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Montana Field Guide

Montana Field Guides

Conical Spot - Punctum randolphi

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Native Species

Global Rank: G5
State Rank: SNR

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General Description
A very small shell, to 1.4 mm diameter and 1.0 mm in height, flattened heliciform with a moderately-high conic to nearly dome-shaped spire, with up to 4 1/4 whorls. The whorls increase in width gradually. Shell is translucent yellowish-brown, with a series of small closely-spaced axial riblets on both surfaces; periphery rounded. The aperture is ovately rounded, broader than high; umbilicus is somewhat narrow (about 0.25 mm) and deep, about 1/4 to 1/5 the shell diameter. The body is short and barely visible beyond the shell when crawling (Forsyth 2004, Burke 2013).

Diagnostic Characteristics
The spots are very small, yellowish brown shells (less than 3.0 mm diameter) with 4.0 to 4.25 whorls, which separates them from other species. Species of Pristiloma are more tightly coiled with more whorls, slightly larger in diameter with a narrow to absent umbilicus, and lack riblets. Punctum minutissimum is flatter in profile (lower conic) with a wider umbillicus than P. randolphi though similar in diameter, P. californicum is similar in shape to P. randolphi but larger with more solid riblets, Paralaoma caputspinulae is larger still (to > 2.0 mm diameter) with rather high cuticular riblets visible under magnification but with a flatter profile like P. californicum.

Species Range
Montana Range


Range Comments
Mostly northern California through western Oregon and Washington to British Columbia, east in the Pandhandle of Idaho (Burke 2013). All Punctum in British Columbia considered P. randolfi by Forsyth (2004). In Montana, most Punctum are either P. californicum or P. minutissimum (Hendricks 2012). Reports exist for tentative P. randolphi from two western counties: Missoula and Sanders. Most or all specimens of Punctum in Montana west of the Continental Divide are probably the same species (P. californicum), but P. randolphi cannot be discounted given their range in adjacent Idaho. The distributions and abundances of all Punctum species in Montana are poorly documented; current status needs investigation. Usually only a few individuals are encountered, but 30 were reported at one Glacier County site in late August (Berry 1919).

Observations in Montana Natural Heritage Program Database
Number of Observations: 6

(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 Conical Spot (Punctum randolphi). 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
Found in mesic to moist mixed conifer forest. Canopy species include Engelmann spruce, grand fir, Douglas-fir, black cottonwood with a secondary canopy of alder, ferns, devil's club, thimbleberry; on the ground under moist dead leaves downed wood and other litter (Forsyth 2004, Burke 2013).

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Citation for data on this website:
Conical Spot — Punctum randolphi.  Montana Field Guide.  .  Retrieved on , from