Montana Field Guide

A small shell, to about 6.5 mm diameter but usually smaller, to about 3.5 mm in height, flattened heliciform, with up to 4 1/2 whorls. Shell is translucent brown to dark brown, the inner 1 1/2 whorls are smooth, later whorls with prominent evenly-spaced axial ribs on both upper and lower surfaces, the periphery sometimes rounded, sometimes angular. Aperture lip is oval to angular and not thickened or flared, the umbilicus wide, about 1/3 the diameter. The animal is pale gray on the sides and tail, darker to blackish on the head, neck, and tentacles (Hendricks 2012, Burke 2013).

A combination of shell shape (flattened heliciform or flattened conic), shell dimensions, number of whorls, shell color (olive-brown and chitinous), the presence of ribbing on the shell, lack of reflected lip, and absence of teeth in the aperture distinguish this from all other Montana land snails except other Discus (Hendricks 2003). Discus shimekii has much weaker ribbing on the base of the shell (may appear almost smooth), Discus brunsoni has a range restricted to the Mission Mountains, is not strongly rib-striate on any surface, and is larger.

Forested habitats, from mesic (western redcedar, western hemlock, Engelmann spruce, Douglasfir, black cottonwood, secondary canopy including alder, Pacific yew, paper birch, mountain maple, dogwood, willow) to relatively dry (ponderosa pine and Rocky Mountain juniper, but usually in moister sites, such as imbedded pockets of aspen). Found under woody debris and rocks, in downed rotten wood, leaf litter and duff (Hendricks 2012).