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Collignoniceras woollgari, Prionocyclus hyatti, P. macombi, P. wyomingensis, P. novimexicanus, P. quadratus, and P. germari have been designated as zonal species in many publications, but aside from C. woollgari, P. hyatti, and P. wyomingensis, most of these species have not been well described in terms of their intraspecific variation. In many species, there is a gracile and a robust form, which may represent sexual dimorphs, but in most instances, the sample size of adult specimens is too small to prove this.