Dimensions.—Shell 0.1 long, 0.09 broad; horn 0.13 long.
Habitat.—Central Pacific, Station 274, depth 2750 fathoms.
Genus 533. Archicapsa,[[150]] Haeckel, 1881, Prodromus, p. 428.
Definition.—Archicapsida without apical horn.
The genus Archicapsa differs from the preceding Halicapsa in the absence of the apical horn, and represents the simplest form among the Monocyrtida clausa, a simple, ovate, or pear-shaped shell without any appendages. The characteristic structure of the basal lattice-plate (with three or four large cortinar pores) makes it probable that Archicapsa has been derived either from Halicapsa, by loss of the horn, or directly from Dictyospyris, by reduction of the sagittal ring and constriction.
1. Archicapsa triforis, n. sp. (Pl. [53], figs. 1, 2).
Shell pear-shaped, smooth, one and a half times as long as broad. Pores subregular, circular, of about the same breadth as the bars. Basal plate (fig. 2) with three very large pores, three times as broad as the others (one odd sternal pore and two paired cardinal pores).
Dimensions.—Shell 0.09 long, 0.06 broad.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
2. Archicapsa quadriforis, n. sp.