Page:Scientific results HMS Challenger vol 18 part 2.djvu/822

1698 the other bears numerous curved bristles around each spine, and at the distal end a verticil of five curved branches. The mouth of the shell (placed in the centre of one of the triangular plates) is in both species armed with six teeth. Both species are found in the Tropical Atlantic.

1. Circogonia icosahedra, n. sp. (Pl. 117, figs. 1, 1a).

Shell regularly icosahedral, with twenty equal, triangular, nearly plane faces, which are separated by thirty prominent, crest-like edges, and covered by a delicate network of small, subregular, hexagonal plates. Twelve radial spines nearly conical, one and a half times as long as the radius of the shell, with five prominent edges. The campanulate base of each spine is covered with numerous long, curved bristles, surrounded by a circle of twelve to sixteen ovate, irregular pores, and separated from the slender distal part by a constriction. The distal apex of the spine is surrounded by a verticil of five conical, curved branches (corresponding to the five edges of the spines, and to the five faces, which meet in the corners where the spine arises). Mouth hexagonal, with six conical, spinulate teeth, convergent towards its centre (fig. 1a).

Dimensions.—Diameter of the shell 0.75, length of the spines 0.5.

Habitat.—Tropical Atlantic, Station 347, depth 2250 fathoms.

2. Circogonia dodecacantha, n. sp. (Pl. 115, figs. 8, 9).

Shell subregularly icosahedral, with twenty equal, triangular, slightly convex faces, which are not separated by prominent crests, but covered with a network of irregular polygonal plates. Twelve radial spines, cylindrical, smooth, about as long as the diameter of the shell, forked at the distal end with two divergent, slightly curved branches (similar to Circospathis furcata, Pl. 115, figs. 4-6). The conical base of each spine is surrounded by a circle of nine to twelve irregular, ovate pores (fig. 9). Mouth circular, with a corona of six conical, smooth teeth which arise perpendicularly from the shell (in the upper part of fig. 8, at left).

Dimensions.—Diameter of the shell 0.6, length of the spines 0.7.

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

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

The genus Circorrhegma is remarkable for the geometrical regularity of its shell, which represents a pentagonal dodecahedron. The shell is therefore composed of twelve equal and regular pentagonal plates, and these are separated in the only known