Principal Genera.—Obolus, Obolella, Kutorgina, Linnarssonia, Siphonotreta, Acrotreta, Neobolus.
Family. Discinidae
Shell rounded, valves more or less conical, fixed by pedicle passing through slit or tubular foramen in ventral valve. No calcified brachial supports. Shell structure chitino-calcareous.
Range.—Ordovician to Recent.
Principal Genera.—Discina, Orbiculoidea, Trematis.
Family. Craniidae
Shell calcareous, subcircular; fixed by surface of ventral valve; dorsal valve the larger, depressed-conical. Shell structure punctate.
Four principal muscular scars in each valve, with central triangular protuberance in ventral valve (see p. [476]).
Range.—Ordovician to Recent.
Principal Genus.—Crania.