| Orthothecida Temporal range:
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|---|---|
Scientific classification 
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| Kingdom: | Animalia |
| Superphylum: | Lophotrochozoa |
| Class: | †Hyolitha |
| Order: |
†Orthothecida Marek, 1966 |
| Families | |
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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: