428 THE VOYAGE OF H.M.S. CHALLENGER. 



Genus 185. Sethostylus, 1 Haeckel, 1881, Prodromus, p. 457. 



Definition. P hacodiscida with simple medullary shell and with two radial 

 spines on the margin of the disk, opposite in the equatorial axis. 



The genus Sethostylus opens the series of the Heliosestrida or of those Phaco- 

 discida in which a constant number of radial spines (two to eight) is more or less 

 regularly disposed on the margin of the disk. All these marginal spines lie in the 

 equatorial plane of the lens, and have in the same species a rather constant number 

 and similar size, a certain form and disposition. Sometimes they incline more or less 

 to irregular variations. In Sethostylus there are only two spines, opposite in the 

 equatorial diameter of the lens ; it corresponds to Stylocyclia among the Cocco- 

 discida, and to Xiphodictya among the Porodiscida. 



Subgenus 1. Sethostylium, Haeckel. 



Definition. Margin of the disk simple, smooth, without equatorial girdle and 

 without a corona of marginal spines. 



1. Sethostylus distyliscus, n. sp. (PI. 31, fig. 9). 



Disk with smooth surface, three times as broad as the medullary shell. Pores regular, circular ; 

 nine to ten on the radius of the disk, in the outer rows cylindrical. Margin of the disk simple, 

 smooth, thin, without peculiar equatorial girdle, radially striped by the prominent beams of the 

 peripheral series of pores. Both marginal spines of equal size, pyramidal, sulcated, about as long 

 as the radius of the disk and as broad at the base as the medullary shell. 



Dimensions. Diameter of the disk 013 to 016, of the medullary shell 0'04 to 0'05 ; length 

 of both opposite spines 0'06 to 0'08, basal breadth 0'03 to 0'04. 



Habitat. Pacific, central area, Station 265, depth 2900 fathoms. 



2. Sethostylus dicylindrus, n. sp. (PL 31, fig. 10). 



Disk with smooth surface, four times as broad as the medullary shell. Pores regular, circular ; 

 sixteen to eighteen on the radius of the disk. Margin of the disk simple, smooth, thick, without 

 peculiar equatorial girdle. Both marginal spines cylindrical, about as long as the diameter of the 

 disk, about twice as broad as a single pore. (Walls of the disk in the central part twice to three 

 times as thick as in the peripheral part.) 



Dimensions. Diameter of the disk 0'2, of the medullary shell 0'05 ; length of both spines 0'2 

 to 0-3, breadth O'OOS. 



Habitat. Pacific, central area, Station 268, depth 2900 fathoms. 



1 Sethostylus = Sieve with styles ; 



