Published online by Cambridge University Press: 20 May 2016
Agnostus acadicus Hartt (in Dawson, 1868) was the first agnostoid trilobite to be described from Middle Cambrian strata in North America. It was later designated the type species of Acadagnostus Kobayashi, 1939. The original description of A. acadicus was based on two specimens, a cephalon and a pygidium, which are illustrated for the first time with photographs. These are interpreted to represent separate species, each belonging to a different genus. The holotype cephalon is reassigned as Peronopsis acadica, which is judged to be a subjective senior synonym of Agnostus fallax Linnarsson, 1869, and several other species-group taxa. Consequently, Acadagnostus is suppressed as a subjective junior synonym of Peronopsis. Agnostus fallax is the type species of Axagnostus Laurie, 1990, which also is suppressed as a subjective junior synonym of Peronopsis. The pygidium on which the original description of A. acadicus was based is reassigned to Hypagnostus parvifrons (Linnarsson, 1869).
Peronopsis acadica, as emended, is a common, cosmopolitan agnostoid in open-marine lithofacies. It has an observed stratigraphic range from the Ptychagnostus praecurrens Interval-zone to the Ptychagnostus punctuosus Interval-zone.