Subfamily Hexacromyida, Haeckel, 1881, Prodromus, pp. 449, 453

Definition.—Cubosphærida with four concentric spherical lattice-shells.

Genus 82. Hexacromyum, Haeckel, 1881, Prodromus, p. 453.

Definition.—Cubosphærida with four concentric lattice-spheres and six simple spines of equal size.

The genus Hexacromyum possesses four concentric, spherical, or octahedral lattice-shells; two inner medullary shells within the central capsule, two outer cortical outside it. The four spheres are connected by six radial beams, which are prolonged outside into simple spines of equal size, opposite in pairs in the three dimensive axes. This genus can be derived from Hexacontium by duplication of the cortical shell.

1. Hexacromyum elegans, n. sp. (Pl. , fig. 9).

Shell composed of four concentric shells, with radial proportion = 1 : 2.5 : 7.5 : 10. First (innermost) shell with very small circular pores, second shell with larger circular pores. Third shell (inner cortical shell) with large, subregular, circular, hexagonally framed pores (eight to nine on the radius), twice as broad as the bars; from the elevated nodal-points of the hexagonal frames (between every three pores) arise thin bristle-shaped radial beams, which are united at the distal end by vaulted branches forming the delicate fourth shell. Surface smooth. Radial spines three-sided pyramidal, as long as the radius, as broad at the base as the innermost shell.

Dimensions.—Diameter of the four shells—(A) 0.02, (B) 0.05, (C) 0.15, (D) 0.2; length of the six radial spines 0.1, basal breadth 0.02.

Habitat.—Central Pacific, Station 271, surface.

2. Hexacromyum quadrigatum, n. sp.

Shell composed of four concentric shells, with radial proportion = 1 : 3 : 8 : 10. Structure of all four shells the same, with regular, circular pores, twice to three times as broad as the bars; size of the pores gradually increasing from the innermost to the outermost shell; surface smooth. Radial spines three-sided pyramidal, as long as the radius, half as broad at the base as the innermost shell.

Dimensions.—Diameter of the four shells—(A) 0.025, (B) 0.08, (C) 0.20, (D) 0.25; length of the spines 0.12, basal breadth 0.012.

Habitat.—North Pacific, Station 253, depth 3125 fathoms.

3. Hexacromyum arachnoides, n. sp.

Hexacromidium arachnoides, Haeckel, 1881, Prodromus, p. 453.

Shell composed of four concentric shells, with radial proportion = 1 : 2 : 3 : 4. Innermost shell with regular, hexagonal meshes; the three other shells with irregular, polygonal meshes; bars between the large meshes in all four shells very thin, cobweb-like. Surface covered with thin bristle-shaped by-spines, as long as the radius. Six radial main spines three-sided prismatic, longer than the diameter of the whole shell.

Dimensions.—Diameter of the four shells—(A) 0.08, (B) 0.16, (C) 0.24, (D) 0.32; length of the spines 0.4, breadth 0.01.

Habitat.—South Pacific, Station 300, depth 1375 fathoms.

4. Hexacromyum octahedrum, n. sp. (Pl. , fig. 2).

Shell composed of four concentric shells which are not spherical (as in the three preceding species), but regular octahedral. Radial proportion = 1 : 2.5 : 6 : 9. Network in all four shells delicate, with irregular polygonal meshes and thin bars; the thickness of the bars and size of the meshes increasing from the innermost to the outermost shell. Six radial spines three-sided prismatic, increasing slowly in breadth towards the distal end, much longer than the shell diameter.

Dimensions.—Diameter of the four shells—(A) 0.02, (B) 0.05, (C) 0.12, (D) 0.18; length of the radial spines 0.2 to 0.3 and more, breadth 0.01.

Habitat.—Central Pacific, Station 263, depth 2650 fathoms.