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, 11a).
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.
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.
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.
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.
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.
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 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.
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.
Definition.—Arms of different size, one odd arm opposite to the angle between both arm-pairs.
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.
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 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.
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.
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.
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. 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 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.
|
I. Subfamily Triopylida. Three gates between three simple arm-chambers. |
brace | Three gates open, without a barring equatorial girdle, | 245. Triolena. | ||
| Three gates barred by a latticed equatorial girdle. | brace | Gate-faces simple, | 246. Triopyle. | ||
| Gate-faces latticed, | 247. Triodiscus. | ||||
|
II. Subfamily Hexapylida. Six gates between three double arm-chambers (three inner and three outer); no chambered marginal girdle. |
brace | Three outer gates open, without a barring equatorial girdle, | 248. Pylolena. | ||
| Three outer gates barred by a latticed (second) equatorial girdle. | brace | Both faces of the outer gates simple, | 249. Hexapyle. | ||
| Both faces of the outer gates latticed, | 250. Pylodiscus. | ||||
|
III. Subfamily Discopylida. Six gates between three double arm-chambers. |
brace | In the equatorial plane on the margin of the Pylodiscus-shell is a chambered equatorial girdle. | brace | No peculiar osculum on the margin of the disc, | 251. Discozonium. |
| One peculiar osculum (with a corona of spines) on the margin of the disc, | 252. Discopyle. | ||||
| 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 latticed equatorial girdle. | ||||||
| Gate-faces simple, | ||||||
| 246. Triopyle. | ||||||
| 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. | ||||||
| Three outer gates barred by a latticed (second) equatorial girdle. | ||||||
| Both faces of the outer gates simple, | ||||||
| 249. Hexapyle. | ||||||
| Both faces of the outer gates latticed, | ||||||
| 250. Pylodiscus. | ||||||
| III. Subfamily Discopylida. Six gates between three double arm-chambers. | ||||||
| In the equatorial plane on the margin of the Pylodiscus-shell is a chambered equatorial girdle. | ||||||
| No peculiar osculum on the margin of the disc, | ||||||
| 251. Discozonium. | ||||||
| One peculiar osculum (with a corona of spines) on the margin of the disc, | ||||||
| 252. Discopyle. | ||||||
Definition.—Pylodiscida with a simple, spherical or lenticular, central chamber, surrounded by three simple arm-chambers, which are separated by three notches or gates.
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.
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.
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 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.
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 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).
Definition.—Pylodiscida with triopyle-shaped medullary shell, surrounded by three distal arm-chambers, which are separated by three open notches or gates.
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.