Shell campanulate, spinulate, similar to _Euscenium tricolpium_ (Pl. 53, fig. 12) with three vaulted hemispherical bosoms between the three delicate arachnoidal vertical wings. Network loose, very irregular, with polygonal meshes of very different sizes. Columella with three or four triradiate verticils, as long as the slender pyramidal horn. Three feet twice as long, somewhat curved, widely divergent, with three or four pairs of lateral branches. The three edges of the feet and of the horn are elegantly denticulated. _Dimensions._--Shell 0.06 long, 0.09 broad. _Habitat._--Central Pacific, Station 266, depth 2750 fathoms. Genus 508. _Peridium_,[125] Haeckel, 1881, p. 429. _Definition._--#Archiperida# (vel Monocyrtida triradiata clausa) without internal columella, with an apical horn. The genus _Peridium_ (or _Archiperidium_, Prodromus, _loc. cit._) and the two following nearly allied genera form together the small group of Archibursida, differing from the four preceding genera (the Euscenida) in the absence of an internal free columella. The three basal feet have therefore no direct connection with the apical horn. _Peridium_, the ancestral form of the Archibursida, may have originated either from _Euscenium_ by reduction and loss of the columella, or directly from _Tripospyris_ by loss of the sagittal ring and the longitudinal constriction. Subgenus 1. _Peridarium_, Haeckel. _Definition._--Feet smooth, simple, neither spiny nor branched. {1154}1. _Peridium lasanum_, n. sp. Shell subspherical, smooth, with subregular circular pores. Basal plate with three large meshes. Horn apical, vertical, of the same size and form as the three divergent smooth feet, slender pyramidal, twice to three times as long as the shell. _Dimensions._--Shell-diameter 0.06 to 0.07, length of the three spines 0.15 to 0.2. _Habitat._--North Pacific, Station 253, depth 3125 fathoms. 2. _Peridium papillatum_, n. sp. Shell subspherical, papillate, with subregular circular pores. Basal plate with four meshes (two major cardinal and two minor jugular pores). Horn occipital, oblique, three-sided pyramidal, half as long as the shell and as the three widely divergent, subhorizontal, smooth feet. _Dimensions._--Shell diameter 0.07 to 0.08, length of the horn 0.04, of the feet 0.08. _Habitat._--Central Pacific, Station 268, depth 2900 fathoms. 3. _Peridium curvipes_, n. sp. Shell hemispherical, thorny, with irregular roundish pores. Basal plate with six pores (two larger middle cardinal, two smaller anterior jugular and two smaller posterior cervical pores, as in Pl. 53, figs. 13, 18). Horn conical, oblique, arising from the middle of the occipital face, half as long as the shell. Feet slender, conical, curved, smooth, twice as long as the shell. _Dimensions._--Shell 0.04 long, 0.06 broad; horn 0.02 long; feet 0.08 long. _Habitat._--Tropical Pacific, Station 206, depth 2100 fathoms. Subgenus 2. _Archiperidium_, Haeckel.