Orthothecida

The orthothecids are one of the two hyolith orders.
Marek diagnoses the order thus: Conchs with a flat or concave ventral surface — opercula with large, flat cardinal processes but without clavicles – tightly sigmoidal, sediment-filled
intestine – helens absent.
Sometimes the Circothecidae and Tetrathecidae are split out into a separate order 'Circothecida', which is defined by the bottom surface not being flat, the cardinal processes being pronounced, and a circular rim sometimes showing hints of differentiation into clavicles.
Internal taxonomy.
Marek gives the following diagnoses:
Kouchinsky lists the following taxonomic criteria:
Elsewhere is stated:
Additional genera are not assigned to a family: