Page:Scientific results HMS Challenger vol 18 part 2.djvu/64

Synopsis of the Genera of Stephanida.

I. Subfamily Lithocircida. No typical basal feet on the base of the ring (no cortinar feet).	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Ring diphragmatic or amphithect; dorsal and ventral bow equal.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Ring smooth or thorny, without branched spines.	401. Archicircus.
				Ring armed with branched spines,	402. Lithocircus.
		Ring dipleuric, bilateral; dorsal and ventral bow different.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Ring smooth or thorny, without branched spines,	403. Zygocircus.
				Ring armed with branched spines.	404. Dendrocircus.
II. Subfamily Cortinida. Three or four typical basal feet on the ring (cortinar feet).	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Three basal feet (and an apical horn).		One caudal and two lateral feet,	405. Cortina.
		Four basal feet (and an apical horn).		Two sagittal and two lateral feet,	406. Stephanium.

Subfamily 1. Lithocircida, Haeckel.

Definition.—Stephanida without typical basal feet or cortinar feet.

Genus 401. Archicircus,^[1] n. gen.

Definition.—Stephanida with a simple amphithect or diphragmatic ring, smooth or thorny, without branched spines and basal feet.

The genus Archicircus is the most primitive and simplest form of all Stephoidea, and probably the common ancestral form, not only of this suborder, but of the greater number of all Nassellaria (compare above, p. 893). The skeleton consists only of a quite simple sagittal ring, in which commonly a slight difference of both poles of the main axis (basal and apical pole) is visible, but no difference between the dorsal and the ventral bow of the ring. In my Prodromus (1881, p. 447) the species of Archicircus were disposed partly in the subgenus Monostephus, partly in the genus Lithocircus.

Subgenus 1. Monostephus, Haeckel, 1881, Prodromus, p. 447.

Definition.—Ring circular, elliptical, or ovate, without prominent corners.

1. Archicircus princeps, n. sp.

Gate circular. Ring circular, smooth, its transverse section also circular. The simplest form of all Stephoidea.

Dimensions.—Diameter of the gate 0.05 to 0.08; thickness of the ring 0.006 to 0.008.

Habitat.—Central Pacific, Stations 265 to 274, depth 2350 to 2925 fathoms.

1. Archicircus = Primordial ring; ἀρχί, κίρκος.