Brachiopoda with chitinophosphatic or calcareous valves, punctate or impunctate, secondary layer never completely fibrous, periostracum chitinous in living species with phosphatic shells, of protein in calcareous-shelled forms; valves commonly held together only by muscles and body wall, rarely articulated and never by hinge teeth and dental sockets; lophophore never supported by shelly outgrowths from dorsal posterior margin; muscle system for opening and closing valves consisting of adductors and obliques (latter not demonstrable in acrotretaceans and neither pair identifiable in paterinaceans) with bases commonly located peripherally in body cavity. Pedicle of living species developing from ventral mantle and inferred to have originated in same manner in extinct groups. Coelom described as schizocoelic; shell, alimentary canal, and lophophore with median tentacle developing in larval stages of growth; settling without mantle reversal. Alimentary canal with functional anus, trocholophous stage of lophophore development with single (adlabial) or double (adlabial and ablabial) row of filaments.
Source: Moore, 1965.
The following taxa that belong to this class occur in this region: