Hi Diane, thanks for the information. I found this paper below that addresses some of this but more work appears to be needed. Since C. aculeata seems more common in Montana, and especially at lower elevations (which this was), I will go with that too. Also, page 104 of Macrolichens of the PNW "Some of our material is intermediate between the two species." The split between them doesn't seem very definitive, not enough for me to say that "almost certainly" I know which one I am looking at! :)
Thanks again!
Printzen C, Domaschke S, Fernández-Mendoza F, Pérez-Ortega S (2013) Biogeography and ecology of Cetraria aculeata, a widely distributed lichen with a bipolar distribution. MycoKeys 6: 33-53.