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Montana Field Guide

Montana Field Guides

Forest Disc - Discus whitneyi
Other Names:  Discus cronkhitei, Helix striatella, Helix whitneyi, Helix cronkhitei, Pyramidula cronkhitei


Global Rank: G5
State Rank: S5

Agency Status
USFWS:
USFS:
BLM:


 

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General Description
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).

Diagnostic Characteristics
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.

Species Range
Montana Range

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Range Comments
Across Alaska and Canada south to California, New Mexico, Texas and Kentucky. In Montana, reported across the state from 26 counties on both sides of the Continental Divide. Elevation range is 655 to 2582 m (2150 to 8470 ft). Perhaps the most common and wide-spread disc in Montana. May be locally abundant; 62 live animals were observed at one Flathead County site in mid-October (Hendricks 2012).

Summary of Observations Submitted for Montana
Number of Observations: 270

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Relative Density

Recency

 

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


Predicted Distribution in Montana
Predicted distribution model for Forest Disc (Discus whitneyi). 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
Habitat
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).

References
  • 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.
  • Additional ReferencesLegend:   View Online Publication
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    • Beetle, D. E. 1961. Mollusca of the Big Horn Mountains. The Nautilus 74:95-102.
    • Beetle, D.E. 1957. The Mollusca of Teton County, Wyoming. Nautilus 71:12-22.
    • Berry, S.S. 1913. A list of Mollusca from the Mussellshell Valley, Montana. Nautilus 26:130-131.
    • Berry, S.S. 1916. Notes of Mollusca of central Montana. Nautilus 29:124-128.
    • Berry, S.S. 1919. Mollusca of Glacier National Park, Montana. Proceedings of the Academy of Natural Sciences of Philadelphia 71:195-205.
    • Elrod, M.J. 1902. Daphnia pond, a study in environment. University of Montana Bulletin #16, Biological Series 5: 230-233.
    • Forsyth, R.G. 2004. Land snails of British Columbia. Royal British Columbia Museum: Victoria, British Columbia, Canada. 188 pp.
    • 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.
    • Hendricks, P. 2009. Terrestrial mollusk surveys in Glacier National Park during 2008, including an illustrated key to all documented species. Helena, Mont: Montana Natural Heritage Program.
    • Pilsbry, H.A. 1948. Land Mollusca of North America (north of Mexico), Volume II Part 2. The Academy of Natural Sciences of Philadelphia Monograph Number 2(2): 521-1113.
    • Russell, R.H. and R.B. Brunson. 1967. A check-list of molluscs of Glacier National Park, Montana. Sterkiana 26:1-5.
    • Squyer, H. 1894. List of shells from the vicinity of Mingusville, Montana. The Nautilus 8:63-65.
    • Vanatta, E.G. 1914. Montana shells. Proceedings of the Academy of Natural Sciences of Philadelphia 66:367-371.
  • Web Search Engines for Articles on "Forest Disc"
  • Additional Sources of Information Related to "Snails / Slugs"
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Citation for data on this website:
Forest Disc — Discus whitneyi.  Montana Field Guide.  .  Retrieved on , from