The general form of the single valves is very varied in the different species, in the majority boat-shaped or hat-shaped, more or less laterally compressed, in a few forms hemispherical. In Conchopsis (Pl. [125]) and Conchoceras (Pl. [124], figs. 15, 16) the lateral parts of the valves (right and left) are vaulted, whilst their median parts are so strongly compressed that they form a sharp sagittal keel, and then the shell in the dorsal or ventral view appears spindle-shaped (Pl. [123], fig. 8a; Pl. [125], fig. 8). Often the frontal margins of the valves are somewhat constricted (Pl. [124], fig. 7).
The junction between the two valves of the shell is always loose, but not so loose as in the two following families. In the Cœlodendrida and Cœlographida the two valves are either perfectly free and separated by a frontal zone of jelly, or in very loose contact on the frontal margins. In the Concharida, however, the margins of both valves seem to be usually in contact, and their connection is effected in a double way. In the subfamily Conchasmida (comprising the genera Concharium and Conchasma, Pl. [123], figs. 1-6) the lateral margins of both valves are smooth, not dentated, and fit one into another like the two parts of a box, or like the two valves of a Diatom (Navicula). In the second subfamily, however, Conchopsida (comprising the five other genera, Pls. [124], [125]), the lateral margins of the valves are dentate, usually provided with a series of numerous strong conical teeth, and the teeth of both valves so catch into one another, that their union is rather firm (like the margin of the shells of Tridacna, Pecten, and other Lamellibranchiata). Besides, a more solid junction is often effected on the posterior or aboral part of the margins, which we shall call the hinge. Here often peculiar strong teeth catch one into another, and in the majority of species two aboral spines are developed, the caudal horns (a dorsal and a ventral); these are very large in Conchoceras (Pl. [124], figs. 15, 16). But a peculiar and most interesting kind of junction is effected in some Concharida by a true ligament between the valves (Pl. [123], figs. 8, 9; Pl. [125], fig. 2). This ligament is always placed on the aboral hinge, is of dark brown colour, and is not dissolved by mineral acids unless long applied. It may preserve the connection of the posterior parts of both valves, when their anterior parts are removed one from another, just as in the Brachiopoda. I observed this interesting ligament mainly in the genus Conchopsis, but not in all species, and it is not yet certain whether it is a constant organ in these and some other Concharida.
In the majority of Concharida the lateral margins of the two valves project slightly inwards into the cavity, and in some species of Conchopsis these inner borders are so broadened that they form a broad, horizontal, fenestrated inner shelf, comparable to the deck of a boat or to the velum of the Hydromedusæ or Craspedotæ (Pl. [125], fig. 9). In this case the velum surrounds the ovate aperture through which the two lobes of the central capsule (dorsal and ventral) enter into the cavity of both valves.
The mouth of the shell lies on the oral pole of the main axis, and is therefore opposed to the aboral hinge. The two valves are here usually more or less emarginate, so as to form a transverse mouth with an upper and a lower lip (Pl. [124], figs. 6, 7, 11). The form of these two lips is often very different and characteristic of particular species (Pl. [124], figs. 3, 15, 16). The mouth remains in many species constantly open, even when the frontal fissure is closed (figs. 7, 16). Since the centre of the shell mouth lies in the prolongation of the proboscis arising from the operculum of the central capsule, probably the main stream of sarcode, issuing from the latter, becomes protruded by the former.
Apophyses of the shell (besides the teeth of the margins) are completely wanting in three genera, Concharium, Conchellium, and Conchopsis (Pl. [123], figs. 1-4, 7; Pl. [125]). The four other genera possess free apophyses or spines, which we call horns. They are probably important as the beginnings of those large hollow tubes which are characteristic of the two following families, Cœlodendrida and Cœlographida. We distinguish two different forms of horns, apical horns on the poles of the sagittal axis, and caudal horns on the aboral pole of the main axis; the former probably correspond to the sagittal tubes and the latter to the caudal tubes of the two following families. Apical horns are found in a single genus only, Conchonia (Pl. [124], figs. 10-14). Here either on one pole or on poles of the sagittal axis a horn is developed, usually curved backwards. Sometimes the base of this conical horn is inflated and fenestrated, and may represent the beginning of the formation of the galea or apical cupola of the Cœlodendrida.
