1. Rhopalatractus pentacanthus, n. sp. (Pl. 68, fig. 11).
Shell very thick-walled, with two sharp strictures. Length of the three joints = 1 : 6 : 8, breadth = 1 : 6 : 7. Cephalis hemispherical, with conical cavity, and a conical horn of twice the length. Thorax subspherical, separated from the abdomen by a deep lumbar stricture, with small, regular, circular, hexagonally-framed pores. Abdomen three-sided prismatic, with three strong, prominent, wing-shaped edges, which are slightly divergent towards the base, and prolonged into three pyramidal feet about as long as the thorax. Pores of the abdomen disposed in longitudinal rows, which are separated by denticulate crests. Abdominal base inversely pyramidal, and prolonged into a very large, cylindrical, basal, axial spine, nearly as long as the whole shell.
Dimensions.—Length of the three joints, a 0.02, b 0.12, c 0.15; breadth, a 0.03, b 0.11, c 0.14.
Habitat.—Central Pacific, Station 268, depth 2900 fathoms.
2. Rhopalatractus foveolatus, n. sp. (Pl. 68, fig. 10).
Shell slenderly ovate, thick-walled, with rough, dimpled surface. Length of the three joints = 1 : 3 : 5, breadth = 2 : 4 : 4. Cephalis hemispherical, with spherical cavity, and a thick, conical spine of the same length. Thorax subspherical, with subregular, circular pores. Abdomen inversely conical, with larger, irregular, roundish pores, prolonged at the distal end into a cylindrical, axial, basal spine of about half its length. From the middle part of the abdomen arise three lateral, conical feet, which are slightly curved and divergent downwardly, scarcely as long as the basal spine. All five spines are dimpled.
Dimensions.—Length of the three joints, a 0.02, b 0.06, c 0.1; breadth a 0.04, b 0.08, c 0.08.
Habitat.—Central Pacific, Station 265, depth 2900 fathoms.
3. Rhopalatractus fusiformis, n. sp.
Shell slender, spindle-shaped, nearly of the same form as in the preceding species. Length of the three joints = 1 : 2 : 6, breadth = 1 : 2 : 2. Cephalis subspherical, with a cylindrical horn three times the length. Pores subregular, circular, of equal breadth in the thorax and in the abdomen, which is prolonged into a cylindrical, axial, basal spine of the same length. From the upper third of the abdomen arise, from a broad triangular base, three conical wings of half the length, divergent and slightly curved inwards.
Dimensions.—Length of the three joints, a 0.025, b 0.05, c 0.15; breadth, a 0.03, b 0.06, c 0.06.
Habitat.—Central Pacific, Station 263, depth 2650 fathoms.
4. Rhopalatractus fenestratus, n. sp. (Pl. 68, fig. 12).
Shell nearly spindle-shaped, with two distinct strictures. Length of the three joints = 1 : 3 : 6, breadth = 1 : 4 : 4. Cephalis hemispherical, with a large, club-shaped horn, nearly as long as the abdomen, sulcated by longitudinal ribs, which are elegantly denticulate in the distal half. Thorax
