The genus Trissopilium and the following nearly allied Archipilium differ from the other Archipilida in the absence of terminal feet, the peristome being quite simple, and truncate. But there are three lateral wings, arising either from the apex or from the lateral sides of the monothalamous shell. Trissopilium may perhaps be derived from Lithomelissa by reduction of the cephalis.
1. Trissopilium tetraplecta, n. sp.
Shell in the upper half three-sided pyramidal, in the lower half inversely hemispherical, of equal length and breadth. Pores irregular, roundish, in the upper half larger. From the apex there diverge four equal, straight and stout, three-sided pyramidal spines, one of which is vertically ascending (the horn), the three others obliquely descending (the wings). The distal half of the latter is free, whilst the proximal half forms three ribs, enclosed in the shell-wall. Mouth constricted, half as broad as the shell.
Dimensions.—Shell 0.08 long and broad, horn and wings 0.1 long.
Habitat.—Central Pacific, Station 268, depth 2900 fathoms.
2. Trissopilium lithomelissa, n. sp.
Shell ovate, one and one-third times as long as broad. Pores circular, of different sizes, small and numerous. Apical horn very large, straight, cylindrical, vertical, longer than the shell. Three lateral wings shorter, arising from the middle of the shell, conical, curved, divergent. Mouth truncate, two-thirds as broad as the shell.
Dimensions.—Shell 0.11 long, 0.08 broad; horn 0.13 long, wings 0.09 long.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.