| 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, | 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. |  | 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. | | 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. |  | Radial spines of the margin two, three, or four, usually quite regularly disposed. | | 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). | | Margin simple, without an equatorial girdle, | 221. Stylodictya. | ||||||
| Margin with a thin porous equatorial girdle, | 222. Stylochlamydium. | ||||||||
5. Subfamily Euchitonida. Margin of the disk with radial chambered arms (or hollow chambered appendages), on the distal end of the arms with or without solid radial spines. |  | Two arms, opposite in one main axis. |  | Two arms simple, undivided. | | Without a patagium, | 223. Amphibrachium. | ||
| With a patagium, | 224. Amphymenium. | ||||||||
| Two arms forked (or one simple, other forked). | | Without a patagium, | 225. Amphirrhopalum. | ||||||
| With a patagium, | 226. Amphicraspedum. | ||||||||
| Three arms (either regular, equal, or bilateral, with unequal arms). |  | Three arms simple, undivided. |  | Without a patagium. | | Regular, | 227. Dictyastrum. | ||
| Bilateral, | 228. Rhopalastrum. | ||||||||
| With a patagium. | | Regular, | 229. Hymeniastrum. | ||||||
| Bilateral, | 230. Euchitonia. | ||||||||
| Three arms forked. | | Either regular or bilateral. | | Without a patagium, | 231. Chitonastrum. | ||||
| With a patagium, | 232. Trigonastrum. | ||||||||
| Four arms (either regular, with four equal crossed arms, or bilateral, with paired arms). |  | Four arms simple, undivided. | | Without a patagium. | | Regular, | 233. Stauralastrum. | ||
| Bilateral, | 234. Hagiastrum. | ||||||||
| With a patagium. | | Regular, | 235. Histiastrum. | ||||||
| Bilateral, | 236. Tessarastrum. | ||||||||
| With a terminal patagial girdle, | 237. Stephanastrum | ||||||||
| Four arms forked. | | Without a patagium. | | Regular, | 238. Dicranastrum. | ||||
| Bilateral, | 239. Myelastrum. | ||||||||
| Five arms (equal or unequal). | | Five arms simple, undivided. | | Without a patagium, | 240. Pentalastrum. | ||||
| With a patagium, | 241. Pentinastrum. | ||||||||
| Five arms forked. | Without a patagium, | 242. Pentophiastrum. | |||||||
| Six arms (equal or unequal). | | Six arms simple, undivided. | | Without a patagium, | 243. Hexalastrum. | ||||
| With a patagium, | 244. Hexinastrum. | ||||||||
| 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, | ||||||||||
| 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. | ||||||||||
| 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. | ||||||||||
| 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. | ||||||||||
| Radial spines of the margin two, three, or four, usually quite regularly disposed. | ||||||||||
| 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). | ||||||||||
| Margin simple, without an equatorial girdle, | ||||||||||
| 221. Stylodictya. | ||||||||||
| Margin with a thin porous equatorial girdle, | ||||||||||
| 222. Stylochlamydium. | ||||||||||
| 5. Subfamily Euchitonida. Margin of the disk with radial chambered arms (or hollow chambered appendages), on the distal end of the arms with or without solid radial spines. | ||||||||||
| Two arms, opposite in one main axis. | ||||||||||
| Two arms simple, undivided. | ||||||||||
| Without a patagium, | ||||||||||
| 223. Amphibrachium. | ||||||||||
| With a patagium, | ||||||||||
| 224. Amphymenium. | ||||||||||
| Two arms forked (or one simple, other forked). | ||||||||||
| Without a patagium, | ||||||||||
| 225. Amphirrhopalum. | ||||||||||
| With a patagium, | ||||||||||
| 226. Amphicraspedum. | ||||||||||
| Three arms (either regular, equal, or bilateral, with unequal arms). | ||||||||||
| Three arms simple, undivided. | ||||||||||
| Without a patagium. | ||||||||||
| Regular, | ||||||||||
| 227. Dictyastrum. | ||||||||||
| Bilateral, | ||||||||||
| 228. Rhopalastrum. | ||||||||||
| With a patagium. | ||||||||||
| Regular, | ||||||||||
| 229. Hymeniastrum. | ||||||||||
| Bilateral, | ||||||||||
| 230. Euchitonia. | ||||||||||
| Three arms forked. | ||||||||||
| Either regular or bilateral. | ||||||||||
| Without a patagium, | ||||||||||
| 231. Chitonastrum. | ||||||||||
| With a patagium, | ||||||||||
| 232. Trigonastrum. | ||||||||||
| Four arms (either regular, with four equal crossed arms, or bilateral, with paired arms). | ||||||||||
| Four arms simple, undivided. | ||||||||||
| Without a patagium. | ||||||||||
| Regular, | ||||||||||
| 233. Stauralastrum. | ||||||||||
| Bilateral, | ||||||||||
| 234. Hagiastrum. | ||||||||||
| With a patagium. | ||||||||||
| Regular, | ||||||||||
| 235. Histiastrum. | ||||||||||
| Bilateral, | ||||||||||
| 236. Tessarastrum. | ||||||||||
| With a terminal patagial girdle, | ||||||||||
| 237. Stephanastrum | ||||||||||
| Four arms forked. | ||||||||||
| Without a patagium. | ||||||||||
| Regular, | ||||||||||
| 238. Dicranastrum. | ||||||||||
| Bilateral, | ||||||||||
| 239. Myelastrum. | ||||||||||
| Five arms (equal or unequal). | ||||||||||
| Five arms simple, undivided. simple, | ||||||||||
| Without a patagium, | ||||||||||
| 240. Pentalastrum. | ||||||||||
| With a patagium, | ||||||||||
| 241. Pentinastrum. | ||||||||||
| Five arms forked. | ||||||||||
| Without a patagium, | ||||||||||
| 242. Pentophiastrum. | ||||||||||
| Six arms (equal or unequal). | ||||||||||
| Six arms simple, undivided. | ||||||||||
| Without a patagium, | ||||||||||
| 243. Hexalastrum. | ||||||||||
| With a patagium, | ||||||||||
| 244. Hexinastrum. | ||||||||||
Subfamily 1. Archidiscida, Haeckel.
Definition.—Porodiscida with a simple spherical or lenticular latticed central chamber, surrounded by a single concentric latticed ring, which is divided by radial beams into two to six or more radial chambers.
Genus 212. Archidiscus,[[251]] n. gen.
Definition.—Porodiscida with a simple central chamber, surrounded by a single concentric ring, which is divided by radial beams into two to six or more radial chambers, without radial spines on the margin.
The genus Archidiscus begins the long and polymorphous series of the Cyclodiscaria or of those Discoidea which do not possess the peculiar "phacoid shell" characteristic of the three preceding families, united as "Phacodiscaria." As already mentioned above, both these groups are probably of independent origin, derived from the Sphæroidea in different ways (compare pp. [402], [405], &c.). Among all Cyclodiscaria Archidiscus is the most simple, and probably the common ancestral form, from which the other genera may be derived.
Subgenus 1. Dioniscus, Haeckel.
Definition.—Ring with two chambers, separated by two radial beams.
1. Archidiscus dioniscus, n. sp.