Montana Field Guide

A moderately small shell, diameter to about 6.5 mm, height to about 3.5 mm. The chitonous shell is heliciform of up to 5 3/4 whorls, flattened on the upper surface, rounded on the under surface; color is light chocolate-brown and almost opaque. The inner two whorls have fine spiral striae, visible with a hand lens, that abruptly switch to a series of small and closely-spaced but well-defined axial riblets. The umbilicus is deep and relatively narrow, with almost vertical walls, and about 1.0 mm in diameter. The aperture is crescent-shaped and slightly oblique. Head and tentacles are black (Hendricks 2012, Burke 2013). Internal anatomy is described by Baker (1930) and Pilsbry (1948).

Similar superficially to Discus whitneyi and D. shimekii, but differs in having a combination of minute spiral striations on the first (embryonic) 1.5 whorls that end abruptly where the ribbing begins, a flat spire, a deep and narrow umbilicus, and a crescent-shaped aperture (in contrast to a rounded or ovate aperture).

Sedentary.

Found in a variety mixed conifer forests, but usually in moist mesic sites; canopy species include western redcedar, western hemlock, grand fir, western white pine, Douglas-fir, Engelmann spruce, subalpine fir, western larch, ponderosa pine, lodgepole pine, black cottonwood, water and paper birch, with a secondary canopy including aspen, Pacific yew, and alder. Found under woody debris, rocks, leaf litter, and bryophyte mats (Hendricks 2012).

The species is hermaphroditic, based on internal anatomy (Pilsbry 1948). Age/size at reproductive maturity is unknown, but Pilsbry (1948) indicated the type specimen (diameter 4.9 mm, 5 whorls) was immature.

Documented Montana sites are on lands administered by the Confederated Salish and Kootenai Tribes (3 sites), State of Montana (1 site, but bounded closely by the Missoula Ranger District, Lolo National Forest), Superior Ranger District, Lolo National Forest (1 site), Darby Ranger District, Bitterroot National Forest (3 sites), Cabinet Ranger District, Kootenai National Forest (2 sites), Fisher River Ranger District, Kootenai National Forest (1 site), Libby Ranger District, Kootenai National Forest (1 site), and the Rexford Ranger District, Kootenai National Forest (1 site).

Logging and grazing over most of the known range are probably the greatest threats, through alteration of appropriate habitat. However, alteration of habitat from fire, highway and road construction, rural home development and land clearing could represent threats, as could fire suppression retardants and chemical methods of weed control.