A brief review of the chief characteristics of fossil Brachiopoda is given below. Those genera which have the greatest zoological or geological importance can alone be noticed owing to the exigencies of space.
I. ECARDINES
External Characters
A considerable diversity of external form is met with even in this division, from the limpet-like Discina to the flattened tongue-shaped Lingula. The valves have most commonly a smooth external surface with delicate growth-lines; but sometimes pittings (Trematis) or radiating ribs (Crania) are present, and in a few forms the shell is furnished with spines (Siphonotreta), which perhaps serve to anchor it in the soft mud of the sea-bottom. The usual mode of fixation was by means of the pedicle (= peduncle or stalk), which either (1) passed out simply between the posterior gaping portion of the valves (Lingula), or (2) lay in a slit in the ventral valve (Lingulella), or (3) pierced the substance of the latter valve by a definite foramen (Discina). The first-mentioned condition of the pedicle seems the most primitive. Rarely the pedicle was absent, and the shell was attached by the whole surface of the ventral valve (Crania, p. [467]).
The two valves in the fossil Ecardines were held together by muscular action, though in some families (Trimerellidae) we see traces of articulating processes. The “hinge line,” or line along which the valves worked as on a hinge, is in most forms more or less curved. A “hinge area” (i.e. that portion of the shell generally smoother than other parts of the valves, more or less triangular in form, and lying between the beaks on one or both sides of the hinge line), is usually absent in the Ecardines.
Fig. 322.—Muscle-scars of Lingula anatina. Inner surface of A, Pedicle-valve or ventral valve. B, Brachial or dorsal valve; p.s, parietal scar; u, umbonal muscle; t, transmedians; c, centrals; a.m.e, laterals (a, anteriors; m, middles; e, externals).
Fig. 323.—Trimerella. (After Davidson and King.) A, Inner surface of pedicle-valve or ventral valve: a, pseudo-deltidium; b, deltidial slope; c, deltidial ridges; d, areal borders; e, cardinal callosities; f, cardinal facet; g, lozenge; i, umbonal chambers separated by cardinal buttress; j, platform; k, platform vaults; l, median plate; m, median scars; n, anterior scars; o, lateral scars; p, post-median scars; q, crown crescent; r, side or lateral crescent; s, end or terminal crescent; t, transverse scars; u, archlet (vascular sinuses); w, sub-cardinal scars; x, umbo-lateral scars. B, Brachial or dorsal valve: e, cardinal sockets; j, platform; k, platform vaults; l, median plate; m, median scars; n, anterior scars; q, crown crescent; r, side or lateral crescent; s, end or terminal crescent; t, transverse scars; u, archlet (vascular sinuses); v, cardinal scars; w, sub-cardinal scars.