Montana Field Guide

A small shell, to about 3.5 mm diameter and 2.1 mm in height, depressed heliciform (low conic), with up to 6.25 whorls. Shell is transparent brown, appearing darker brown when animal is alive, with faint growth wrinkles on the surface (visible under magnification). It is tightly coiled, imperforate (lacks an open umbilicus), and has a distinct shoulder on the body whorl with a thin crescent-shaped aperture (Burke 2013).

Very small size and brown color separates Pristiloma from other shells except the spots (Punctum and Paralaoma); Pristiloma is slightly larger with more whorls, a narrow or absent umbilicus, and lacks riblets. Pristiloma idahoense differs from P. wascoense, the only other Pristiloma documented in Montana, by the absence of an open umbilicus (shell is imperforate), a distinct shoulder on the body whorl, tight coiling with more whorls, and slightly larger size.

Often found in Idaho at lower elevations in moist forest zones under mature closed-canopy ponderosa pine, Douglas-fir, grand fir, and Pacific yew. Sometimes in mossy talus under coarse organic debris, and sometimes associated with limestone and basalt (Frest and Johannes 1995, 1997, 2001). Habitat in Montana was under 70% closed canopy of Engelmann spruce, subalpine fir, and Douglas-fir with a ground cover of ferns and moss; on the underside of woody debris atop the ground and relatively near a small seepage (Hendricks 2016). Habitat in Montana is poorly documented.