48-i 



THE VOYAGE OF H.M.S. CHALLENGER. 



differences as in the foregoing famil}-. 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 peculiai-ities 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 theii* distal end bears a terminal radial spine. 



Tlie 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. 



Synojisis of the Genera of the Porodiscida. 



I. Section of the Porodiscida — Arcliidiscaria. 

 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. Archidiscns. 



213. Axodiscus. 



IL 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. 



^largin simple, without 



an equatorial girdle, . 214. Porodiscus. 



2. Subfamily 



Trematodiscida. 

 Margin of the disk quite simple, without radial append- 

 ages (spines or chambered arras), without peculiar j Margin with a thin por- 

 oscula. 



3. Subfamily 



Ommatodiscida. 

 IMargin of the disk without chambered arms, but dis- 

 tinguished by one or two large oscula, or wide 

 openings armed with a crown of spines. 



4. Subfamily 



Stylodictyida. 



Margin of the disk without 

 peculiar oscular openings - 

 and without chambered 

 arras, but armed with 

 solid radial spines. 



Radial spines of the 

 margin two, three, or 

 four, usually quite 

 regularly disposed. 



Earlial spines of the 

 margin five or more, 

 commonly irregularly 

 disposed (generally ten 

 to twelve). 



ous equatorial girdle, . 215. Perichlamydium. 



Disk with a single 



marginal osculum, . 216. Ommatodiscus. 



Disk with two opposite 



marginal oscula, . 217. Stomatodiseus. 



Two opposite sjsines, . 218. Xiphodichja. 



Three equidistant spines, 219. Tripodictya. 



Four crossed spines, . 220. Staurodictya. 



Margin simple, without 



an equatorial girdle, . 221. Stylodictya. 



Margin with a thin por- 

 ous equatorial girdle, . 222. Sfylocldamydi im . 



