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

940

Definition.— without typical basal feet or cortinar feet.

Definition.— 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, and probably the common ancestral form, not only of this suborder, but of the greater number of all (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.

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.

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.