Page:Scientific results HMS Challenger vol 18 part 2.djvu/414

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Definition.— (vel Dicyrtida eradiata aperta) with conical or campanulate, gradually dilated thorax and wide open mouth. Cephalis with one or more horns.

The genus Sethoconus is the oldest and simplest form of the Sethocorida, or of those Dicyrtida in which the simple thorax bears no apophyses and has the mouth open and not closed by a basal lattice-plate. Probably all those eradiate Sethocorida have been derived either from triradiate Sethopilida (Lychnocanium), or from multiradiate Sethophormida (Sethophormis, Anthocyrtis ?) by loss of the radial ribs and feet. In Sethoconus the thorax is more or less conical, sometimes more campanulate, commonly smooth, in some species covered with spines. The cephalis is usually small, with rudimentary collar septum, at other times large, with distinct cortinar septum. Correspondingly, the genus may be divided into different groups, which here are enumerated as subgenera.

Definition.—Cephalis relatively large, with distinct collar septum and numerous pores. Thorax smooth.

1. Sethoconus trochus, Haeckel.

Eucyrtidium trochus, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 293, Taf. vii, fig. 17.

Conarachnium trochus, Haeckel, 1881, Prodromus, p. 430.

Cephalis ovate, relatively large, with a pyramidal horn of the same length, and three pairs of large opposite pores, on each side of a vertical septum. Thorax wide, conical, about as long as broad, with nearly straight outlines; its pores regular, circular, hexagonally framed, of the same size as the cephalic pores.

Dimensions.—Cephalis 0.03 long, 0.02 broad; thorax 0.06 long, 0.06 broad.

Habitat.—Tropical Pacific, Stations 200 to 281, surface.

2. Sethoconus cucullaris, Haeckel.

Cornutella cucullaris, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 68, Taf. ii. fig. 7.

Ceratocyrtis cucullaris, Bütschli, 1882, Zeitschr. f. wiss Zool., Bd. xxvi. p. 536, Taf. xxxiii. fig. 36, a., b.

Cephalis subspherical, thorny, relatively large, with a conical horn of the same length, and small circular pores. Thorax wide, conical, enveloping the lower hidden half of the cephalis, about as