Edited 8-7-2016: added photos of live snail.

Edited 1-15-2018: ID narrowed further from O. strigosa to O. alpina. Shell sutures are too impressed for O. strigosa, and the diameter and height to diameter ratio, as well as the umbilicus/diameter ratio all suggest O. alpina over strigosa, to me. According to Pilsbry, O. alpina also matches this shell in that 'The last whorl is strongly angular at periphery but the angle disappears near the end, where the whorl descends slightly.' In addition, O. alpina has been documented from the Bob Marshall on nearby Scapegoat Mountain to the SSE, as well as in the Mission Mountains to the west.

Using "A Guide to the Land Snails and Slugs of Montana", and Pilsbry, 1939, I have narrowed this down to (I believe): Oreohelix alpina.

The snail was found live on a north-facing steep rocky slope a few hundred feet below a ~200' limestone cliff. Elevation of the site is ~ 7500', vegetation is low with a few small evergreen trees.

Color of the live animal is very dark, nearly entirely black.

These photos are of the live snail after collection, but the snail had dried slightly and retreated into its shell.

Despite approximately 10 man-hours of sporadic searching in the region over a 7-day period, this was the only snail or shell found, and I was keeping a pretty good eye out for snails. I discovered it on a damp rock, two days after a heavy rainstorm, with rainy and cooler than average weather during the entire week leading up to finding it. Temps were in the 50s to 60s F during the day and high 30s at night.

Source:

https://archive.org/details/Aa103edf-3eb4-41a9-95a1-12046928fe85

Hendricks, Paul. 2012. A Guide to the Land Snails and Slugs of Montana. A report to the U.S.
Forest Service - Region 1. Montana Natural Heritage Program, Helena, MT. vii + 187 pp. plus
appendices.