The orthothecids are one of the two hyolith orders.

[2] 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.

[3] Marek [3] gives the following diagnoses: Kouchinsky[4] lists the following taxonomic criteria: Elsewhere[5]: 243  is stated: Additional genera are not assigned to a family:

This prehistoric protostome article is a stub.