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. Genus 500. _Archipilium_,[117] Haeckel, 1881, Prodromus, p. 427. _Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) with three lateral ribs or wings. Mouth truncate, without terminal feet. Apex without horn. The genus _Archipilium_ differs from the nearly allied _Trissopilium_, its probable ancestral form, in the absence of the apical horn. The three lateral wings of the ovate shell are stout spines. It may also be derived from the similar _Sethopilium_ by reduction and loss of the cephalis. 1. _Archipilium orthopterum_, n. sp. (Pl. 98, fig. 7). Shell ovate, smooth, nearly cylindrical, scarcely broader than the truncate, widely open mouth. Pores numerous, irregular, roundish, of very different sizes. Three lateral wings arising from the middle zone, twice as long as the shell, straight, divergent, stout, cylindrical, longitudinally striped. _Dimensions._--Shell 0.08 long, 0.06 broad; wings 0.15 long. _Habitat._--Central Pacific, Station 272, depth 2600 fathoms. 2. _Archipilium sigmopterum_, n. sp. Shell ovate, smooth, nearly campanulate, of equal length and breadth, one and one-third times as broad as the truncate, slightly constricted mouth. Pores scarce, irregularly formed and {1140}scattered, of very different sizes. Three lateral wings, stout, prismatic, about as long as the shell widely divergent and S-shaped, curved. _Dimensions._--Shell 0.08 diameter, wings 0.08 long. _Habitat._--Central Pacific, Station 266, depth 2750 fathoms. 3. _Archipilium cyrtopterum_, n. sp. Shell ovate, rough, twice as broad as the constricted mouth. Pores small and numerous, subregular, circular, smaller than the bars. Three lateral wings cylindrical, crescentic, in the upper half divergent, in the lower convergent, one and one-half times as long as the shell.