Family. Lingulidae
Shell elongated, composed of alternating chitinous and calcareous layers, the latter of which are perforated. Attached by a pedicle passing between apices of valves.
Arms have no calcified supports.
(For muscles see Fig. [322].)
Range.—Lower Cambrian to Recent.
Principal Genera.—Lingula, Lingulella, Lingulepis.
Family. Obolidae
Shell varies in shape. Ventral valve provided with pedicular groove or foramen. Cardinal border thickened. No brachial supports. Shell composed of alternating chitinous and calcareous layers.
(For muscles see p. [496].)
Range.—Lower Cambrian to Devonian.