REPORT ON THE RADIOLARIA. 473 



the diameter of the central disk and at the base half as broad, slowly decreasing in breadth 

 towards the distal end, which is armed with a strong pyramidal spine. 



Dimensions. Diameter of the phacoid shell 01, of the medullary shell 005 ; length of the 

 arms 012, basal breadth 0'07. 



Habitat. Pacific, central area, Station 268, depth 2900 fathoms ; also fossil in Barbados. 



7. Trigonactura trixiphos, n. sp. 



Phacoid shell circular, twice as broad as the medullary shell, without a completely surrounding 

 chambered ring, with four pores on its radius. Arms club-shaped, about as long as the diameter 

 of the central disk, at the base half as broad as long, at the rounded distal end broader, and 

 armed with a strong pyramidal terminal spine, nearly as long as the arm itself. 



Dimensions. Diameter of the phacoid shell 0'08, of the medullary shell 0'04 ; length of the 

 arms (without terminal spine) 0'08, basal breadth 0'04, distal breadth 0'06. 



Habitat. Fossil in the rocks of Barbados. 



Genus 207. Hymenactura, 1 Haeckel, 1881, Prodromus, p. 459. 



Definition. C occodiscida with three chambered arms on the margin of the 

 circular or triangular disk, connected by a spongy patagium. 



The genus Hymenactura differs from the foregoing Trigonactura in the spongy 

 patagium between the arms, and bears therefore to it the same relation that among the 

 Porodiscida Hymeniastrum does to the simpler Dictyastrum. The oldest known 

 species of this genus is Hymenactura pythagorve, described by Ehrenberg as Hymeni- 

 astrum pythagorce, but differing from this in the structure of the central disk. 



Subgenus 1. Hymenacturium, Haeckel. 

 Definition. Distal end of the arms blunt or truncated, without terminal spines. 



1. Hymenactura archimedis, n. sp. (PI. 38, fig. 8). 



Phacoid shell three times as broad as the medullary shell, with eight pores on its radius. 

 Arms nearly trapezoidal, somewhat longer than the diameter of the central disk, at the truncated 

 distal end nearly as broad, at the base only half as broad. In each arm eleven to twelve 

 transverse rows of square chambers, each of which exhibits on the surface one large pore. 

 Patagium enveloping the basal half of the arms, with four to five convex rows of chambers, forming 

 together a circular concentric disk. 



Dimensions. Diameter of the phacoid shell Oil, of the medullary shell 0'035 ; length of the 

 arms 012, basal breadth 0'05, distal breadth 01. 



Habitat. Western Tropical Pacific, Station 225, depth 4475 fathoms. 



1 Hymenactura = Star-shaped shell with membrane between the rays ; tp.yi', din';, ovt>ti. 



(ZOOL. CHALL. EXP. PART XL. 1885.) Rr 60 



