[8] Some hyoliths had helens, long structures that taper as they coil gently in a logarithmic spiral in a ventral direction.

[8] Encrusting organisms have been found on helens, and also on both sides of the main shell, all of which are therefore supposed to have been raised above the sea bed.

Hyolitha have dorso-ventrally differentiated opercula, with the ventral surface of the shell extending forwards to form a shelf termed the ligula.

[8] They have a straight (planar) opening, sometimes with a notch on the bottom side, and sealed with an operculum that has no ligula, clavicles, furrow or rooflets.

[10] Orthothecids fall into two groups: one, the orthothecida sensu stricto,[11] is kidney or heart shaped in cross-section due to a longitudinal groove on its ventral surface, and its opercula bear cardinal processes; the other has a rounded cross-section and often lacks cardinal processes, making them difficult to distinguish from other cornet-shaped calcareous organisms.

[12] Their grade of organization was historically considered to be of the 'mollusc-annelid-sipunculid' level,[13] consistent with a Lophotrochozoan affinity, and comparison was primarily drawn with the molluscs or sipunculids.

[13][14] Older studies (predating the Lophotrochozoan concept) consider hyoliths to represent a stem lineage of the clade containing (Mollusca + Annelida + Arthropoda).

In the Cambrian, their global distribution shows no sign of provinciality, suggesting a long-lived planktonic larval life stage (reflected by their protoconchs); but by the Ordovician distinct assemblages were becoming evident.

[10] Some orthothecids are preserved in vertical [life] orientation, suggesting a sessile suspension-feeding habit; hyolithids tend to be flat on the bottom, and their shape and the occurrence of epibionts are consistent with a sessile suspension feeding habit via orientation relative to passive currents.