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.
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.