Adrain, J.M., Westrop, S.R., Karin, T.S., Landing, E., 2014. Trilobite Biostratigraphy of the Stairsian Stage (upper Tremadocian) of the Ibexian Series, Lower Ordovician, Western United States. Memoirs of the Associaiton of Australasian Palaeontologists  45, 167?214.