Elements of species of Drepanoistodus Lindstrom, Panderodus Ethington, and Protopanderodus Lindstrom  systematically vary from symmetrical to pronouncedly asymmetrical (symmetry-transition series) within each species, and a group of elements with systematic increase in curvature of the cusp can be recognized for each element type; each such group is named a curvature-transition series. Panderodus and Protopanderodus, both of the Panderodontacea , have essentially identical apparatuses with regard to symmetry and curvature-transition series whereas Drepanoistodus , of the Distacodontacea, differs in type of symme try-transition series a nd number of curvature-transit ion series . The curvature-transition series is considered to represent a taxonomically distinct character at the suprageneric level.

Key words: conodont apparatuses, Ordovician, classification