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



« 

 Genus 183. Periphcena,^ Elireuberg, 1873, Monatsber. d. k. preuss. Akad. d. 



Wiss. Berlin, p. 246. 



Definition. — P hacodiscida with simple medullary shell, without radial spines, 

 Ijut with a solid equatorial girdle around the margin of the lenticular disk. 



The genus PenphcBna, founded by Ehreubei-g in 1873 for the fossil Pervphcena 

 decora of Barbados, differs fi-om its ancestral form Sethodiscus in the development of a 

 very thin siliceous sohd girdle around the margin of the lenticular disk ; this girdle lies 

 in the equatorial plane of the shell, and reappears in similar form in Perichla'mydiuvi 

 among the Porodiseida, in Spongophacus among the Spongodiscida, and in Zonodiscus 

 aniono- the Cenodiscida. 



-'ti 



1. Periphcena cincta, n. sj). (PI. 33, fig. 4). 



Disk with smooth surface, four times as broad as the meduUary shell. Pores regularly circular; 

 fourteen to sixteen on the radius of the disk. Gii-dle of the maroin about half as broad as the 

 radius of the medullary shell, in the distal half structureless, in the proximal half with seventy to 

 eighty short radial ribs. 



Bwiensions. — Diameter of the disk 0-2, of the medullary shell 0'05, of the pores 0'005. 



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



2. Periphcena decora, Ehrenberg. 



Periphcena decora, Ehreiibei'g, 1875, Abhandl. d. k. Akad. d. "Wiss. Berlin, p. 80, Taf. xxviii. fig. 6. 



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

 twenty to twenty-two on the radius of the disk, disposed in radial series. Girdle of the margin 

 nearly as broad as the radius of the meduUary shell, in the distal half structureless, in the proximal 

 half with eighty to ninety short radial riljs. 



Dimensions. — Diameter of the disk 0-25, of the medullary shell 0-08, of the pores 0-06. 



Habitat. — Fossil in Barbados. 



3. Periphcena statoblastus, n. sp. 



Disk with smooth surface, five times as broad as the medullary shell. Pores regularly circular; 

 twenty-four to twenty-six on the radius of the disk, chsposed in radial series, those in the distal half 

 of the disk being separated by piercing radial beams. Girdle of the margin twice as broad as the 

 radius of the medullary shell, in the whole breadth with one hundred and twenty to one hundred 

 and thirty piercing radial ribs, the prolongations of the beams. 



DimcnsioTis. — Diameter of the disk 0-3, of the medullary shell 0-06, of the pores 0-04. 



Habitat. — South Pacific, Station 300, depth 1375 fathoms. 



' Periphcena— Hhell with transparent girdle ; !rs;;<pa(»«. 



