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



Dimensions. — Diameter of the disk 0'4, of the outer medullary shell 0"08, of the inner 0"04 ; 

 length of the polar spines 0'5, basal breadth 0'04. 

 Habitat. — ^Fossil in the rocks of Barbados. 



Genus 187. Triactiscus} Haeckel, 1881, Prodromus, p. 457. 



Definition. — P hacodiscida with simple medullary shell, and with three radial 

 spines on the margin of the disk, placed in the equatorial plane. 



The genus Triactiscus exhibits on the margin of the lenticular phacoid shell three 

 radial solid spines, the distance between which is sometimes equal, at other times 

 unequal. Either all the three spines are of equal size and similar form, or one odd arm 

 is smaller than the two paired arms. The triradial form of this genus is repeated in 

 Tripodocyclia among the Coccodiscida, and in Tripodictya among the Porodiscida. 

 The medullary shell of Triactiscus is simple. 



1. Triactiscus tripyramis, n. sp. (PI. 33, fig, 6). 



Disk with smooth surface and smooth margin, three times as broad as the medullary shell. 

 Pores regular, circular ; twelve to thirteen on the radius of the disk. Three radial spines of nearly 

 equal size and equidistant, or one odd spine a Httle smaller, and the opposite angle (between the 

 paired spines) also smaller. Spines pyramidal, with broad prominent edges, one to one and a half 

 times as long as the radius of the disk, as broad at the base as the radius of the meduUary shell. 



Dimensions. — Diameter of the disk 015 ; of the medullary shell 0'05, pores 0'006. 



Habitat. — Pacific, central area, Station 272, depth 2600 fathoms 



2. Triactiscus tricuspis, n. sp. (PI. 33, fig. 5). 



Disk with smooth surface and smooth margin, five times as broad as the medullary shell. 

 Pores irregular, roundish ; ten to eleven on the radius of the disk. Three radial spines of unequal 

 size and at unequal distances, very short, conical, not longer than the radius of the meduUary shell. 



Dimensions. — Diameter of the cUsk 0'15, of the medullary shell 0"03 ; pores 0'004 to O^OOS. 



Habitat. — Central Pacific, Station 268, depth 2900 fathoms. 



3. Triactiscus tripodiscus, Haeckel. 



Haliomma triadia, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 236, Taf. xxviii. 

 fig. 4. 



Disk with thorny surface and spiny margin, three times as broad as the medullary shell. 

 Pores regular, chcular ; eight to nine on the radius of the disk. Three radial spines of different 



' Triaciiscus = SkA\ with three rays ; Tg<«xTiVxof. 