The two caudal horns are opposite on the aboral hinge of the shell, one arising from the posterior end of each valve. Usually they are short and thick, pyramidal, the ventral horn larger than the dorsal (Pl. [124], figs. 3, 6). Rarely the two caudal horns are fenestrated at the base and reach a considerable size, as in Conchoceras (Pl. [124], figs. 15, 16).
The walls of the bivalved shell usually exhibit in the Concharida a rather solid shape and regular structure, with an elegant network of regularly arranged pores. But in some species the walls of the shell become very thin and fragile, and assume the same shape (with very irregular network), as in the Cœlodendrida and Cœlographida. The pores are usually small and numerous, circular, often hexagonally framed (Pl. [125], figs. 4-6). They pierce the thick shell-wall either in a radial or in an oblique direction. Sometimes each pore is armed with six radial teeth (Pl. [123], fig. 7a). At other times each pore represents an oblique ampullaceous canal, dilated in its middle part, with two narrow openings (Pl. [125], figs. 5a, b, c, 6). The pores are so arranged in the majority of species that they form regular curved series, which are separated by prominent crests, and converge towards the poles of the main axis. Usually the marginal pores (along the frontal margin of the valves) are much smaller (compare Pls. [123]-[125]).
The central capsule of the Concharida, very well preserved in numerous specimens of the Challenger collection, constantly possesses the same situation and structure. It is always enclosed in the aboral or posterior half of the shell-cavity, whilst the oral or anterior half is filled up by the phæodium. The free spaces between both and between the inner surface of the shell are completely filled up by the jelly of the calymma, which also covers the whole shell as a thin outer jelly-envelope. The form of the central capsule is sometimes nearly spherical, usually somewhat compressed in the direction of the main axis, and sometimes also in the direction of the frontal axis (Pl. [123], figs. 1-9). In some species it becomes bilobed, with an upper dorsal and a lower ventral lobe, and in some others it becomes triangular (Pl. [125], fig. 7). Its two membranes (inner and outer) are often separated by a broad colourless interval, containing a clear fluid or jelly (Pl. [123], figs. 8, 9). The nucleus is usually about half as large as the central capsule and ellipsoidal, its longer axis lying in the sagittal diameter of the body. Several specimens (of different genera) contained two separate nuclei, one placed in the dorsal, the other in the ventral half of the capsule (Pl. [124], fig. 6). This duplication of the nucleus is probably the preparation for the division of the capsule. The division will be probably effected in the frontal plane, so that each half of the bisected capsule gets one nucleus and one valve, and the other valve becomes newly formed (in a way similar to that in the bivalved Diatomaceæ). The astropyle, or the main-opening of the capsule, is closed by a radiate operculum, from which arises a tubular proboscis; this lies in the main axis of the body, is directed towards the anterior mouth of the shell, and surrounded by the phæodium. The two shorter tubes of the paired parapylæ, or the accessory lateral openings, lie on the posterior or caudal side of the capsule, at right and left, and are directed half backwards, half outwards (towards the frontal fissure between the valves, Pl. [123], figs. 1, 8a).
The phæodium exhibits in all Concharida the same characteristic shape, and represents a dark conglomeration of phæodellæ, filling up the anterior or oral half of the shell-cavity. Usually it is bilobed, divided into a dorsal and a ventral lobe or wing, which fills up the corresponding valve of the shell (Pl. [123], figs. 8, 9). The phæodium is commonly more voluminous than the capsule, and surrounds its anterior half, more rarely it encloses nearly the entire capsule (Pl. [124], figs. 6, 10). Its colour is usually olive, sometimes more greenish, at other times more brownish, in some species nearly black. The phæodellæ, or the roundish granules which compose the phæodium, exhibit the same shape as in all other Phæodaria (compare above, p. [1535]). Sometimes peculiar rather oblong nucleated cells are scattered in great numbers between the phæodellæ, probably parasites or symbiontes (Pl. [123], figs. 7-9, 9a).