Page:Scientific results HMS Challenger vol 18 part 1.djvu/543

Rh from them in the multiplication of the cortical shell, and may be considered as the most highly developed form of this amphistylous series, which proceeds from Ellipsoxiphus to Lithatractus and Stylatractus.

Definition.—Shell composed of two medullary shells and two cortical shells.

1. Cromyatractus tetraphractus, n. sp. (Pl. 15, fig. 2).

Stylocromium tetraphractum, Haeckel, 1879, Atlas (pl. xv. fig. 2).

Proportion of the main axes of the four concentric shells about = 1 : 3 : 5 : 7. Network of all four shells nearly of the same structure, subregular, with circular pores of equal size (in one and the same shell). The absolute size of the pores increases gradually from the innermost to the outermost shell. The bars between the pores are smaller and quite smooth. Surface of the outermost shell smooth. Polar spines cylindrical, with conical apex, of variable length, nearly as broad as the innermost shell.

Dimensions.—Main axes of the four shells—(A) inner medullary shell 0.03, (B) outer medullary shell 0.08, (C) inner cortical shell 0.15, (D) outer cortical shell 0.2; equatorial axes of them—(A) 0.025, (B) 0.06, (C) 0.12, (D) 0.15; pores of (A) 0.004, (B) 0.007, (C) 0.01, (D) 0.013, bars 0.002 to 0.004; length of the polar spines 0.1 to 0.3 (and more), breadth 0.015.

Habitat.—Antarctic (Indian) Ocean, Station 157, depth 1950 fathoms.

2. Cromyatractus tetralepas, n. sp.

Proportion of the main axes of the four concentric shells about = 1 : 2 : 4 : 6. Network of the two medullary shells regular, with small circular pores, little larger than the bars. Network of the two cortical shells irregular, with much larger polygonal pores, three to nine times as broad as the thin bars. (Somewhat similar to Cromyatractus ceparius, Pl. 15, fig. 4.) Surface of the outermost shell smooth or a little thorny. Polar spines cylindrical, very large, twice to three times as long as the main axis of the outermost shell, about as broad as the innermost shell, with conical apex; smooth or a little thorny.

Dimensions.—Main axes of the four shells—(A) 0.03, (B) 0.05, (C) 0.13, (D) 0.18; equatorial axes of them—(A) 0.2, (B) 0.45, (C) 0.11, (D) 0.14; pores of the two medullary shells 0.004 and 0.006, of both cortical shells 0.01 to 0.03, bars 0.003.

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

3. Cromyatractus tetracelyphus, n. sp. (Pl. 15, figs. 1, 1a).

Proportion of the main axes of the four concentric shells about = 1 : 3 : 10 : 11. Network of the two spherical medullary shells (fig. 1a) regular, with small circular pores, twice as broad as