Arms supported by a calcareous loop, usually bent back on itself.
(For muscular impressions see p. 502, Figs. [328], [329].)
Beak of ventral valve perforated by foramen, furnished with deltidium.
Range.—Devonian to Recent; maximum development in Mesozoic times.
Principal Genera.—Terebratula, Terebratulina, Waldheimia, Terebratella, Kingena, Magas, Centronella.
Family. Argiopidae
Large foramen for passage of pedicle. Marginal septa present in both valves. Calcareous brachial loop follows margin of shell and is more or less fused with the septa. Shell structure punctate.
Range.—Jurassic to Recent.
Principal Genera.—Argiope, Cistella.