Page:Scientific results HMS Challenger vol 18 part 2.djvu/367

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Definition.— with the basal mouth of the shell open (vel Dicyrtida multiradiata aperta).

Definition.— (vel Dicyrtida multiradiata aperta) with numerous radial ribs in the wall of the flat and broad campanulate or nearly discoidal thorax. Cephalis also flat and broad, cap-shaped without horn.

The genus Sethophormis comprises a great number of widely distributed and large Dicyrtida, all agreeing in the flat cap-shaped or nearly discoidal form of the thorax, which has constantly a very delicate network, and is pierced by numerous radial beams. Four of these are commonly primary and direct prolongations of the four cortinar rods in the collar septum, whilst the others are secondary or interpolated ribs. Sethophormis may be derived either from the triradial Lamprodiscus by multiplication of the ribs (or interpolation of secondary ribs between the three primary), or from Halicalyptra by development of transverse rods connecting its terminal feet, and forming a ribbed thorax. Corresponding to the number of the ribs, we may distinguish as subgenera (separate genera in my Prodromus, 1881, p. 432) Tetraphormis with four, Pentaphormis with five, Hexaphormis with six, Octophormis with eight, Enneaphormis with nine, and Astrophormis with ten to twenty or more ribs. The cephalis bears no horn, and is usually large, flat, cap-shaped or hemispherical.

Definition.—Thorax with four radial ribs opposite in pairs in two meridional planes, perpendicular to one another. Collar septum with four large pores.

1. Sethophormis cruciata, n. sp.

Cephalis very large, hemispherical with irregular, delicate network of small square meshes. Thorax flatly campanulate, with delicate, regular network of small hexagonal meshes. All bars very thin, thread-like. Four stout radial thoracic ribs, crossed in pairs, lie in two meridional planes, perpendicular to one another. Very similar to Sethophormis aurelia (Pl. 55, figs. 3, 4), but not so flat, and with only four thoracic ribs. Peristome or margin of the thoracic mouth smooth, circular.

Dimensions.—Cephalis 0.04 long, 0.08 broad; thorax 0.12 long, 0.36 broad.

Habitat.—Central Pacific, Station 271, surface.