458 



THE VOYAGE OF H.M.S. CHALLENGER. 



(perradii), and reduced in the direction of the alternating rays (interradii). In some 

 Astracturida the chambered arms exhibit a structure different from the more irregular 

 chamber-work between them, so that we can distinguish the latter, connecting the arms 

 like a web-membrane, as a peculiar " patagium " (as in many Porodiscida) 

 (PL 38, figs. 8, 9). Often the distal ends of the chambered arms are armed with a 

 radial spine (PI. 38, figs. 5, 6, 9). 



The Central Capsule of the Coccodiscida is originally always of the same form and 

 shape as in their ancestral group, the Phacodiscida ; a circular lenticular disk, which 

 envelops the simple or double medullary shell and is enclosed by the cortical phacoid 

 shell. But whilst in the Phacodiscida the phacoid shell envelops the central capsule 

 perfectly, in the Coccodiscida it envelops only the capsule from the two flat sides (by the 

 upper and lower sieve-plates) ; the marginal part of the lenticular capsule overgrows the 

 margin of the phacoid shell by peripheral extension, and fills out the chambered cavity 

 of the concentric girdles. In the Lithocyclida and the Stylocyclida, where there are no 

 chambered arms, the central capsule remains a simple circular lens or disk ; in the 

 Astracturida, where chambered radial arms surround the margin of the circular central 

 disk, the capsule enters also into these arms and fills out the greatest part of their 

 chambered cavities. 



I. Subfamily 



Lithocyclida. 



Margin of the disk simple, 

 circular, without radial 

 appendages. 



IL Subfamily 



Stylocyclida. 



Margin of the disk armed 

 with solid radial spines. 



Synopsis of the Genera of Coccodiscida. 



circular margin. 

 Two opposite spines. 



Neither radial spines nor | Medullary shell simple, 

 chambered arms on the 



Medullary shell double, 



Three radial spines. 

 Four crossed spines. 



( Medullary shell simple, 

 ( Medullary shell double, 

 Medullary shell simple, 

 Medullary shell simple, 



Five to ten or more radial j Medulla T she11 sim P le > 

 spmes ' \ Medullary shell double, 



ill. Subfamily 



Astracturida. 



Margin of the disk with 

 two to five or more 

 (commonly three or four) 

 hollow radial chambered 

 arms (with or without 

 a connecting patagium). 

 (Medullary shell com- 

 monly simple.) 



,,, ... . ( Without patagium, 



Two arms, opposite in one ) 



With patagium, . 



axis. 



rp, I- i / . i ( Without patagium, 



Ihree radial arms (at equal ) 



distances). "j With patagium> 



-r, ,. , C Without patagium 



rour arms (in two crossed ) 



diameters). 



I Wi 

 ( Wi 



With patagium, 



-r,. ,. , . ,, ( Without patagium, 



Five radial arms (at variable J 



distances). 



With patagium, 



196. Lithocydia. 



. 197. Coccodiscus. 



198. Stylocydia. 



199. Amphicydia. 



. '200. Trigonocydia, 



. 201. Staurocydia. 



202. Astrocydia. 



. 203. Coccocijdia. 



. 204. Diplactura. 



. 205. Amphiactura. 



206. Trif/onactura. 



207. Hymenadura. 

 . 208. Astradura. 



. 209. Stauractura. 



. 210. Pentactura. 



211. Ei-liinnctura. 



