Shell smooth, slender, pyramidal. Cephalis small, spherical, with few small pores, and a stout, vertical, conical horn twice the length. Thorax very prolonged, conical, with straight lateral outlines and numerous (twenty to fifty) radial beams (in the upper third twelve, in the middle third twenty-four, in the lower third forty to fifty, by interpolation); they are connected by very numerous interrupted transverse bars. Meshes subregular, quadrangular. Surface smooth.
Dimensions.—Cephalis 0.02 diameter; thorax 1.05 long, 0.45 broad.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
Genus 561. Plectopyramis,[[178]] Haeckel, 1881, Prodromus, p. 432.
Definition.—Sethophormida (vel Dicyrtida multiradiata aperta) with numerous straight or slightly curved radial ribs in the wall of the pyramidal thorax. Network double, the large primary meshes being fenestrated by fine secondary network. Cephalis commonly without horn.
The genus Plectopyramis differs from the preceding Sethopyramis, its ancestral form, only in the peculiar double fenestration of the shell, the large primary meshes of which are separated by strong bars, and filled up by a very delicate arachnoidal network, composed of small pores and very thin threads between them. This double lattice-work often exhibits a very remarkable regularity. As in the preceding genus, the number of radial ribs is commonly six or nine, rarely twelve to twenty or more.
Subgenus 1. Hexapleuris, Haeckel.
Definition.—Pyramidal shell with six radial main beams (sometimes five or seven in individual varieties).
1. Plectopyramis magnifica, Haeckel.
Polycystina magnifica, Bury, 1862, Polycystins of Barbados, pl. xi. fig.