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Montana Field Guide

Montana Field Guides

Humped Coin - Polygyrella polygyrella
Other Names:  Helix polygyrella, Helicodiscus polygyrella

Species of Concern
Native Species

Global Rank: G3
State Rank: S1S2

Agency Status

External Links

General Description
A medium sized shell, to 13 mm diameter and 6 mm in height. The shell is heliciform (discoidal), relatively flattened, greenish or yellowish brown in color, and somewhat translucent and glossy; umbilicus is wide and well-like, about one-third of the diameter, enlarging in the last half whorl. The whorls are closely coiled, about 7 to 8 1/2 in number; the initial 2 to 3 whorls are smooth, the rest with strong radial ribs which become more obscure near the aperture. Aperture is lunate-triangular, with an erect parietal tooth opposite the ends of the lip and triangular in shape, the lip thickened within. Within the last whorl there are one or two radial rows of three teeth each, which are visible as lighter patches through the shell (Hendricks 2012, Burke 2013). Internal anatomy is described by Pilsbry (1932) and (1939).

Diagnostic Characteristics
A combination of size, low to flattened and closely-ribbed spire, large umbilicus, and large parietal tooth are unique. Unlike any other snail species in Montana.

Species Range
Montana Range


Range Comments
Extreme northeastern Oregon and southeastern Washington through northern Idaho and northwestern Montana (Burke 2013). In Montana, 23 records from three counties west of the Continental Divide: Mineral (8), Ravalli (3), Sanders (12). Elevation range is 812 to 1526 m (2665 to 5005 ft). Original description based on individuals collected in 1860 on the east slope of the Coeur d’Alene Mountains, probably Mineral County, Montana, but possibly Shoshone County, Idaho. The Ravalli County populations appear to be isolated. May be locally abundant; 45 and 50 have been found at sites in Ravalli County (at three closely spaced sites) and Sanders County in mid-September and mid-October, respectively (Hendricks 2012).

Observations in Montana Natural Heritage Program Database
Number of Observations: 23

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

Predicted Distribution in Montana
Predicted distribution model for Humped Coin (Polygyrella polygyrella). Records were spatially unique and had a locational uncertainty of ≤ 400 meters.  Hotter colors indicate areas that are predicted to have more suitable habitat for the species.  Black dots are positive data used to build the model.  Gray dots are locations where a survey capable of detecting the species has been performed.  Landownership, a shaded relief map, and county lines are included for reference.  Details of the modeling effort, a description of the environmental layers used, and a more thorough interpretation of model outputs can be found in the report Land Mollusk Surveys and Predicted Distribution Models on USFS Northern Region Lands: 2007.

Predicted Distribution Map

More model output for this species

Occupies mesic mixed conifer forest, often relatively close to water such as streams and seeps; canopy species include western redcedar, western hemlock, grand fir, Engelmann spruce, Douglas-fir, subalpine fir, black cottonwood, and western white pine; secondary canopy includes alder and mountain maple. Found under woody debris and rocks in damp soil and humus (Hendricks 2012).

Food Habits
There is no information on food habits.

Documented Montana sites are on lands administered by the Thompson Falls Ranger District, Lolo National Forest with private in-holdings (1 site), Superior Ranger District, Lolo National Forest (1, possibly 2 sites), and 1 site on private land near Thompson Falls. The vague locality for the Deer Lodge Valley site precludes assigning ownership to the lands where it was found.

Threats or Limiting Factors
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.

  • Literature Cited AboveLegend:   View Online Publication
    • Burke, T. E. 2013. Land snails and slugs of the Pacific Northwest. Corvallis, OR: Oregon State University Press. 344 p.
    • Hendricks, P. 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.
    • Pilsbry, H.A. 1932. The land snail genus Polygyrella. Proceedings of the Academy of Natural Sciences of Philadelphia 84:15-19.
    • Pilsbry, H.A. 1939. Land Mollusca of North America (North of Mexico), Volume 1, Part 1. Monograph of the Academy of Natural Sciences of Philadelphia Monograph Number 3 (1): 1-573.
  • Additional ReferencesLegend:   View Online Publication
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    • Ancey, C.F. 1887. Description of North American shells. The Conchologists’ Exchange 2:79-80.
    • Bland, T. and J.G. Cooper. 1861. Notice of land and freshwater shells collected by Dr. J.G. Cooper in the Rocky Mountains, etc, in 1860. Annals of the Lyceum of Natural History of New York 7:362-370.
    • Cooper, J.G. 1868. The shells of Montana. American Naturalist 2:486-487.
    • Frest, T.J. and E.J. Johannes. 1995. Interior Columbia Basin mollusk species of special concern. Final report to the Interior Columbia Basin Ecosystem Management Project, Walla Walla, WA. Contract #43-0E00-4-9112. 274 pp. plus appendices.
    • Frest, T.J. and E.J. Johannes. 2001. An annotated checklist of Idaho land and freshwater mollusks. Journal of the Idaho Academy of Science 36(2):1-51.
    • Henderson, J. 1924. Mollusca of Colorado, Utah, Montana, Idaho, and Wyoming. University of Colorado Studies 13(2):65-223.
    • Henderson, J. 1936. Mollusca of Colorado, Utah, Montana, Idaho, and Wyoming, supplement. University of Colorado Studies 23(2): 81-145.
    • Hendricks, P. 2005. Surveys for animal species of concern in northwest Montana. Section 4: Terrestrial mollusk surveys in northwestern Montana; and section 5: Plum Creek owl and mollusk surveys. Unpublished report to the Montana Department of Fish, Wildlife & Parks, Montana Natural Heritage Program, Helena, Montana, May 2005. 53 p.
    • Hendricks, P., B.A. Maxell, and S. Lenard. 2006. Land mollusk surveys on USFS Northern Region lands. A report to the USDA Forest Service, Northern Region. Montana Natural Heritage Program, Helena, Montana. 11 pp. plus appendices.
    • Hendricks, P., B.A. Maxell, S. Lenard, and C. Currier. 2007. Land mollusk surveys on USFS Northern Region lands: 2006. A report to the USDA Forest Service, Northern Region. Montana Natural Heritage Program, Helena, Montana. 11 pp. plus appendices.
    • Hendricks, P., B.A. Maxell, S. Lenard, and C. Currier. 2008. Surveys and predicted distribution models for land mollusks on USFS Northern Region Lands: 2007. Report to the USDA Forest Service, Northern Region. Helena, MT: Montana Natural Heritage Program. 12 pp. + appendices.
    • Smith, A.G. 1943. Mollusks of the Clearwater Mountains, Idaho. Proceedings of the California Academy of Sciences, fourth series, 23:537-554.
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Citation for data on this website:
Humped Coin — Polygyrella polygyrella.  Montana Field Guide.  .  Retrieved on , from