Biometry of 381 specimens from a two-foot carbonate bed low in the Tunnel Mountain Formation near Flathead Pass reveals a unimodal population of A. curvilateralis. Related species A. opimus (Hall), A. matheri (Dunbar and Condra), and A. occiduus (Sadlick) are separable from it by biometric and qualitative criteria. A. curvilateralis is thought to have evolved from Spirifer pellaensis cavecreekensis Hernon.

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