REPORT ON THE RADIOLARIA. 471 



Genus 206. Trigonactura,^ Haeckel, 1881, Prodromus, p. 459. 



Definition. — C occodiscida 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 'pythagorce, Haeckel. 



Astromma pythagorce, 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-tliirds as long as the diameter of 

 the central disk. 



Dimensions. — Diameter of the phacoid shell O'l, of the medullary shell O'OS ; length of the 

 arms 0-07, distal breadth 0-07. 



Habitat. — Cosmopohtan ; Atlantic, Indian, Pacific, in various depths ; also fossil in Tertiary rocks 

 of Barbados and Nicobar. 



2. Trigonactura rJiopalastrella, n. sp. 



Stephanastrum sp.. Bury, 1562, Polycystins of Barbados, pi. 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 ; tlieir basal breadth is one-fourth, their distal breadth one-half of 

 its length. 



Dimensions. — Diameter of the phacoid shell O'l, of the medullary shell 0'05 ; length of the 

 arms 01, basal breadth 0-025, distal breadth 005. 



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 ; T^iyoi/ar, cixris, oi^x. 



