Definition.—Sethopilida (vel Dicyrtida triradiata aperta) with three complete thoracic ribs, which are prolonged into three solid divergent feet on the peristome. Cephalis smooth, without a horn.
The genus Sethopilium differs from its probable ancestral form Dictyophimus by the complete loss of the cephalic horn, and of the collar septum between both joints, which are only separated by the slight external collar stricture.
1. Sethopilium orthopus, n. sp. (Pl. [97], fig. 8).
Shell smooth, with sharp collar stricture, Relative length of the two joints = 5 : 4, breadth = 7 : 6. Cephalis large, subspherical, with numerous regular circular pores, twice as broad as the bars. Thorax with three stout, straight, widely divergent ribs, prolonged into three straight pointed feet of the same length. Between every two ribs, in the centre of the collar stricture, a single large triangular mesh, and beyond this three to six rows of smaller irregular meshes.
Dimensions.—Cephalis 0.05 long, 0.07 broad; thorax 0.04 long, 0.06 broad.
Habitat.—Central Pacific, Station 266, depth 2750 fathoms.
2. Sethopilium cyrtopus, n. sp.
Shell smooth, with sharp collar stricture. Relative length of the two joints = 4 : 6, breadth = 7 : 11. Cephalis hemispherical, with large irregular roundish pores, four times as broad as the bars. Thorax with three stout, curved, widely divergent ribs, prolonged into three cylindro-conical, curved feet of twice the length, which are convex in the proximal half, concave in the distal half. Between every two ribs, beyond the collar stricture, two large ovate meshes, and beyond this two rows of smaller irregular meshes.
Dimensions.—Cephalis 0.04 long, 0.07 broad; thorax 0.05 long, 0.11 broad.
Habitat.—Central Pacific, Station 268, depth 2900 fathoms.