Page:Scientific results HMS Challenger vol 18 part 2.djvu/115

Rh

Definition.— with two bisected horizontal rings, connected by the complete vertical sagittal ring. (The upper mitral ring and the lower basal ring become divided by the complete primary or sagittal ring into two horizontal symmetrical gates, two mitral gates on the apical and two basal gates on the basal pole.)

Definition.— with two bisected horizontal rings, connected by two vertical columellæ.

The genus Protympanium, is the most simple form of all Tympanida, and probably the common ancestral form of this family. It arises from Lithocircus by the development of two horizontal rings, perpendicular to the primary sagittal ring. The upper or mitral ring arises by lateral union of two pairs of superior branches, the lower or basal ring by union of two pairs of basal branches of the primary vertical ring.

1. Protympanium primordiale, n. sp.

Mitral and apical ring of equal size, elliptical, smooth, somewhat smaller than the connecting thorny sagittal ring.

Dimensions.—Diameter of the two horizontal rings 0.12, of the sagittal ring 0.15.

Habitat.—Central Pacific, Stations 263 to 274, at various depths.