The Central Capsule of the Phacodiscida is everywhere circular, lenticular, envelops the medullary shell, and is enclosed by the phacoid shell, perforated by the radial beams, which connect the latter to the former.
Synopsis of the Genera of the Phacodiscida.
III. Subfamily Sethodiscida. Margin of the disk without radial spines. |  | Margin simple, without equatorial girdle. |  | Medullary shell simple, | 181. Sethodiscus. |
| Medullary shell double, | 182. Phacodiscus. | ||||
| Margin surrounded by a hyaline equatorial girdle. | | Medullary shell simple, | 183. Periphæna. | ||
| Medullary shell double, | 184. Perizona. | ||||
III. Subfamily Heliosestrida. Margin of the disk with two to eight solid radial spines, usually quite regularly disposed. (Number usually constant.) |  | Two radial spines (opposite in one axis). | | Medullary shell simple, | 185. Sethostylus. |
| Medullary shell double, | 186. Phacostylus. | ||||
| Three radial spines. | Medullary shell simple, | 187. Triactiscus. | |||
| Four radial spines (in cross form). | | Medullary shell simple, | 188. Sethostaurus. | ||
| Medullary shell double, | 189. Phacostaurus. | ||||
| Six radial spines. | Medullary shell simple, | 190. Distriactis. | |||
| Eight radial spines. | | Medullary shell simple, | 191. Heliosestrum. | ||
| Medullary shell double, | 192. Astrosestrum. | ||||
III. Subfamily Heliodiscida. Margin of the disk with numerous (ten to twenty or more) radial spines, usually irregularly disposed. (Number variable.) |  | Radial spines all simple, not branched. | | Medullary shell simple, | 193. Heliodiscus. |
| Medullary shell double, | 194. Astrophacus. | ||||
| Radial spines all or partly | Medullary shell simple, | 195. Heliodrymus. |
| I. Subfamily Sethodiscida. Margin of the disk without radial spines. | ||||||
| Margin simple, without equatorial girdle. | ||||||
| Medullary shell simple, | ||||||
| 181. Sethodiscus. | ||||||
| Medullary shell double, | ||||||
| 182. Phacodiscus. | ||||||
| Margin surrounded by a hyaline equatorial girdle. | ||||||
| Medullary shell simple, | ||||||
| 183. Periphæna. | ||||||
| Medullary shell double, | ||||||
| 184. Perizona. | ||||||
| II. Subfamily Heliosestrida. Margin of the disk with two to eight solid radial spines, usually quite regularly disposed. (Number usually constant.) | ||||||
| Two radial spines (opposite in one axis). | ||||||
| Medullary shell simple, | ||||||
| 185. Sethostylus. | ||||||
| Medullary shell double, | ||||||
| 186. Phacostylus. | ||||||
| Three radial spines. Medullary shell simple, | ||||||
| 187. Triactiscus. | ||||||
| Four radial spines (in cross form). | ||||||
| Medullary shell simple, | ||||||
| 188. Sethostaurus. | ||||||
| Medullary shell double, | ||||||
| 189. Phacostaurus. | ||||||
| Six radial spines. Medullary shell simple, | ||||||
| 190. Distriactis. | ||||||
| Eight radial spines. | ||||||
| Medullary shell simple, | ||||||
| 191. Heliosestrum. | ||||||
| Medullary shell double, | ||||||
| 192. Astrosestrum. | ||||||
| III. Subfamily Heliodiscida. Margin of the disk with numerous (ten to twenty or more) radial spines, usually irregularly disposed. (Number variable.) | ||||||
| Radial spines all simple, not branched. | ||||||
| Medullary shell simple, | ||||||
| 193. Heliodiscus. | ||||||
| Medullary shell double, | ||||||
| 194. Astrophacus. | ||||||
| Radial spines all or partly branched. | ||||||
| Medullary shell simple, | ||||||
| 195. Heliodrymus. | ||||||
Subfamily 1. Sethodiscida, Haeckel, 1881, Prodromus, p. 457.
Definition.—Phacodiscida without radial spines on the margin of the disk.
Genus 181. Sethodiscus,[[220]] Haeckel, 1881, Prodromus, p. 457.
Definition.—Phacodiscida with simple medullary shell and simple margin of the circular disk, without surrounding equatorial girdle and without radial spines.
The genus Sethodiscus is the most simple and primitive form of all Phacodiscida, and may be regarded as the common ancestral form of this family. The simple spherical medullary shell is connected by a variable number of radial beams with the lenticular or discoidal cortical shell (or "phacoid shell"). The margin of this latter is quite simple, circular, without solid equatorial girdle or radial spines. From the nearly allied genus Carposphæra of the Sphæroidea, its probable ancestral form, Sethodiscus can be derived simply by lenticular compression of the spheroidal cortical shell.