422 THE VOYAGE OF H.M.S. CHALLENGEK. 



Subfamily 1. Sethodiscida, Haeckel, 1881, Prodromus, p. 457. 

 Definition. — P hacodiscida mthout radial spines on the margin of the disk. 



Genus 181. Sethodiscus,^ Haeckel, 1881, Prodromus, p. 457. 



Definition. — P hacodiscida with simple medullary shell and simple margin of th;' 

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



The genus Sethodiscus is the most simple and primitive form of all Phacodiscida, and 

 may be regarded as the common ancestral form of this family. The simple spherical 

 medullary shell is connected by a variable number of radial beams with the lenticular or 

 discoidal cortical shell (or " phacoid shell "). The margin of this latter is quite simple, 

 circular, without solid equatorial girdle or radial spines. From the nearly allied genus 

 Carposphceixt of the S j) hier o i de a, its probable ancestral form, Sethodiscus can be 

 derived simply by lenticular compression of the spheroidal cortical shell. 



Subgenus 1. Sethodiscinus, Haeckel. 

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



1. Sethodiscus phacoides, n. sp. 



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

 fourteen to fifteen on the radius of the disk. (Very similar to PcripJia-na cincta, PI. 33, fig. 4, but 

 without the girdle of the margin.) 



Dimensions. — Diameter of the disk 0-2, of the medullary shell Omi7, of the pores 0'005. 



Habitat. — Pacific, central area, Stations 270 to 274, in various depths. 



2. Sethodiscus macroporus, n. sp. 



Disk with smooth surface, twice as broad as the medullary shell. Pores regular, circular, very 

 lai'ge ; five to six on the radius of the disk. (Eemarkahle for the extraordinary size of the pores, 

 which reaches half the radius of the medullary shell.) 



Dimensions. — Diameter of the disk O^l, of the meduUary shell 0'05, of the poi-es 0"012. 



Habitat. — North Atlantic, Gulf Stream, Fferoe Channel, John Murray. 



3. Sethodiscus microporus, n. sp. 



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

 very small ; twenty -two to twenty-four on the radius of the disk. (The small pores are scarcely 

 half as broad as the thick bars between them.) 



' &(Aorfiscits= Sieve-disk ; (r»)()o'f, SiV^oj. 



