The "fork-thicket" of the Cœlotholida is identical with that of the Cœlodendrida, and is composed only of the innumerable dichotomous branches of the hollow tubes. It envelops the two central valves and the enclosed central capsule in the same way as in the Cœlodendrida. But the Cœlotholida differ from these latter in constantly possessing a rhinocanna and two frenulæ. The entire form of this thicket, which in the few species observed was never complete, but always more or less destroyed, is usually probably subspherical or polyhedral, sometimes cordate or kidney-shaped. Its surface is densely studded with thousands of spathillæ. Its diameter is about four to eight times as great as that of the enclosed bivalve shell.
The "lattice-mantle" of the Cœloplegmida, which replaces in this subfamily the fork-thicket of the Cœlotholida, is always produced by the anastomoses of the distal ramules of the brushes, and of those branches of the styles which do not proceed over the surface of the mantle. Its network is always very irregular, and composed of polygonal meshes of very different sizes. Usually it is quite simple, and may be compared with the cortical shell of the Disphærida. More rarely it is more or less spongy. Its surface is densely studded with thousands of spathillæ or anchor-pencils. The entire form of the lattice-mantle is always symmetrically polyhedral, since its dorsal and ventral halves are symmetrically developed on both sides of the equatorial plane, and therefore correspond perfectly to the enclosed smaller halves of the central bivalve shell. The two valves of the lattice-mantle (dorsal and ventral valves) are never really united and grown together, but are in loose contact in the equatorial plane; here the free edges of both valves catch into one another by means of free ramules (Pl. [128], figs. 1, 7). This loose connection is similar to what occurs in the Conchopsida (or in the Concharida with dentate edges), but never so regular. The special form of the polyhedral lattice-mantle depends on the number, arrangement, and development of the styles, which proceed over its surface; it preserves the polyhedral form of the calymma, on the surface of which it is deposed.
The characteristic styles of the Cœlographida (which are never found in the preceding Cœlodendrida) are longer hollow tubes, symmetrically disposed on both valves. They are prominent over the surface of the fork-thicket in the Cœlotholida, of the lattice-mantle in the Cœloplegmida. They bear in these latter a peculiar terminal coronet on their distal end, whilst in the former this end is armed with large pencils of spathillæ. The styles may be forked once or twice at their base, but in their greatest part they are verticillate, and not dichotomously branched like the brushes. The lateral branches of the styles are usually very numerous and regularly cruciate in alternating opposite pairs. In the odd nasal style, e.g., the first and third pairs of opposite lateral branches usually lie in the frontal plane, the second and fourth in the sagittal plane, perpendicular to the former, and so on. A similar regular disposition of the lateral branches is found also in other styles, but not in all. There are certain styles in which the lateral branches are not opposite in pairs, but alternate or verticillate, and others in which they represent unequal branches of forks, so that each single segment of the branched style represents the stouter branch of a fork, and the appertaining lateral branch the thinner branch of the fork. Further accurate examinations are required to recognise the different laws of the ramification of the styles in the different forms of Cœlographida. The lateral branches of the styles are usually again dichotomously branched inside the lattice-mantle, and their distal ends pass over into its network. But the verticillate or cruciate branches, which arise from the free part of the styles outside the lattice-mantle, are always armed with the same elegant pencils of spathillæ which cover the surface of the fork-thicket in the Cœlotholida, the surface of the lattice-mantle in the Cœloplegmida. These pencils also are often regularly opposite in pairs, and the pairs alternate in two planes perpendicular one to another (Pl. [128], figs. 1, 4).
The terminal coronets are peculiar ornaments which protect the distal ends of the styles in the Cœloplegmida, whilst in the Cœlotholida these are armed with the usual pencils of spathillæ (Pl. [122], fig. 8). Each coronet is usually produced by the double, triple, or quadruple furcation of the free distal end of the style; therefore composed of four, eight, or sixteen terminal branches, which, on account of their peculiar form and function, we may call "fingers." More rarely the ramification of the coronets is more or less irregular, and sometimes the number of the fingers exceeds twenty or even thirty.
In the majority of species eight fingers are regularly disposed (Pl. [127], figs. 1-3; Pl. [128], figs. 1-8). Often too sixteen occur, rarely four only. Sometimes the fingers are placed nearly in one plane and form a hand. The form of the fingers is very variable and most characteristic of the individual species. Very often they have the shape of a human finger, and are smooth, spinulate, or armed with recurved hooks. The distal end of each finger often again bears a small coronet or a spathilla (Pl. [128], figs. 5-9), and sometimes it is arrow-shaped (Pl. [126], fig. 2a). All these apophyses of the terminal coronets as well as the anchor-pencils of the mantle and the finest branches of the tubes, are hollow and filled up by jelly.
