Page:Scientific results HMS Challenger vol 18 part 2.djvu/821

Rh inflated distal apex a trident or fuscina, similar to that of Circoporus sexfuscinus (Pl. 115, fig. 2). Mouth with nine convergent, conical, denticulate teeth (fig. 3a).

Dimensions.—Diameter of the shell 0.6, length of the spines 0.4.

Habitat.—South Atlantic, Station 318 (east of Patagonia), depth 2040 fathoms.

3. Circospathis tetradeca, n. sp.

Shell spherical, covered with irregular, polygonal plates. Nine radial spines, cylindrical, as long as the radius of the shell, bristly, armed at the distal apex with a trident. (In one specimen six spines possessed three terminal branches, two spines two branches, and one spine four branches.) The broader base of each spine is surrounded by a corona of nine pores and a circle of curved bristles. Mouth with nine smooth conical teeth.

Dimensions.—Diameter of the shell 0.55, length of the spines 0.3.

Habitat.—South Atlantic, Station 332 (west of Tristan da Cunha), depth 2200 fathoms.

4. Circospathis tetrodonta, n. sp. (Pl. 115, fig. 10).

Shell subspherical or polyhedral, with fourteen triangular, convex faces, covered with irregular polygonal plates. Nine radial spines cylindrical, nearly smooth, about as long as the diameter of the shell; at the distal apex with four strong curved horns, like those of Circoporus hexastylus, Pl. 117, fig. 4. Each spine is surrounded at the base by a corona of nine irregular pores. Mouth with four large, prominent, conical teeth, which are spinulate, with vertical inner edge (fig. 10).

Dimensions.—Diameter of the shell 0.5, length of the spines 0.45.

Habitat.—South Atlantic, Station 323 (east of Buenos Ayres), depth 1900 fathoms.

Definition.— with a regular icosahedral shell, composed of twenty congruent, triangular plates, with twelve corners, from which arise twelve radial spines.

The genus Circogonia is remarkable for the regular icosahedral form of its shell, a geometrical fundamental form, which occurs very rarely in organised bodies (as in Aulacantha icosahedra, and in some Astrosphærida with twelve regularly disposed radial spines). The shell is composed of twenty triangular, equilateral and congruent plates, which are sometimes separated by prominent crests; their surface is panelled by smaller hexagonal or polygonal secondary plates. The thirty edges between the plates are sometimes thickened. From the twelve corners of the regular icosahedron (where every five triangular plates meet), twelve equal radial spines arise, each of which is surrounded at the base by a corona of nine to sixteen pores. Two species only of Circogonia have been observed; the one has smooth spines, forked at the distal end,