Genus 206. Trigonactura,[1] Haeckel, 1881, Prodromus, p. 459.
Definition.—Coccodiscida 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.
Subgenus 1. Trigonacturium, Haeckel.
Definition.—Distal end of the arms blunt or truncated, without a terminal spine.
1. Trigonactura pythagoræ, Haeckel.
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.
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.
- ↑ Trigonactura = Triangular shell with three rays; τρίγονον, ἀκτίς, ὀυρά.
