Piper's Windflower
Anemone piperi Britton ex Rydb

Collected along the Lolo Trail possibly along Eldorado Creek near the mouth of Lunch Creek in Idaho Co., Idaho, on 15 Jun 1806. (Source)

A perennial herb with alternate divided leaves. Flowers are white, with r petal-like sepals and no petals. The fruits are tiny, beaked seed-like achenes in dense heads. It blooms in May-July.  The plant prefers cool, shady site with a humus, moist soil
alpine house, moist, humus soil,  and shade.  It grows in  thicket and wood at elevations from 400-3000m in the Western United States.

Caution: All anemones are somewhat poisonous.