Page:Scientific results HMS Challenger vol 18 part 2.djvu/109

Rh 2. Tricyclidium semantrum, n. sp.

Sagittal ring ovate, smaller than the violin-shaped frontal ring and larger than the elliptical basal ring. All three rings of nearly equal thickness, armed with short irregular branches, which are partly connected, and forming small irregular meshes along the rings. The four basal gates are of different sizes; the two anterior (jugular) gates only half as large as the two posterior (cardinal) gates.

Dimensions.—Height of the frontal ring 0.12, breadth 0.18.

Habitat.—South Atlantic, Station 348, depth 2450 fathoms.

Definition.— with eight large simple gates of equal size. Skeleton composed of three simple complete rings, perpendicular to one another.

The genus Trissocircus, and the following nearly allied Trissocyclus, differ from the two preceding ancestral genera in the remarkable growth of the four basal gates, which reach the size of the lateral gates. Therefore both hemispheres of the shell (the upper or apical and the lower or basal) here become equal and the basal ring becomes equatorial. Sometimes even all three rings attain the same size, so that it is difficult or impossible to distinguish them. In this curious case the Coronida exhibit a striking resemblance to some.

Definition.—Sagittal ring smaller than the two other rings, which are both elliptical.

1. Trissocircus lentellipsis, n. sp. (Pl. 93, fig. 10).

All three rings elliptical, of different sizes, smooth, without spines. The sagittal ring is the smallest, but two to three times as thick as the other two rings, which have the larger (transverse) axis common. The smaller (sagittal) axis of the sagittal ring is also the smaller axis of the equatorial ring, whilst the larger (principal) axis of the former is the smaller axis of the frontal ring.

Dimensions.—Height of the frontal ring 0.07, breadth 0.1.

Habitat.—Western Tropical Pacific, Station 224, depth 1850 fathoms.

2. Trissocircus binellipsis, n. sp. (Pl. 83, fig. 6).

Sagittal ring circular, smaller than the other two rings, which are both equal, elliptical, slightly constricted on the poles of the principal and transverse axes. The axis of the circular sagittal