The different number and arrangement of the styles offers the best means for the distinction of genera in the Cœlographida. The minimum number is six (Cœlographis, Pl. [126], fig. 1), the maximum number sixteen (Cœlothamnus, Pl. [122], fig. 3, and Cœlagalma, Pl. [126], fig. 4). Since the arrangement of the styles in both valves is constantly symmetrical, the fundamental form of the whole body is in all Cœloplegmida "amphithect," as in the Ctenophora. The longitudinal or main axis of the body is vertical, with two distinct poles; the proboscis of the central capsule and the two rhinocannæ are directed upwards, towards the oral pole; the caudal tube of each valve is directed downwards, towards the aboral pole. The two other axes of the body are unequal, horizontal, and perpendicular one to the other; each has two equal poles. On the poles of the sagittal axis lie the galeæ of the dorsal and ventral valves; on the poles of the frontal axis lie the two secondary openings or parapylae of the central capsule. The frontal fissure or the large cleft between the dorsal and ventral valves of the skeleton lies in the vertical frontal plane of the body, which is perpendicular to the vertical sagittal plane; the equatorial plane, however, is horizontal.
The central capsule of the Cœlographida exhibits the same shape and position as in the preceding Cœlodendrida. It is subspherical, slightly depressed in the direction of the main axis, and lies enclosed between the two central valves of the lattice-shell. Its three constant openings lie in the frontal plane, and therefore in the frontal fissure between the two valves. The astropyle, or the main-opening of the capsule, lies on the oral pole of the main axis, and its radiate operculum (d) is directed upwards; the curved proboscis arising from it (o) is prominent between the mouths of the two opposed rhinocannæ. The two lateral parapylæ or accessory openings lie on both sides of the aboral pole, on the right and left (Pl. [127], figs. 4-6). The large spheroidal or somewhat lenticular nucleus (n) is usually about half as broad as the capsule, and contains numerous nucleoli. The protoplasm around the nucleus contains many vacuoles, and in the oral part of the capsule (between nucleus and operculum) often numerous groups of crystals (Pl. [127], figs. 4-6k, 7). The double membrane of the central capsule exhibits the same shape as in the other Phæodaria.
The calymma, or the extracapsular jelly-veil, is in the Cœlographida very voluminous, and includes the entire skeleton, the fork-thicket of the Cœlotholida, the lattice-mantle of the Cœloplegmida, and also the prominent large styles. Only the distal ends of the latter (with the terminal coronets), and the anchor pencils, covering the surface of the thicket and the mantle, seem to project over the surface of the calymma. The entire form of the latter is therefore a symmetrical polyhedron. The phæodium fills up a small part only of the calymma, and is usually enclosed in the two galeæ and their rhinocannæ (Pl. [127], figs. 5, 9), but often also a part of the phæodium is scattered around the oral half of the central capsule.
Synopsis of the Genera of Cœlographida.
I. Subfamily Cœlotholida. Rhinocanna of each valve with two paired lateral frenula. The distal ends of the dichotomous brushes are not united by anastomoses, and form an outer bivalved fork-thicket. |  | Eight paired styles (four on each valve), | 731. Cœlotholus. | ||
| Twelve paired styles (six on each valve), | 732. Cœlothauma. | ||||
| Sixteen paired styles (eight on each valve), | 733. Cœlothamnus. | ||||
II. Subfamily Cœloplegmida. Rhinocanna of each valve with an odd sagittal frenulum. The distal ends of the dichotomous brushes are united by anastomoses, and form an outer bivalved lattice-mantle. |  | Mantle with six styles. |  | One odd and two paired styles on each valve, | 734. Cœlographis. |
| Mantle with eight styles. | | Two odd and two paired styles on each valve, | 735. Cœlospathis. | ||
| Mantle with ten styles. | | One odd and four paired styles on each valve, | 736. Cœlodecas. | ||
| Mantle with twelve styles. | | Two odd and four paired styles on each valve, | 737. Cœlostylus. | ||
| Mantle with fourteen styles. | | One odd and six paired styles on each valve, | 738. Cœloplegma. | ||
| Mantle with sixteen styles. | | Two odd and six paired styles on each valve, | 739. Cœlagalma. | ||