484 



THE VOYAGE OF H.M.S. CHALLENGER, 



differences as in the foregoing family. In the subfamily of Stylodictyida it bears a 

 certain number of solid radial spines, often regularly disposed (as in the Stylocyclida). 

 In the subfamily of Euchitonida the margin is distinguished by the possession of two 

 to six or more (commonly three or four) chambered arms, also situated in the plane of 

 the disk, and of the same structure (sometimes more or less irregular, spongy). These 

 arms are very variable in size, form, and structure, exhibit the same peculiarities as in 

 the Astracturida, and are sometimes free, at other times connected by a " patagium " 

 or an interbrachial spongy skeleton of different structure, like a web membrane 

 (compare above, p. 458). In some genera the arms become forked or branched on the 

 distal end. Sometimes their distal end bears a terminal radial spine. 



The Central Capsule of the Porodiscida assumes generally the form of the including 

 shell, with or without arms, but is constantly somewhat smaller, as it remains enclosed 

 by the sieve-plates of the disk surface. Often the capsule is filled with many coloured 

 oil-globules, disposed regularly in the chamber rows. The nucleus of it is enclosed by 

 the central chamber, and in many cases by this and the innermost concentric rings. 



Synopsis of the Genera of the Porodiscida. 



I. Section of the Porodiscida Archidiscaria. 

 Central chamber of the disk surrounded only by one single chambered ring. 



1. Subfamily 



Archidiscida. 



Only one single chambered girdle surrounds the central 

 chamber; margin smooth or spiny. 



Margin smooth, without 

 spines, 



Margin armed with radial 

 spines, 



212. Archidiscus. 



213. Axodiseus. 



II. Section of the Porodiscida Astrodiscaria. 



Central chamber of the disk surrounded by two or more (commonly three to six) concentric 



chambered rings or radiated girdles. 



2. Subfamily 



Trematodiscida. 



( Margin simple, without 



an equatorial girdle, . 214. Porodiscus. 



Margin of the disk quite simple, without radial append- - 



ages (spines or chambered arms), without peculiar | Margin with a thin por- 



oseula. 



3. Subfamily 



Ommatodiscida. 



Margin of the disk without chambered arms, but dis- 

 tinguished by one or two large oscula, or wide 

 openings armed with a crown of spines. 



ous equatorial girdle, . 215. Perichlamydium. 



4. Subfamily 



Stylodictyida. 



Margin of the disk without 

 peculiar oscular openings - 

 and without chambered 

 arms, but armed with 

 solid radial spines. 



I 



Radial spines of the 

 margin two, three, or 

 four, usually quite 

 regularly disposed. 



Disk with a single 

 marginal osculum, 



Disk with two opposite 

 marginal oscula, 



Two opposite spines, 



216. Ommatodiscus. 



217. Stomatodiscus. 



218. Xipliodidya. 



Three equidistant spines, 219. Tripodidya. 



Four crossed spines, 



220. Staurodidya. 



Radial spines of the f Margin simple, without 



margin five or more, an equatorial girdle, . 221. Stylodictya. 



commonly irregularly -I 



disposed (generally ten I Margin with a thin por- 



to twelve). oils equatorial girdle, . 22?. Sfyloclilamydi 'in. 



