REPORT ON THE RADIOLARIA. 529 



9. Rhopalastrum triceros, n. sp. (PL 43, fig. 4). 



Distance between the paired arms about two-thirds as large as their distance from the odd arm. 

 All three arms club-shaped, three times as broad at the thickened distal part as at the base, and 

 armed with one single, conical, terminal spine. Odd arm of the same breadth, but twice as long 

 as the paired arms. 



Dimensions. Eadius of the odd arm 0'35, of the paired arms 0'2 ; basal breadth 0'04, distal 

 breadth 012. 



Habitat. Pacific, central area, Station 274, surface. 



10. Rhopalastrum hexaceros, n. sp. (PI. 43, fig. 3). 



Distance between the paired arms equals four-fifths of their distance from the odd arm. All three 

 arms nearly of the same size, about square, a little broader at the truncated distal end, which is 

 armed at both corners with a strong, conical, radial spine. 



Dimensions. Radius of each arm 0'2, basal breadth O'l, distal breadth 0'12. 



Habitat. Indian Ocean, Ceylon, Belligemma, Haeckel, surface. 



11. Rhopalastrum arcticum, n. sp. (PL 43, fig. 6). 



Distance between the paired arms half as large as their distance from the odd arm, which is a little 

 larger. All three arms of the same form, lanceolate, twice to three times as long as broad, twice 

 as broad in the middle as at either end. Each arm with twelve to fourteen transverse septa, at the 

 distal end with a bunch of conical spines, and one single, very large, pyramidal, terminal spine. 



Dimensions. Eadius of each arm (without spine) 0'17, greatest breadth of it 0'05 to 0'06. 



Habitat. Arctic Ocean, lat. 83 19' N., North Polar expedition of the " Alert." 



Genus 229. Hymeniastrum, 1 Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. 



d. Wiss. Berlin, p. 54. 



Definition. P orodiseida with three simple, undivided, chambered arms, 

 connected by a patagium ; triangular shell regular, with three equal arms and three 

 equal angles. 



The genus Hymeniastrum was founded by Ehrenberg (1847) with a very incomplete 

 diagnosis, and hitherto known only by one single species, figured by him as Hymeni- 

 astrum pythagorce (Mikrogeol., 1854, Taf. xxxvi. fig. 31). This form occurs in two 

 different states, externally quite identical ; in one state the central disk (as figured, loc. 

 cit.J, is a simple lens or hollow disk, containing a medullary shell or "central chamber"; 

 in the other state the central disk is composed of two concentric rings surrounding the 

 " central chamber." We retain here the name Hymeniastrum for this latter state, 



1 Hymeniastrum = Membranous star ; vft^r, aarfon. 

 (ZOOL. CHALL. Exp. PART XL. 1885.) Rr 67 



