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. {1192}2. _Archicapsa quadriforis_, n. sp. Shell ovate, rough, a little longer than broad. Pores subregular, circular, twice as broad as the bars. Basal plate with four larger ovate cortinar pores (two smaller jugular and two larger cardinal pores). _Dimensions._--Shell 0.12 to 0.15 long, 0.1 to 0.12 broad. _Habitat._--Central Pacific, Station 265 to 268, depth 2700 to 2900 fathoms. 3. _Archicapsa nonaforis_, n. sp. Shell pear-shaped, thorny, one and a half times as long as broad. Pores regular, circular, small, hexagonally framed, twice as broad as the bars. Basal plate with nine cortinar pores (three larger alternating with three pairs of smaller pores, as in Pl. 87, fig. 2). _Dimensions._--Shell 0.14 long, 0.09 broad. _Habitat._--Central Pacific, Station 272, depth 2600 fathoms. ---- Section II. DICYRTIDA, Haeckel, 1862, Monogr. d. Radiol., pp. 280, 296. _Definition._--#Cyrtoidea# dithalamia, with two-jointed shell, being divided by a transverse collar stricture into an apical joint (or cephalis) and a basal joint (or thorax). _Synopsis of the three Families and six Subfamilies of Dicyrtida._ Family LXII. Tripocyrtida. { Mouth open, 1. Sethopilida. { Three radial apophyses. { Mouth closed, 2. Sethoperida. Family LXIII. Anthocyrtida. { Mouth open, 3. Sethophormida. { Numerous radial apophyses. { Mouth closed, 4. Sethophænida. Family LXIV. Sethocyrtida. { Mouth open, 5. Sethocorida. { No radial apophyses. { Mouth closed, 6. Sethocapsida. Family LXII. #TRIPOCYRTIDA#, n. fam. _Sethopilida et Sethoperida_, Haeckel, 1881, Prodromus, pp. 431, 433. _Definition._--#Dicyrtida# triradiata. (#Cyrtoidea# with a two-jointed shell, divided by a transverse constriction into cephalis and thorax, and bearing three radial apophyses.)