Genus 141. Cromyatractus,[[180]] n. gen.
Definition.—Druppulida with four or more concentric shells (two medullary shells and two or more cortical shells), in the main axis with two large opposite polar spines of equal size and similar form.
The genus Cromyatractus is very nearly allied to all the foregoing genera, which bear two equal spines opposite to one another on the poles of the main axis; it differs 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.
Subgenus 1. Cromyatractium, Haeckel.
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.