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



8. Sethodiscus echinatus, Haeckel. 



Haliomma eclrinatum, Ehrenberg, 1875, AbhandL d. k. Akad. d. Wiss. Berlin, p. 74, 

 Taf. xxvii. fig. 2. 



Disk with radiated thorny surface, three times as broad as the medullary shell. Pores 

 regular, circular ; eleven to twelve on the radius of the disk, arranged in radial series (about sixty), 

 which are separated by thorny crests, prominent a little on the dentated margin. 



Dimensions. Diameter of the disk 0'18, of the medullary shell 0'06, of the pores O'Ol. 



Habitat. Fossil in the rocks of Barbados. 



9. Sethodiscus cristatus, n. sp. 



Disk with radiated surface, four times as broad as the medullary shell. Pores regularly 

 circular ; sixteen to seventeen on the radius of the disk, arranged in radial series (about eighty), 

 which are separated in the distal half by smooth elevated crests, strongly prominent on the dentated 

 margin. (Very similar to Phacodiscus cristatus, PI. 35, fig. 6, but with simple medullary shell.) 



Dimensions. Diameter of the disk 0'2, of the medullary shell 005, of the pores 0'005. 



Habitat. South Pacific, Station 285, depth 23*75 fathoms. 



Genus 182. Phacodiscus, 1 Haeckel, 1881, Prodromus, p. 457. 



Definition. P hacodiscida with double medullary shell and simple margin of 

 the circular disk, without surrounding equatorial girdle and without radial spines. 



The genus Phacodiscus differs from the ancestral genus Sethodiscus only in the 

 duplication of the medullary shell, and has therefore the same relation to it that Theco- 

 sphcera in the Sphseroidea exhibits to Carposphcera. Both former discoid al genera 

 differ from the two latter spheroidal by the lenticular flattening of the cortical shell. 



Subgenus 1. Phacodiscinus, Haeckel. 

 Definition. Surface of the disk smooth, without radial ribs or spines. 



1. Phacodiscus rotula, n. sp. (PI. 35, fig. 7). 



Disk with smooth surface, four and a half tunes as broad as the outer and fourteen times as 

 broad as the inner medullary shell. Pores regularly circular ; sixteen to eighteen on the radius of 

 the disk. Margin of the lens very thick, truncated, nearly as broad as the outer medullary shell. 



Dimensions. Diameter of the disk 0'21, of the outer medullary shell 0'045, of the inner 0'015; 

 pores 0-008. 



Habitat. North Pacific, Station 224, depth 1850 fathoms. 



1 Pliacodiscus = Lenticular disk ; 



