Only in few Coccodiscida the girdle-building remains restricted to the equatorial planes, so that all the chambers lie in it. Commonly on both sides of this plane become developed several layers, and often the number of these (three to six or more) increases towards the periphery; in other cases not their number, but their height increases. Therefore very often the margin of the discoidal shell is much thickened, as thick as the centre of the lenticular phacoid shell (or even more); whilst between the latter and the former (on the proximal girdles) the disk is considerably thinner (Pl. [36], figs. 2, 4; Pl. [37], figs. 7, 8; Pl. [38], figs. 2, 4). The stratified layers communicate by large openings between their chambers. The radial beams are commonly more or less regular and piercing, but also frequently irregular and interrupted; often their number increases towards the margin by intercalation of new beams.
The Pores of the sieve-plates, which cover both sides of the chambered disk, appear on the margin of the phacoid shell as direct continuations of the pores of the latter, and sometimes they are so regularly disposed that one single circular pore is situated on the surface of each chamber (Pl. [36], fig. 7; Pl. [37], fig. 1). But commonly the pores are of variable size and number, two to three on each chamber, and often quite irregularly scattered.
The Margin of the chambered disk exhibits many differences, which afford characters for the distinction of genera. In the first subfamily, the Lithocyclida, the margin is quite simple without radial appendages. In the second subfamily, the Stylocyclida, it is armed with solid radial spines lying in the equatorial plane, and often regularly disposed in the same manner as in the Phacodiscida (compare above, p. [421]). In the third subfamily, the Astracturida, the margin bears two or more (commonly three or four) chambered arms, also situated in the plane of the disk, and of the same structure as the circular chambered girdles (Pl. [38]). In some cases even the whole system of chamber-girdles is represented only by the radial arms, which are inserted immediately on the margin of the phacoid shell. We may regard therefore these formations as imperfect chambered disks, which are developed only in the direction of certain rays (perradii), and reduced in the direction of the alternating rays (interradii). In some Astracturida the chambered arms exhibit a structure different from the more irregular chamber-work between them, so that we can distinguish the latter, connecting the arms like a web-membrane, as a peculiar "patagium" (as in many Porodiscida) (Pl. [38], figs. 8, 9). Often the distal ends of the chambered arms are armed with a radial spine (Pl. [38], figs. 5, 6, 9).
The Central Capsule of the Coccodiscida is originally always of the same form and shape as in their ancestral group, the Phacodiscida; a circular lenticular disk, which envelops the simple or double medullary shell and is enclosed by the cortical phacoid shell. But whilst in the Phacodiscida the phacoid shell envelops the central capsule perfectly, in the Coccodiscida it envelops only the capsule from the two flat sides (by the upper and lower sieve-plates); the marginal part of the lenticular capsule overgrows the margin of the phacoid shell by peripheral extension, and fills out the chambered cavity of the concentric girdles. In the Lithocyclida and the Stylocyclida, where there are no chambered arms, the central capsule remains a simple circular lens or disk; in the Astracturida, where chambered radial arms surround the margin of the circular central disk, the capsule enters also into these arms and fills out the greatest part of their chambered cavities.
Synopsis of the Genera of Coccodiscida.
I. Subfamily Lithocyclida. Margin of the disk simple, circular, without radial appendages. |  | Neither radial spines nor chambered arms on the circular margin. |  | Medullary shell simple, | 196. Lithocyclia. |
| Medullary shell double, | 197. Coccodiscus. | ||||
II. Subfamily Stylocyclida. Margin of the disk armed with solid radial spines. |  | Two opposite spines. |  | Medullary shell simple, | 198. Stylocyclia. |
| Medullary shell double, | 199. Amphicyclia. | ||||
| Three radial spines. | Medullary shell simple, | 200. Trigonocyclia. | |||
| Four crossed spines. | Medullary shell simple, | 201. Staurocyclia. | |||
| Five to ten or more radial spines. | | Medullary shell simple, | 202. Astrocyclia. | ||
| Medullary shell double, | 203. Coccocyclia. | ||||
III. Subfamily Astracturida. Margin of the disk with two to five or more (commonly three or four) hollow radial chambered arms (with or without a connecting patagium). (Medullary shell commonly simple.) |  | Two arms, opposite in one axis. | | Without patagium, | 204. Diplactura. |
| With patagium, | 205. Amphiactura. | ||||
| Three radial arms (at equal distances). | | Without patagium, | 206. Trigonactura. | ||
| With patagium, | 207. Hymenactura. | ||||
| Four arms (in two crossed diameters). | | Without patagium, | 208. Astractura. | ||
| With patagium, | 209. Stauractura. | ||||
| Five radial arms (at variable distances). | | Without patagium, | 210. Pentactura. | ||
| With patagium, | 211. Echinactura. |
| I. Subfamily Lithocyclida. Margin of the disk simple, circular, without radial appendages. | ||||||
| Neither radial spines nor chambered arms on the circular margin. | ||||||
| Medullary shell simple, | ||||||
| 196. Lithocyclia. | ||||||
| Medullary shell double, | ||||||
| 197. Coccodiscus. | ||||||
| II. Subfamily Stylocyclida. Margin of the disk armed with solid radial spines. | ||||||
| Two opposite spines. | ||||||
| Medullary shell simple, | ||||||
| 198. Stylocyclia. | ||||||
| Medullary shell double, | ||||||
| 199. Amphicyclia. | ||||||
| Three radial spines. | ||||||
| Medullary shell simple, | ||||||
| 200. Trigonocyclia. | ||||||
| Four crossed spines. | ||||||
| Medullary shell simple, | ||||||
| 201. Staurocyclia. | ||||||
| Five to ten or more radial spines. | ||||||
| Medullary shell simple, | ||||||
| 202. Astrocyclia. | ||||||
| Medullary shell double, | ||||||
| 203. Coccocyclia. | ||||||
| III. Subfamily Astracturida. Margin of the disk with two to five or more (commonly three or four) hollow radial chambered arms (with or without a connecting patagium). (Medullary shell commonly simple.) | ||||||
| Two arms, opposite in one axis. | ||||||
| Without patagium, | ||||||
| 204. Diplactura. | ||||||
| With patagium, | ||||||
| 205. Amphiactura. | ||||||
| Three radial arms (at equal distances). | ||||||
| Without patagium, | ||||||
| 206. Trigonactura. | ||||||
| With patagium, | ||||||
| 207. Hymenactura. | ||||||
| Four arms (in two crossed diameters). | ||||||
| Without patagium, | ||||||
| 208. Astractura. | ||||||
| With patagium, | ||||||
| 209. Stauractura. | ||||||
| Five radial arms (at variable distances). | ||||||
| Without patagium, | ||||||
| 210. Pentactura. | ||||||
| With patagium, | ||||||
| 211. Echinactura. | ||||||
Subfamily 1. Lithocyclida, Haeckel, 1881, Prodromus, p. 458.
Definition.—Coccodiscida with simple circular disk, without any radial appendages of the margin (either solid radial spines or chambered arms).