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



2. Cenodiscus rotula, n. sp. 



Disk with smooth surface, without radial ribs or spines. Margin of the clisk blunt, very thick, 

 rounded ; both faces little convex. Pores regular, circiilar ; thirteen to fourteen on the radius of 

 the disk. (Similar to Phacodiscus rotula, PI. 35, fig. 7, but without medullary shell.) 



Dimensions. — Diameter of the disk 0'16, of the pores O^OOG. 



Eahitat. — North Pacific, Station 241, depth 2300 fathoms. 



3. Cenodiscus lenticula, n. sp. 



Disk with thorny surface, scattered with small, conical spines. Margin of the lenticular 

 biconvex disk thin. Pores irregular, roundish ; ten to eleven on the radius of the disk. 

 Dimensions. — Diameter of the disk 015, of the pores O'OOS. 

 Habitat. — South Pacific, Station 295, depth 1500 fathoms. 



Genus 176. Zonodiscus,^ n. gen. 



Definition. — C enodiscida with surrounding solid equatorial girdle on the 

 margin of the lenticular disk, without radial spines. 



The genus Zonodiscus differs from the preceding Cenodiscus only in the develop- 

 ment of a solid siliceous girdle around the keen margin of the lenticular disk. This 

 form can also be derived from Periphcena or Perizona by reduction and loss of the 

 medullary shell. The same girdle formation returns not only in both these Phacodiscida 

 (PI. 32, fig. 7 ; PI. 33, fig. 4), but also in the Porodiscid Perichlamydium. 



1. Zonodiscus saturnalis, n. sp. 



Disk with smooth surface, without radial spines. Pores regular, circular, fifteen to sixteen on the 

 radius of the disk, iu its distal half arranged in fifty to sixty radial series, which are separated by 

 prominent radial crests or ribs. The crests are prolonged into the proximal half of the thin solid 

 equatorial girdle, which is one-third as broad as the radius of the disk. 



Dimensions. — Diameter of the disk 0'25, of the pores 0'05. 



Hahitat. — South Atlantic, Station 335, depth 1425 fathoms. 



Subfamily 2. Trochodiscida, Haeckel. 



Definition. — C enodiscida with radial spines on the margin of the disk, disposed 

 in the equatorial plane. 



Genus 177. Stylodisctcs,^ n. gen. 



Definition. — C enodiscida with two radial spines on the margin of the disk, 

 opposite in one equatorial axis. 



Zonodiscus = Disk with girdle ; ^atin, Hhkos. ^ Stylodiscus = Disk with styles ; jtwAoj, S/okoj. 



