Page:Scientific results HMS Challenger vol 18 part 1.djvu/692

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.					$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Margin smooth, without spines,			212. Archidiscus.
						Margin armed with radial spines,			213. Axodiscus.
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 of the disk quite simple, without radial appendages (spines or chambered arms), without peculiar oscula.					$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Margin simple, without an equatorial girdle,			214. Porodiscus.
						Margin with a thin porous equatorial girdle,			215. Perichlamydium.
3. Subfamily Ommatodiscida. Margin of the disk without chambered arms but distinguished by one or two large oscula, or wide openings armed with a crown of spines.					$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Disk with a single marginal osculum,			216. Ommatodiscus.
						Disk with two opposite marginal oscula,			217. Stomatodiscus.
4. Subfamily Stylodictyida. Margin of the disk without peculiar oscular openings and without chambered arms, but armed with solid radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Radial spines of the margin two, three, or four, usually quite regularly disposed.			$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Two opposite spines,			218. Xiphodictya.
						Three equidistant spines,			219. Tripodictya.
						Four crossed spines,			220. Staurodictya.
		Radial spines of the margin five or more, commonly irregularly disposed (generally ten to twelve).			$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Margin simple, without an equatorial girdle,			221. Stylodictya.
						Margin with a thin porous equatorial girdle,			222. Stylochlamydium.