Distance between all three arm-points nearly the same; but the odd arm is one and a half times as large as both paired arms, and is perpendicular to the common axis of the latter, therefore the shell has the form of a hammer. Each arm is twice as broad at the distal end as at the base, and armed with a conical terminal spine; the latter is vertical in the odd arm, horizontal in the paired arms.
Dimensions.—Radius of the odd arm 0.3, of the paired arms 0.15; distal breadth of the former 0.12, of the latter 0.06.
Habitat.—North Atlantic, Station 354, surface.
9. Rhopalastrum triceros, n. sp. (Pl. [43], fig. 4).
Distance between the paired arms about two-thirds as large as their distance from the odd arm. All three arms club-shaped, three times as broad at the thickened distal part as at the base, and armed with one single, conical, terminal spine. Odd arm of the same breadth, but twice as long as the paired arms.
Dimensions.—Radius of the odd arm 0.35, of the paired arms 0.2; basal breadth 0.04, distal breadth 0.12.
Habitat.—Pacific, central area, Station 274, surface.
10. Rhopalastrum hexaceros, n. sp. (Pl. [43], fig. 3).
Distance between the paired arms equals four-fifths of their distance from the odd arm. All three arms nearly of the same size, about square, a little broader at the truncated distal end, which is armed at both corners with a strong, conical, radial spine.
Dimensions.—Radius of each arm 0.2, basal breadth 0.1, distal breadth 0.12.