Shell three-sided pyramidal, smooth. Length of the three joints = 1 : 3 : 2, breadth = 1 : 4 : 6. Cephalis small, subspherical, with a large conical horn three times the length. Thorax and abdomen with small, regular, circular pores of equal size, densely crowded together, with thin bars. Abdomen shorter, with three concave basal edges. Feet very thick, straight, from the girdle strongly divergent.
Dimensions.—Length of the three joints, a 0.015, b 0.05, c 0.025; breadth a 0.02, b 0.08, c 0.12.
Habitat.—Fossil in Barbados.
5. Pterocanium pyramis, n. sp. (Pl. [68], fig. 7).
Shell three-sided pyramidal, smooth. Length of the three joints = 1 : 4 : 3, breadth = 2 : 4 : 6. Cephalis hemispherical, with a conical horn of the same length. Thorax campanulate, with fifteen to twenty prominent longitudinal ribs, separating as many rows of regular, circular pores. Abdomen shorter, with three concave basal bays, and irregular, roundish pores. Feet very strong, pyramidal, straight, gradually divergent from the girdle.
Dimensions.—Length of the three joints, a 0.025, b 0.08, c 0.06; breadth, a 0.04, b 0.08, c 0.12.
Habitat.—Central Pacific, Station 266, depth 2750 fathoms.
6. Pterocanium virgineum, n. sp. (Pl. [73], fig. 6).
Shell nearly three-sided pyramidal, smooth. Length of the three joints = 1 : 4 : 8, breadth 2 : 6 : 9. Cephalis hemispherical, with a conical straight horn three times the length. Thorax campanulate, with three slight swellings between the three knee-shaped ribs, and with regular circular pores. Abdomen longer, with three concave basal bays, and with very small and numerous pores (much smaller than those of the thorax). At both sides of the three prominent abdominal crests, two rows of larger, nearly square pores occur. Feet slender, nearly straight, divergent.
Dimensions.—Length of the three joints, a 0.15, b 0.06, c 0.12; breadth, a 0.03, b 0.09, c 0.14.