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, 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.

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. , 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, 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).

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. , 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.

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, 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. , 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

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.