Montana Field Guide

The shell is medium to large, up to 23 mm diameter but usually less than 21 mm (newborn young about 2 or 3 mm diameter), and to 11 mm in height but usually less than 10 mm; flattened heliciform (spire only moderately elevated) with up to 5 1/2 whorls, the sutures generally filled by the keel of the previous whorl. Umbilicus is broad, a little larger than 1/4 the shell diameter, aperture is very oblique and oval, periphery of body whorl keeled but more weakly on the last quarter turn. Shell opaque and chalky, color is brownish-gray to whitish (dead shells to pearly white), with two reddish-brown spiral bands, one on the upper surface and a prominent band near the periphery on the lower surface. The last whorl has a cord-like keel on the periphery and sometimes descends below the periphery to the aperture. The surface is wrinkled with fine spiral striae (Hendricks 2012, Burke 2013). Internal anatomy is described by Pilsbry (1939).

There are two subspecies: O. y. extremitatis and O. y. mariae.

Medium to large size, calcareous whitish to gray, flattened spire, variable banding, broad umbilicus, absence of refelcted lip, surface sculpture (including an obvious keel), and oblique oval aperture help distinguish this from other larger shells, including typical O. subrudis and O. strigosa.

Associated primarily with dry limestone outcrops and rocky soils. Tree canopy absent to scattered; canopy species where present include scattered Douglas-fir, Rocky Mountain juniper, and Utah juniper. Ground cover scattered but includes sagebrush and bunch grasses. Live animals occur mostly under junipers in duff or soil accumulations under rocks; sun-bleached shells may be found on the surface (Beetle 1987, Hendricks 2012).