Range.—Silurian to Permian. Genus Productus very characteristic of the Carboniferous.
Principal Genera.—Productus, Chonetes, Strophalosia, Proboscidella, Aulosteges.
Family. Strophomenidae
Shell very variable in shape; concavo-convex, plano-convex, or biconvex; hinge line usually straight; frequently with an area on each valve; foramen may or may not be present. Shell structure near always punctate. Ventral valve usually furnished with hinge-teeth; and dorsal valve with cardinal process.
Brachial supports completely absent or very rudimentary.
(For muscular impressions see p. 502, Fig. [334].)
Range.—Wholly Palaeozoic.
Principal Genera.—Orthis, with many sub-genera, Clitambonites, Skenidium, Strophomena, Orthothetes, Leptaena, Stropheodonta, Plectambonites.
Family. Koninckinidae
Shell plano-convex or concavo-convex. Brachial apparatus composed of two lamellae spirally enrolled in the same plane, or in the form of depressed cones, with the apices directed into the ventral valve.