Page:Scientific results HMS Challenger vol 18 part 1.djvu/679

Rh

Definition.— with three chambered arms on the margin of the circular or triangular disk, without a connecting patagium.

The genus Trigonactura exhibits three radial arms, which in all known species are separated by three equal angles. The terminal points of the arm-axes are the corners of an equilateral triangle. It corresponds therefore to Dictyastrum among the Porodiscida.

Definition.—Distal end of the arms blunt or truncated, without a terminal spine.

1. Trigonactura pythagoræ, Haeckel.

Astromma pythagoræ, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 66, Taf. xxx. fig. 2.

Phacoid shell circular, twice as broad as the medullary shell, with five pores on its radius, without a completely surrounding chambered girdle. Arms nearly square, at the truncated distal end as broad as long, and scarcely broader than at the base, two-thirds as long as the diameter of the central disk.

Dimensions.—Diameter of the phacoid shell 0.1, of the medullary shell 0.05; length of the arms 0.07, distal breadth 0.07.

Habitat.—Cosmopolitan; Atlantic, Indian, Pacific, in various depths; also fossil in Tertiary rocks of Barbados and Nicobar.

2. Trigonactura rhopalastrella, n. sp.

Stephanastrum sp., Bury, 1862, Polycystins of Barbados, pl. xv. fig. 7.

Phacoid shell twice as broad as the medullary shell, with five pores on its radius, without a perfect chambered ring around it. Arms club-shaped, at the blunt distal end rounded, as long as the diameter of the phacoid shell; their basal breadth is one-fourth, their distal breadth one-half of its length.

Dimensions.—Diameter of the phacoid shell 0.1, of the medullary shell 0.05; length of the arms 0.1, basal breadth 0.025, distal breadth 0.05.

Habitat.—Pacific, central area, Stations 270 to 274, depths 2400 to 2800 fathoms; also fossil in Tertiary rocks of Barbados.