Shell smooth, slender, pyramidal. Cephalis large, ovate, with pointed apex, nearly hyaline, with very few and small scattered pores. Thorax very slenderly pyramidal, with straight lateral outlines, and nine stout and straight radial beams, which are connected by fifteen to twenty horizontal complete rings. Meshes subregular, square, simple, without secondary network; their length and breadth equally increasing towards the mouth.
Dimensions.—Cephalis 0.05 long, 0.03 broad; thorax (with twenty transverse rings) 0.24 long, 0.1 broad at the mouth.
Habitat.—Central Pacific, Stations 265 to 268, depth 2700 to 2900 fathoms.
4. Sethopyramis quadrata, n. sp. (Pl. [54], fig. 2).
Shell smooth, slenderly pyramidal, very similar in form and structure to the preceding species; differing from it in the rudimentary cephalis, which is very small, hemispherical, with few small porules. The three internal cortinar bars of the collar stricture are present; when they become lost this species passes over into Bathropyramis quadrata (Pl. [54], fig. 1).
Dimensions.—Cephalis 0.01 long, 0.02 broad; thorax (with ten transverse rings) 0.15 long, 0.1 broad at the mouth.
Habitat.—North Pacific, Station 244, depth 2900 fathoms.
5. Sethopyramis quadratella, Haeckel.
Cornutella quadratella, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 68, Taf. ii. fig. 2.
Litharachnium quadratella, Bütschli, 1882, Zeitschr. f. wiss. Zool., vol, xxxvi. p. 540, Taf. xxxiii. fig. 37.