Orthothecida

Orthothecida Temporal range:
Scientific classification
Kingdom:	Animalia
Class:	†Hyolitha
Order:	†Orthothecida Marek, 1966
Families
Orthothecidae Allathecidae Rozanov ''et al.'', 1969 Rozanov et al., 1969 Syssoiev, 1957 Tetrathecidae Gracilithecidae Sysoyev, 1968 Circothecidae Syssoiev, 1972

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