Page:Scientific results HMS Challenger vol 18 part 1.djvu/896

688 lying opposite in pairs in two crossed diagonal planes. Therefore the number of the septa (four) and the domes (eight) is here doubled. Among the eight cupolas we distinguish four median (on both sides of the sagittal plane) and four lateral (on both sides of the equatorial plane); both groups are of different size and form. Probably Zonidium is derived from Zonarium by lattice-connection between the eight diagonal wing-spines, which in both species of this genus are present, the same as in Tetrapyle octacantha.

1. Zonidium octostylium, n. sp.

Cortical shell nearly quadrangular, with rounded corners and thorny surface. Eight long and thin radial spines on both sides of the lateral plane opposite in pairs and lying in two crossed diagonal planes (as in Tetrapyle octacantha). Sagittal constriction three times as long as the lentelliptical medullary shell. Four medial cupolas (on both sides of the sagittal plane) somewhat larger than the four lateral cupolas (on both sides of the equatorial plane).

Dimensions.—Length of the cortical shell 0.15, breadth 0.12; length of the medullary shell 0.05, breadth 0.03.

Habitat.—Indian Ocean, Madagascar, Rabbe, surface.

2. Zonidium octotholium, n. sp. (Pl. 50, fig. 12).

Cortical shell nearly octangular, with spiny surface; twenty long and stout radial spines between numerous smaller spines; eight wing-spines opposite in pairs in two crossed diagonal planes (as in the preceding species); twelve other strong spines in the lateral plane (four longer opposite in pairs on the poles of the principal and transverse axes, eight others smaller, alternating between these and the diagonal spines). Sagittal constriction twice as long as the hexagonal medullary shell. Four median cupolas somewhat smaller than the four lateral cupolas.

Dimensions.—Length of the cortical shell 0.18, breadth 0.15; length of the medullary shell 0.07, breadth 0.05.

Habitat.—Indian Ocean, Zanzibar, Pullen, depth 2200 fathoms.

Definition.— with symmetrical spiral shell, divided by the spiral plane into two symmetrical halves; all windings of the spiral lie in this plane. Primordial chamber either simple or Larnacilla-shaped.

The family comprises all those  in which the growth of the latticed shell is spirally winding in one plane, Nautilus-like. They agree in the spiral growth with the following family. But in the Streblonida the spiral is screw-shaped,