The shell in all Circoporida exhibits a simple, excentric mouth, which corresponds to the main osculum of the central capsule. The excentric position of this shell-mouth has no influence on the regular form of the shell. The mouth is either circular or polygonal, usually about as large as a basal circle of pores. It is always armed with prominent conical or pyramidal teeth, the number of which is variable and seems to depend partly upon the number of the shell-faces or the radial spines. Circoporus has usually four cruciate teeth (Pl. [115], fig. 1; Pl. [117], figs. 4, 5). Circorrhegma exhibits a pentagonal mouth with five teeth (Pl. [117], fig. 2). Circogonia possesses a hexagonal mouth with six teeth (Pl. [115], fig. 8; Pl. [117], fig. 1). Circospathis has no constant number; some species have five teeth (Pl. [115], fig. 4), others four (Pl. [115], fig. 10), others nine (Pl. [117], fig. 3a). Circostephanus is also variable; one species exhibits eight teeth (Pl. [116], fig. 3a), another ten, another twelve. In Haeckeliana (Pl. [114], fig. 3) the teeth are smaller and more numerous. Usually the teeth arise vertically from the surface of the shell and are spinulate; more rarely their points are directed towards the centre of the mouth. Sometimes numerous thin needles arise between the teeth (Pl. [115], fig. 10).
The radial spines of the Circoporida are tubular, usually cylindrical and conical at the thickened base, more rarely prismatic or slenderly pyramidal, with three or more edges; sometimes the edges are spirally wound around the axis (Pl. [115], fig. 6). In the majority of species their length is about equal to the diameter of the shell; often they are somewhat shorter, more rarely longer. Their tubular structure is always the same as in the Tuscarorida; the wall of the tube is thick, and in the axis of its cavity lies an axial cord or funicle, which is connected with the wall by numerous horizontal, simple, or branched threads (Pl. [115], figs. 6, 7). The axial funicle itself is either a simple thread of silica, or a strand, composed of three to fifteen or more filaments, which are closely twisted like a rope around the axis of the spine, and arise separately from the bridges between the pores of the basal circle (Pl. [115], figs. 7, 9). The number of pores in these circles is very variable, commonly from three to nine, sometimes twelve to fifteen or more; their form is usually irregularly ovate (Pl. [115], figs. 7, 9). The bridges between the pores usually bear numerous siliceous threads or bristles, which are curved downwards or upwards (Pl. [115], figs. 4-6). Sometimes the spines are entirely covered with similar bristles or cilia (Pl. [117], figs. 2-5), more rarely with regular verticils of lateral branches (Pl. [117], fig. 6).
The distal ends of the radial spines are simple in Haeckeliana (Pl. [114]), whilst in the other Circoporida they are usually (or perhaps constantly) either forked or armed with a verticil of terminal branches. When the spines are simply forked, their two terminal branches lie in certain meridional planes (Pl. [117], fig. 5). The same is probably the case when they bear three divergent branches (Pl. [115], figs. 1, 2), or four crossed branches (Pl. [117], figs. 4, 6). The larger species usually bear a corona of five curved branches disposed around the distal apex of each spine (Pl. [116], fig. 3; Pl. [117], figs. 1, 2). Sometimes the formation of these coronas is twice or more often repeated, so that the spines appear verticillate.
The central capsule of the Circoporida (Pl. [115], fig. 8; Pl. [117], fig. 6) is usually about half as large as the enclosing shell, and lies excentrically in that half of its cavity which is remote from the shell-mouth (Pl. [115], fig. 8). The other half (near the mouth) is filled up by the cap-shaped, dark phæodium. The proboscis, or the central tubule of the astropyle, is hidden in the axis of the blackish phæodium, and is directed towards the mouth of the shell; it is often S-shaped, more or less curved. The circular radiate operculum of the astropyle, placed on the base of the proboscis, exhibits numerous branched radial ribs, and closes the main-opening like the lid of a tea-kettle (Pl. [115], fig. 3). The number of the parapylæ, or the accessory openings of the capsule, which in the majority of Phæodaria is two, seems to be usually increased in the Circoporida. In Circoporus the capsule seems to posses six and in Circospathis nine secondary openings, and therefore there is some probability that each circle of pores on the base of a radial spine corresponds to a secondary opening of the capsule. In Haeckeliana, however (Pl. [114]), I could observe no parapylæ at all. These accessory openings are always very small in the Circoporida, and may be easily overlooked. Furthermore, the number of observations respecting this difficult point is too small, and their certainty not satisfactory; it requires therefore further accurate examination, as also does the whole central capsule of the Circoporida.
The numerous and remarkable characters of shape and structure, which we have described above as occurring in the Circoporida, are also found in the following closely allied family, the Tuscarorida; all the species are inhabitants of great depths. Perhaps it may therefore be more convenient to separate these two families from the other Phæogromia as a peculiar order, under the name Phæocalpia.
Synopsis of the Genera of Circoporida.
I. Subfamily Circogonida. Shell spherical or polyhedral with panelled structure and polygonal plates. A certain number of radial spines are geometrically disposed. |  | Shell octahedral (or spherical), | Six radial spines, | 711. Circoporus. | |
| Shell tetradecahedral, | Nine radial spines, | 712. Circospathis. | |||
| Shell icosahedral, | Twelve radial spines, | 713. Circogonia. | |||
| Shell dodecahedral, | Twenty radial spines, | 714. Circorrhegma. | |||
| Shell polyhedral, | Twenty-four to forty or more radial spines, | 715. Circostephanus. | |||
| II. Subfamily Haeckelinida. |  | Shell spherical, with dimpled structure, and a variable number of radial spines (without polygonal plates), | 716. Haeckeliana. | ||
I. Subfamily Circogonida. Shell spherical or polyhedral with panelled structure and polygonal plates. A certain number of radial spines are geometrically disposed. | ||||||
| Shell octahedral (or spherical), | ||||||
| Six radial spines, | ||||||
| 711. Circoporus. | ||||||
| Shell tetradecahedral, | ||||||
| Nine radial spines, | ||||||
| 712. Circospathis. | ||||||
| Shell icosahedral, | ||||||
| Twelve radial spines, | ||||||
| 713. Circogonia. | ||||||
| Shell dodecahedral, | ||||||
| Twenty radial spines, | ||||||
| 714. Circorrhegma. | ||||||
| Shell polyhedral, | ||||||
| Twenty-four to forty or more radial spines, | ||||||
| 715. Circostephanus. | ||||||
| II. Subfamily Haeckelinida. | ||||||
| Shell spherical, with dimpled structure, and a variable number of radial spines (without polygonal plates), | ||||||
| 716. Haeckeliana. | ||||||
Subfamily 1. Circogonida, Haeckel.
Definition.—Circoporida with panelled shell, composed of polygonal plates. The shell is usually polyhedral, more rarely spherical, and the radial spines are usually (or perhaps constantly) branched and regularly arranged.