The hinge line is either curved or straight, and the valves are articulated by means of a pair of “hinge-teeth” (Fig. [329], t) in the ventral valve, which fit into corresponding sockets in the opposite valve. Some genera have the teeth very rudimentary, or have lost them altogether. The teeth are frequently supported by “dental plates,” and the sockets by “socket plates” (e.g. Conchidium, Figs. [324], [325]). A few genera with a long hinge line have the whole of it denticulated (Stropheodonta). In the dorsal valve medianly close under the hinge line is a shelly protuberance—the “cardinal process”—to which the diductor muscles are attached. It is sometimes of great length and forked (Stringocephalus, Fig. [326]), or tripartite, or even quadripartite; but in Rhynchonella and some other genera it is rudimentary.
Fig. 324.—Conchidium galeatum. Wenlock Limestone.
Fig. 325.—Conchidium galeatum. Transverse section. d, Dorsal valve; d.s, dorsal septum; s, socket plate; v, ventral valve; v.s, ventral septum; d.p, dental plate.
Fig. 326.—Stringocephalus Burtini. (Modified from Woodward.) Devonian. A, Interior of dorsal valve. B, Side view of interior of shell; a, adductor (= occlusor) scars; c, crura; c.p, cardinal process; d.s, dorsal septum; h.p, hinge plate; l, brachial loop; s.p, shelly processes; t.s, dental sockets; v.s, ventral septum.
A “hinge area” (Fig. [334], c.a) is often present on one or both valves, and may be of great size, as in Clitambonites, but in Productus it is wholly absent. In those genera that possess it a triangular fissure—the “deltidial fissure”—frequently traverses it on both valves; in the dorsal valve the fissure is merely the space between the dental sockets, and may be occupied by the cardinal- process (Fig. [334], C) or covered by a shelly plate—the “chilidium.” In the ventral valve it gives passage to the pedicle, and may be partly or entirely closed by a similar plate (Fig. [334], d) known as the “pseudo-deltidium,” especially large in Clitambonites, or remain open (Orthis). This pseudo-deltidium is a primitive character, and arises in an early stage of the development as a shell-growth on the dorsal side of the animal, becoming attached to the ventral valve subsequently. The pedicle in many genera passes out through a special foramen in the beak of the ventral valve; and its proximal portion is often embraced by a pair of small plates—the deltidial plates or “deltidium”—which are formed on lateral extensions of the ventral mantle lobe, according to Beecher. These plates lie on each side of the pedicle, or grow round and unite in front of it (Rhynchonella, Fig. [327]), or constitute merely its anterior border (Terebratula, Fig. [328]). In some cases this foramen becomes closed in old age.
Fig. 327.—Rhynchonella Boueti. (Cornbrash.) d, Deltidium; f, foramen.