Family. Rhynchonellidae
Shell biconvex, hinge line usually curved.
Beak of ventral valve incurved, with foramen.
Calcareous brachial supports reduced to a pair of short curved crura.
The septa, dental and socket plates may be highly developed and divide up the cavity of the shell into chambers (Stenochisma, Conchidium).
Shell structure fibrous, rarely punctate; muscular impressions as in Terebratulidae.
Range.—Ordovician to Recent: majority of the genera are Palaeozoic.
Principal Genera.—Rhynchonella (Fig. [327]), Stenochisma, Stricklandia, Conchidium.
Family. Terebratulidae
Shell structure punctate.