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:


 * Orthothecidae: Flat or concave bottom surface of conical shell. Transverse aperture.

Kouchinsky lists the following taxonomic criteria:


 * Circothecidae: circular cross-section; same shape and sculpture on upper and lower surfaces.
 * Turcurthecidae: oval or lenticular cross-section; top and bottom identical
 * Allathecidae: top side flat or slightly convex; bottom side convex

Elsewhere is stated:


 * Tetrathecidae: polyhedral cross-section; shell axially twisted
 * Novitatidae: bottom side concave
 * Lenatheca: aperture heart-shaped.

Additional genera are not assigned to a family:
 * Probactrotheca
 * Pedunculotheca