplectida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with three radial spines.

Genus 393. Triplecta,^[12] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with three radial spines, arising from one common central point and lying in one horizontal plane.

The genus Triplecta is the simplest and most primitive of the Plectanida, and may therefore be regarded as the prototype of this family. The skeleton represents a triangular lattice-plate with three radial beams. On the central union of the latter rests the oral pole of the central capsule. Since the axis of the latter is vertical, the lattice-plate must be horizontal, serving for the expansion of the pseudopodia. Triplecta has arisen from Triplagia by union of the branches of its three radial spines. In the simplest case only three large meshes are formed, corresponding probably to the three cortinar meshes in the collar septum of many Cyrtellaria.

1. Triplecta triangulum, n. sp.

Spines straight, equal, smooth, cylindrical, each in the basal half with one pair of divergent straight lateral branches. The opposed branches of every two neighbouring spines are united by a thin convex bow. In this way arises a very simple, equilateral triangular skeleton, with three equal large meshes.

Dimensions.—Length of the spines 0.14, sides of the triangle 0.16.

Habitat.—Tropical Atlantic, Station 348, depth 2450 fathoms.

2. Triplecta triactis, n. sp. (Pl. 91, fig. 7).

Spines straight, equal, three-sided prismatic, pinnate, each with three to four pairs of opposite slender pinnulæ or lateral branches; the pinnulæ of each spine are correspondingly parallel to the main rods of the other two spines. All pinnulæ branched and connected by a few slender bows, marking the sides of a regular triangle.

Dimensions.—Length of the spines 0.15, sides of the triangle 0.17.

Habitat.—Central Pacific, Station 272, depth 2600 fathoms.

Genus 394. Plectophora,^[13] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with three radial spines, arising from one common central point and corresponding to the edges of a three-sided pyramid.

The genus Plectophora has been derived from Plagiacantha by union of the meeting neighbouring branches of the three radial spines, and exhibits therefore to it the same relation that Triplecta bears to Triplagia. The loose framework so produced corresponds to the three sides of a flat pyramid and supports the central capsule.

1. Plectophora triomma, n. sp.

Spines equal, straight, three-sided prismatic, thorny, connected by three convex bows, the meeting branches of lateral spines arising in one pair from the middle part of each spine. Therefore the skeleton exhibits three large ovate meshes only.

Dimensions.—Length of the spines 0.12, of the meshes 0.08.

Habitat.—North Pacific, Station 238, surface.

2. Plectophora arachnoides, Haeckel.