Clitocybe squamulosa  (Pers.) Fr. (1821) var. montana E. Bigelow (1968)

This is the most common Clitocybe species in Sierra Nevada in the Spring. I have seen it in Colorado in the Fall too. As Bigelow points out too, in most cases the pileus is quite glabrous and the name doesn't seem to be very descriptive. 

Collections from Colorado, Rocky Mountains

Spores: 7-12x4-5.5  --  quite polymorphic in shape.